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In this article, we investigate risk return characteristics and diversification benefits when private equity is used as a portfolio component. We use a unique dataset describing 642 US-American portfolio companies with 3620 private equity investments. Information about precisely dated cash flows at the company level enables for the first time a cash flow equivalent and simultaneous investment simulation in stocks, as well as the construction of stock portfolios for benchmarking purposes. With respect to the methodology involved, we construct private equity, stock-benchmark and mixed-asset portfolios using bootstrap simulations. For the late 1990s we find a dramatic increase in the extent to which private equity outperforms stock investment. In earlier years private equity was underperforming its stock benchmarks. Within the overall class of private equity, returns on earlier private equity investment categories, like venture capital, show on average higher variations and even higher rates of failure. It is in this category in particular that high average portfolio returns are generated solely by the ability to select a few extremely well performing companies, thus compensating for lost investments. There is a high marginal diversifiable risk reduction of about 80% when the portfolio size is increased to include 15 investments. When the portfolio size is increased from 15 to 200 there are few marginal risk diversification effects on the one hand, but a large increase in managing expenditure on the other, so that an actual average portfolio size between 20 and 28 investments seems to be well balanced. We provide empirical evidence that the non-diversifiable risk that a constrained investor, who is exclusively investing in private equity, has to hold exceeds that of constrained stock investors and also the market risk. From the viewpoint of unconstrained investors with complete investment freedom, risk can be optimally reduced by constructing mixed asset portfolios. According to the various private equity subcategories analyzed, there are big differences in optimal allocations to this asset class for minimizing mixed-asset portfolio variance or maximizing performance ratios. We observe optimal portfolio weightings to be between 3% and 65%.
We take a simple time-series approach to modeling and forecasting daily average temperature in U.S. cities, and we inquire systematically as to whether it may prove useful from the vantage point of participants in the weather derivatives market. The answer is, perhaps surprisingly, yes. Time-series modeling reveals conditional mean dynamics, and crucially, strong conditional variance dynamics, in daily average temperature, and it reveals sharp differences between the distribution of temperature and the distribution of temperature surprises. As we argue, it also holds promise for producing the long-horizon predictive densities crucial for pricing weather derivatives, so that additional inquiry into time-series weather forecasting methods will likely prove useful in weather derivatives contexts.
Despite powerful advances in yield curve modeling in the last twenty years, comparatively little attention has been paid to the key practical problem of forecasting the yield curve. In this paper we do so. We use neither the no-arbitrage approach, which focuses on accurately fitting the cross section of interest rates at any given time but neglects time-series dynamics, nor the equilibrium approach, which focuses on time-series dynamics (primarily those of the instantaneous rate) but pays comparatively little attention to fitting the entire cross section at any given time and has been shown to forecast poorly. Instead, we use variations on the Nelson-Siegel exponential components framework to model the entire yield curve, period-by-period, as a three-dimensional parameter evolving dynamically. We show that the three time-varying parameters may be interpreted as factors corresponding to level, slope and curvature, and that they may be estimated with high efficiency. We propose and estimate autoregressive models for the factors, and we show that our models are consistent with a variety of stylized facts regarding the yield curve. We use our models to produce term-structure forecasts at both short and long horizons, with encouraging results. In particular, our forecasts appear much more accurate at long horizons than various standard benchmark forecasts. JEL Code: G1, E4, C5
We consider three sets of phenomena that feature prominently - and separately - in the financial economics literature: conditional mean dependence (or lack thereof) in asset returns, dependence (and hence forecastability) in asset return signs, and dependence (and hence forecastability) in asset return volatilities. We show that they are very much interrelated, and we explore the relationships in detail. Among other things, we show that: (a) Volatility dependence produces sign dependence, so long as expected returns are nonzero, so that one should expect sign dependence, given the overwhelming evidence of volatility dependence; (b) The standard finding of little or no conditional mean dependence is entirely consistent with a significant degree of sign dependence and volatility dependence; (c) Sign dependence is not likely to be found via analysis of sign autocorrelations, runs tests, or traditional market timing tests, because of the special nonlinear nature of sign dependence; (d) Sign dependence is not likely to be found in very high-frequency (e.g., daily) or very low-frequency (e.g., annual) returns; instead, it is more likely to be found at intermediate return horizons; (e) Sign dependence is very much present in actual U.S. equity returns, and its properties match closely our theoretical predictions; (f) The link between volatility forecastability and sign forecastability remains intact in conditionally non-Gaussian environments, as for example with time-varying conditional skewness and/or kurtosis.
We extend the important idea of range-based volatility estimation to the multivariate case. In particular, we propose a range-based covariance estimator that is motivated by financial economic considerations (the absence of arbitrage), in addition to statistical considerations. We show that, unlike other univariate and multivariate volatility estimators, the range-based estimator is highly efficient yet robust to market microstructure noise arising from bid-ask bounce and asynchronous trading. Finally, we provide an empirical example illustrating the value of the high-frequency sample path information contained in the range-based estimates in a multivariate GARCH framework.
Financial theory creates a puzzle. Some authors argue that high-risk entrepreneurs choose debt contracts instead of equity contracts since risky but high returns are of relatively more value for a loan-financed firm. On the contrary, authors who focus explicitly on start-up finance predict that entrepreneurs are the more likely to seek equity-like venture capital contracts, the more risky their projects are. Our paper makes a first step to resolve this puzzle empirically. We present microeconometric evidence on the determinants of debt and equity financing in young and innovative SMEs. We pay special attention to the role of risk for the choice of the financing method. Since risk is not directly observable we use different indicators for financial and project risk. It turns out that our data generally confirms the hypothesis that the probability that a young high-tech firm receives equity financing is an increasing function of the financial risk. With regard to the intrinsic project risk, our results are less conclusive, as some of our indicators of a risky project are found to have a negative effect on the likelihood to be financed by private equity.
We study the returns the venture capital and private equity investment from 221 venture capital and private equity funds that are part of 72 venture capital and private equity firms, 5040 entrepreneurial firms (3826 venture capital and 1214 private equity), and spanning 32 years (1971 - 2003) and 39 countries from North and South America, Europe and Asia. We make use of four main categories of variables to proxy for value-added activities and risks that explain venture capital and private equity returns: market and legal environment, VC characteristics, entrepreneurial firm characteristics, and the characteristics and structure of the investment. We show Heckman sample selection issues in regards to both unrealized and partially realized investments are important to consider for analysing the determinants of realized returns. We further compare the actual unrealized returns, as reported to investment managers, to the predicted unrealized returns based on the estimates of realized returns from the sample selection models. We show there exists significant systematic biases in the reporting of unrealized investments to institutional investors depending on the level of the earnings aggressiveness and disclosure indices in a country, as well as proxies for the degree of information asymmetry between investment managers and venture capital and private equity fund managers. Klassifikation: G24, G28, G31, G32, G35
We analyze welfare maximizing monetary policy in a dynamic two-country model with price stickiness and imperfect competition. In this context, a typical terms of trade externality affects policy interaction between independent monetary authorities. Unlike the existing literature, we remain consistent to a public finance approach by an explicit consideration of all the distortions that are relevant to the Ramsey planner. This strategy entails two main advantages. First, it allows an accurate characterization of optimal policy in an economy that evolves around a steady-state which is not necessarily efficient. Second, it allows to describe a full range of alternative dynamic equilibria when price setters in both countries are completely forward-looking and households' preferences are not restricted. In this context, we study optimal policy both in the long-run and along a dynamic path, and we compare optimal commitment policy under Nash competition and under cooperation. By deriving a second order accurate solution to the policy functions, we also characterize the welfare gains from international policy cooperation. Klassifikation: E52, F41 . This version: January, 2004. First draft: October 2003 .
This paper considers a theoretical model of n asymmetric firms that reduce their initial unit costs by spending on R&D activities. In accordance with Schumpeterian hypotheses we obtain that more efficient (bigger) firms spend more in R&D and this leads to a more concentrated market structure. We also find a positive relationship between innovation and market concentration. This calls for a corrective tax on R&D activities to curtail strategic incentives to over-invest in R&D trying to achieve a higher market share. Klassifikation: L11, L52, O31 . February, 2004.
This paper aims to analyze the impact of different types of venture capitalists on the performance of their portfolio firms around and after the IPO. We thereby investigate the hypothesis that different governance structures, objectives and track record of different types of VCs have a significant impact on their respective IPOs. We explore this hypothesis by using a data set embracing all IPOs which occurred on Germany's Neuer Markt. Our main finding is that significant differences among the different VCs exist. Firms backed by independent VCs perform significantly better two years after the IPO compared to all other IPOs and their share prices fluctuate less than those of their counterparts in this period of time. Obviously, independent VCs, which concentrated mainly on growth stocks (low book-to-market ratio) and large firms (high market value), were able to add value by leading to less post-IPO idiosyncratic risk and more return (after controlling for all other effects). On the contrary, firms backed by public VCs (being small and having a high book-to-market ratio) showed relative underperformance. Klassifikation: G10, G14, G24 . 29th January 2004 .
How might retirees consider deploying the retirement assets accumulated in a defined contribution pension plan? One possibility would be to purchase an immediate annuity. Another approach, called the "phased withdrawal" strategy in the literature, would have the retiree invest his funds and then withdraw some portion of the account annually. Using this second tactic, the withdrawal rate might be determined according to a fixed benefit level payable until the retiree dies or the funds run out, or it could be set using a variable formula, where the retiree withdraws funds according to a rule linked to life expectancy. Using a range of data consistent with the German experience, we evaluate several alternative designs for phased withdrawal strategies, allowing for endogenous asset allocation patterns, and also allowing the worker to make decisions both about when to retire and when to switch to an annuity. We show that one particular phased withdrawal rule is appealing since it offers relatively low expected shortfall risk, good expected payouts for the retiree during his life, and some bequest potential for the heirs. We also find that unisex mortality tables if used for annuity pricing can make women's expected shortfalls higher, expected benefits higher, and bequests lower under a phased withdrawal program. Finally, we show that delayed annuitization can be appealing since it provides higher expected benefits with lower expected shortfalls, at the cost of somewhat lower anticipated bequests. Klassifikation: G22, G23, J26, J32, H55 . January 2004.
Life of Varroa destructor, Anderson and Trueman, an ectoparasitic mite of honeybees, is divided into a reproductive phase in the bee brood and a phoretic phase during which the mite is attached to the adult bee. Phoretic mites leave the colony with workers involved in foraging tasks. Little information is available on the mortality of mites outside the colony. Mites may or not return to the colony as a result of death of the infested foragers, host change by drifting of foragers, or removal of mites outside the colony. That mites do not return to the colony was indicated by substantially higher infestation of outflying workers compared to the infestation of returning workers (Kutschker, 1999). The main objective of the study was to provide information whether V. destructor influences flight behaviour of foragers and consequently returning frequency of foragers to the colony. I first repeated the experiment of Kutschker (1999) examining the infestation of outflying and returning workers. Further, I registered flight duration of foragers using a video method. In this experiment I compared also the infestation and flight duration of bees of different genetic origin, Carnica from Oberursel and bees from Primorsky region. I investigated returning time of workers, returning frequency until evening, drifting to other colonies and orientation toward the nest entrance in the experiments in which workers were released in close vicinity of the colony. At last, I measured the loss of foragers in relation to colony infestation using a Bee Scan. Results from this study, listed below, showed considerable influence of V. destructor on flight behavior of foragers translating into loss of mites. Loss of mites with foragers add substantial component to mite mortality and was underestimated in previous studies. Such loss might be viewed as a mechanism of resistance against V. destructor. a) The mean infestation of outflying workers (0.019±0.018) was twice as the mean infestation of returning workers (0.009±0.018). The difference in the infestation between outflying and returning workers was more marked in highly infested colonies. b) Investigation of individually tagged workers by use of a two camera video recording device showed significantly higher infestation of outflying workers compared to returning workers. Mites were lost by the non returning of infested foragers (22%) and by loss of mites from foragers that returned to the colony without the mite (20%). A small portion of mites (1.8%) was gained. Loss of mites significantly exceeded mite gain. c) The flight duration of infested workers determined by using the same two camera video system was significantly higher in infested compared to uninfested workers of the same age that flew closest at time. The median flight duration of infested workers was 1.7 higher (214s) than the median duration of unifested workers (128s). d) Infested workers took 2.3 times longer to return to the colony than uninfested workers of the same age when released from the same locations, closest at time. The returning time increased with the distance of release. In a group of bees released simultaneously the infestation was higher in bees returning later and in those that did not return in the observation period of 15 min. e) Released workers did not return to the colony 1.5 more frequently than uninfested workers in evening. The difference in returning was significant for locations of 20 and 50m from the colony. No difference in returning between infested and uninfested workers were observed for the most distant location of 400m. f) No significant difference was found in returning time and/or in the returning frequency until evening between workers artificially infested overnight and naturally infested workers. Artificially infested workers returned later and less frequently than a control group indicating rapid influence of V. destructor on flight behavior of foragers. g) The orientation ability of infested workers toward the nest entrance was impaired. Infested workers compared to uninfested workers twice as often approached a dummy entrance before finding the nest entrance. h) No significant differences were found in drifting between infested and uninfested workers. Drifting in the neighboring nucleus colony occurred in about 1% occasions after release of marked workers. Similarly, more infested, but not significantly more infested workers (2.6%) entered a different colored hive than the same colored hive (1.9%). However, the number of drifting bees were to low to make results conclusive. i) The comparison between Carnica and Primorsky workers revealed higher infestation in Carnica compared to Primorsky. Further, Primorsky workers lost more mites during foraging due to mite loss from foragers and non returning of infested workers. No significant differences in flight duration were observed between the two bee stocks. j) Loss of foragers, as determined by the Bee Scan counts of outflying and returning foragers, and the infestation of outflying bees increased significantly over a period of 70 days. A colony with 7.7. higher infestation of outflying foragers lost 2.2. time more bees per flight per day compared to a low infested colony. k) The estimates of mite loss with foragers from mite population per day up to 3.1% exceeds approximately mite mortality of 1% within the colony as represented by counting dead mites on bottom board inserts.
Alzheimer’s disease (AD) is the most common neurodegenerative disorder world wide, causing presenile dementia and death of millions of people. During AD damage and massive loss of brain cells occur. Alzheimer’s disease is genetically heterogeneous and may therefore represent a common phenotype that results from various genetic and environmental influences and risk factors. In approximately 10% of patients, changes of the genetic information were detected (gene mutations). In these cases, Alzheimer’s disease is inherited as an autosomal dominant trait (familial Alzheimer’s disease, FAD). In rare cases of familial Alzheimer’s disease (about 1-3%), mutations have been detected in genes on chromosomes 14 and 1 (encoding for Presenilin 1 and 2, respectively), and on chromosome 21 encoding for the amyloid precursor protein (APP), which is responsible for the release of the cell-damaging protein amyloid-beta (ß-amyloid, Aß). Familial forms of early-onset Alzheimer’s disease are rare; however, their importance extends far beyond their frequency, because they allow to identify some of the critical pathogenetic pathways of the disease. All familial Alzheimer mutations share a common feature: they lead to an enhanced production of the Aß, which is the major constituent of senile plaques in brains of AD patients. New data indicates that Aß promotes neuronal degeneration. Therefore, one aim of these thesis was to elucidate the neurotoxic biochemical pathways induced by Aß, investigating the effect of the FAD Swedish APP double mutation (APPsw) on oxidative stress-induced cell death mechanisms. This mutation results in a three- to sixfold increased Aß production compared to wild-type APP (APPwt). As cell models, the neuronal PC12 (rat pheochromocytoma) and the HEK (human embryonic kidney 293) cell lines were used, which have been transfected with human wiltyp APP or human APP containing the Swedish double mutation. The used cell models offer two important advantages. First, compared to experiments using high concentrations of Aß at micromolar levels applied extracellularly to cells, PC12 APPsw cells secret low Aß levels similar to the situation in FAD brains. Thus, this cell model represents a very suitable approach to elucidate the AD-specific cell death pathways mimicking physiological conditions. Second, these two cell lines (PC12 and HEK APPwt and APPsw) with different production levels of Aß may additionally allow to study dose-dependent effects of Aß. The here obtained results provide evidence for the enhanced cell vulnerability caused by the Swedish APP mutation and elucidate the cell death mechanism probably initiated by intracellulary produced Aß. Here it seems likely that increased production of Aß at physiological levels primes APPsw PC12 cells to undergo cell death only after additional stress, while chronic high levels in HEK cells already lead to enhanced basal apoptotic levels. Crucial effects of the Swedish APP mutation include the impairments of cellular energy metabolism affecting mitochondrial membrane potential and ATP levels as well as the additional activation of caspase 2, caspase 8 and JNK in response to oxidative stress. Thereby ,the following model can be proposed: PC12 cells harboring the Swedish APP mutation have a reduced energy metabolism compared to APPwt or control cells. However, this effect does not leads to enhanced basal apoptotic levels of cultured cells. An exposure of PC12 cells to oxidative stress leads to mitochondrial dysfunction, e.g., decrease in mitochondrial membrane potential and depletion in ATP. The consequence is the activation of the intrinsic apoptotic pathway releasing cytochrome c and Smac resulting in the activation of caspase 9. This effect is amplified by the overexpression of APP, since both APPsw and APPwt PC12 cells show enhanced cytochrome c and Smac release as well as enhanced caspase 9 activity as vector transfected control. In APPsw PC12 cells a parallel pathway is additionally emphased. Due to reduced ATP levels or enhanced Aß production JNK is activated. Furthermore, the extrinsic apoptotic pathway is enhanced, since caspase 8 and caspase 2 activation was clearly enhanced by the Swedish APP mutation. Both pathways may then converge by activating the effector enzyme, caspase 3, and the execution of cell death. In addition, caspase independent effects also needs to be considered. One possibility could be the implication of AIF since AIF expression was found to be induced by the Swedish APP mutation. In APPsw HEK cells high chronic Aß levels leads to enhanced apoptotic levels, reduce mitochondrial membrane potential and ATP levels even under basal conditions. Summarizing, a hypothetical sequence of events is proposed linking FAD, Aß production, JNK-activation, mitochondrial dysfunction with caspase pathway and neuronal loss for our cell model. The brain has a high metabolic rate and is exposured to gradually rising levels of oxidative stress during life. In Swedish FAD patients the levels of oxidative stress are increased in the temporal inferior cortex. This study using a cell model mimicking the in vivo situation in AD brains indicates that probably both, increased Aß production and the gradual rise of oxidative stress throughout life converge at a final common pathway of an increased vulnerability of neurons to apoptotic cell death from FAD patients. Presenilin (PS) 1 is an aspartyl protease, involved in the gamma-secretase mediated proteolysis of Amyloid-ß-protein (Aß), the major constituent of senile plaques in brains of Alzheimer’s disease (AD) patients. Recent studies have suggested an additional role for presenilin proteins in apoptotic cell death observed in AD. Since PS 1 is proteolytic cleaved by caspase 3, it has been prosposed that the resulting C-terminal fragment of PS1 (PSCas) could play a role in signal transduction during apoptosis. Moreover, it was shown that mutant presenilins causing early-onset of familial Alzheimer's disease (FAD) may render cells vulnerable to apoptosis. The mechanism by which PS1 regulates apoptotic cell death is yet not understood. Therefore one aim of our present study was to clarify the involvement of PS1 in the proteolytic cascade of apoptosis and if the cleavage of PS1 by caspase 3 has an regulatory function. Here it is demonstrated that both, PS1 and PS1Cas lead to a reduced vulnerability of PC12 and Jurkat cells to different apoptotic stimuli. However a mutation at the caspase 3 recognition site (D345A/ PSmut), which inhibits cleavage of PS1 by caspase 3, show no differences in the effect of PS1 or PSCas towards apoptotic stimuli. This suggest that proteolysis of PS1 by caspase 3 is not a determinant, but only a secondary effect during apoptosis. Since several FAD mutation distributed through the whole PS1 gene lead to enhanced apoptosis, an abolishment of the antiapoptotic effect of PS1 might contribute to the massive neurodegeneration in early age of FAD patients. Here, the regulate properties of PS1 in apoptosis may not be through an caspase 3 dependent cleavage and generation of PSCas, but rather through interaction of PS1 with other proteins involved in apoptosis.
This paper proves correctness of Nocker s method of strictness analysis, implemented for Clean, which is an e ective way for strictness analysis in lazy functional languages based on their operational semantics. We improve upon the work of Clark, Hankin and Hunt, which addresses correctness of the abstract reduction rules. Our method also addresses the cycle detection rules, which are the main strength of Nocker s strictness analysis. We reformulate Nocker s strictness analysis algorithm in a higherorder lambda-calculus with case, constructors, letrec, and a nondeterministic choice operator used as a union operator. Furthermore, the calculus is expressive enough to represent abstract constants like Top or Inf. The operational semantics is a small-step semantics and equality of expressions is defined by a contextual semantics that observes termination of expressions. The correctness of several reductions is proved using a context lemma and complete sets of forking and commuting diagrams. The proof is based mainly on an exact analysis of the lengths of normal order reductions. However, there remains a small gap: Currently, the proof for correctness of strictness analysis requires the conjecture that our behavioral preorder is contained in the contextual preorder. The proof is valid without referring to the conjecture, if no abstract constants are used in the analysis.
Work on proving congruence of bisimulation in functional programming languages often refers to [How89,How96], where Howe gave a highly general account on this topic in terms of so-called lazy computation systems . Particularly in implementations of lazy functional languages, sharing plays an eminent role. In this paper we will show how the original work of Howe can be extended to cope with sharing. Moreover, we will demonstrate the application of our approach to the call-by-need lambda-calculus lambda-ND which provides an erratic non-deterministic operator pick and a non-recursive let. A definition of a bisimulation is given, which has to be based on a further calculus named lambda-~, since the na1ve bisimulation definition is useless. The main result is that this bisimulation is a congruence and contained in the contextual equivalence. This might be a step towards defining useful bisimulation relations and proving them to be congruences in calculi that extend the lambda-ND-calculus.
This Article concerns the duty of care in American corporate law. To fully understand that duty, it is necessary to distinguish between roles, functions, standards of conduct, and standards of review. A role consists of an organized and socially recognized pattern of activity in which individuals regularly engage. In organizations, roles take the form of positions, such as the position of the director. A function consists of an activity that an actor is expected to engage in by virtue of his role or position. A standard of conduct states the way in which an actor should play a role, act in his position, or conduct his functions. A standard of review states the test that a court should apply when it reviews an actor’s conduct to determine whether to impose liability, grant injunctive relief, or determine the validity of his actions. In many or most areas of law, standards of conduct and standards of review tend to be conflated. For example, the standard of conduct that governs automobile drivers is that they should drive carefully, and the standard of review in a liability claim against a driver is whether he drove carefully. Similarly, the standard of conduct that governs an agent who engages in a transaction with his principal is that the agent must deal fairly, and the standard of review in a claim by the principal against an agent, based on such a transaction, is whether the agent dealt fairly. The conflation of standards of conduct and standards of review is so common that it is easy to overlook the fact that whether the two kinds of standards are or should be identical in any given area is a matter of prudential judgment. In a corporate world in which information was perfect, the risk of liability for assuming a given corporate role was always commensurate with the incentives for assuming the role, and institutional considerations never required deference to a corporate organ, the standards of conduct and review in corporate law might be identical. In the real world, however, these conditions seldom hold, and in American corporate law the standards of review pervasively diverge from the standards of conduct. Traditionally, the two major areas of American corporate law that involved standards of conduct and review have been the duty of care and the duty of loyalty. The duty of loyalty concerns the standards of conduct and review applicable to a director or officer who takes action, or fails to act, in a matter that does involve his own self-interest. The duty of care concerns the standards of conduct and review applicable to a director or officer who takes action, or fails to act, in a matter that does not involve his own self-interest.
When performance measures are used for evaluation purposes, agents have some incentives to learn how their actions affect these measures. We show that the use of imperfect performance measures can cause an agent to devote too many resources (too much effort) to acquiring information. Doing so can be costly to the principal because the agent can use information to game the performance measure to the detriment of the principal. We analyze the impact of endogenous information acquisition on the optimal incentive strength and the quality of the performance measure used.
The volume is a collection of papers given at the conference “sub8 -- Sinn und Bedeutung”, the eighth annual conference of the Gesellschaft für Semantik, held at the Johann-Wolfgang-Goethe-Universität, Frankfurt (Germany) in September 2003. During this conference, experts presented and discussed various aspects of semantics. The very different topics included in this book provide insight into fields of ongoing Semantics research.
In dieser Arbeit werden Untersuchungen über die Anwendbarkeit von vier Methoden zur selektiven Einführung von Radikalen in DNA vorgestellt. Hierzu wurde die EPR-Spektroskopie (Elektronen-paramagnetische Resonanz) benutzt. Die selektive Einführung und Erzeugung von Radikalen in DNA ist nötig, um J-Kopplungen in DNA zu untersuchen. Vor dem Fernziel der Bestimmung der Austauschkopplungskonstanten J in biradikalischer DNA und deren Korrelation mit der charge-transfer-Geschwindigkeitskonstanten kCT stellen diese Untersuchungen einen wichtigen Ausgangspunkt dar. Stabile aromatische Nitroxide. Simulationen von Raumtemperatur-CW-X-Band-EPRSpektren fünf verschiedener aromatischer Nitroxide, welche potentielle DNA-Interkalatoren sind, wurden durchgeführt. Die aromatischen Nitroxide zeigen aufgelöste Hyperfeinkopplungen, welche zu dem Schluss führen, dass die Spindichte in hohem Maße delokalisiert ist, was die Verwendung dieser Verbindungen zur Messung von J-Kopplungen in biradikalischer DNA erlaubt. Transiente Guanin-Radikale. Transiente Guanin-Radikale werden in DNA selektiv durch die Flash-Quench-Technik erzeugt, bei der optisch anregbare Ruthenium-Interkalatoren verwendet werden. Transiente Thymyl-Radikale aus UV-bestrahltem 4'-Pivaloyl-Thymidin. Es werden photoinduzierte Prozesse untersucht, welche durch Bestrahlung von Thymin-Nukleosiden, die an der 4’-Position die optisch spaltbare Pivaloyl-Gruppe tragen, erzeugt werden. Dieses Nukleosid wurde speziell dafür entworfen, um Elektronenlöcher in DNA zu injizieren. In dieser Arbeit wird gezeigt, dass diese Verbindung benutzt werden kann, um selektiv eine Thymin-Base zu reduzieren. Transiente Thymyl-Radikale erzeugt durch ein neuartig modifiziertes Thymin nach UV-Bestrahlung. Photoinduzierte Prozesse, welche durch Bestrahlung eines ähnlichen Thymidin-Nukleosids erzeugt wurden, werden hier untersucht. Dieses Thymidin- Nukleosid wurde modifiziert, indem die optisch spaltbare Pivaloyl-Gruppe an eine Seitenkette angehängt wurde, welche an der C6-Position der Thymin-Base sitzt. Die Thymin-Base wurde speziell dafür entworfen, um Elektronen in DNA zu injizieren. In dieser Arbeit wurde bestätigt, dass ein Überschuss-Elektron selektiv auf eine Thymin-Base transferiert werden kann.
Die in Englisch verfasste Dissertation, die unter der Betreuung von Herrn Prof. Dr. H. F. de Groote, Fachbereich Mathematik, entstand, ist der Mathematischen Physik zuzuordnen. Sie behandelt Stonesche Spektren von Neumannscher Algebren, observable Funktionen sowie einige Anwendungen in der Physik. Das abschließende Kapitel liefert eine Verallgemeinerung des Kochen-Specker-Theorems. Stonesche Spektren und observable Funktionen wurden von de Groote eingeführt. Das Stonesche Spektrum einer von Neumann-Algebra ist eine Verallgemeinerung des Gelfand-Spektrums, die observablen Funktionen verallgemeinern die Gelfand-Transformierten. Da de Grootes Ergebnisse zum großen Teil unveröffentlicht sind, folgt nach dem Einleitungskapitel im zweiten Kapitel eine Übersichtsdarstellung dieser Ergebnisse. Das dritte Kapitel behandelt die Stoneschen Spektren endlicher von Neumann-Algebren. Für Algebren vom Typ In wird eine vollständige Charakterisierung des Stoneschen Spektrums entwickelt. Zu Typ-II1-Algebren werden einige Resultate vorgestellt. Das vierte Kapitel liefert. einige einfache Anwendungen des Formalismus auf die Physik. Das fünfte Kapitel gibt erstmals einen funktionalanalytischen Beweis des Kochen-Specker-Theorems und liefert die Verallgemeinerung dieses Satzes, wobei die Situation für alle von Neumann-Algebren geklärt wird.
Die Ermittlung von Proteinstukturen mittels NMR-Spektroskopie ist ein komplexer Prozess, wobei die Resonanzfrequenzen und die Signalintensitäten den Atomen des Proteins zugeordnet werden. Zur Bestimmung der räumlichen Proteinstruktur sind folgende Schritte erforderlich: die Präparation der Probe und 15N/13C Isotopenanreicherung, Durchführung der NMR Experimente, Prozessierung der Spektren, Bestimmung der Signalresonanzen ('Peak-picking'), Zuordnung der chemischen Verschiebungen, Zuordnung der NOESY-Spektren und das Sammeln von konformationellen Strukturparametern, Strukturrechnung und Strukturverfeinerung. Aktuelle Methoden zur automatischen Strukturrechnung nutzen eine Reihe von Computeralgorithmen, welche Zuordnungen der NOESY-Spektren und die Strukturrechnung durch einen iterativen Prozess verbinden. Obwohl neue Arten von Strukturparametern wie dipolare Kopplungen, Orientierungsinformationen aus kreuzkorrelierten Relaxationsraten oder Strukturinformationen, die sich in Gegenwart paramagnetischer Zentren in Proteinen ergeben, wichtige Neuerungen für die Proteinstrukturrechnung darstellen, sind die Abstandsinformationen aus NOESY-Spektren weiterhin die wichtigste Basis für die NMR-Strukturbestimmung. Der hohe zeitliche Aufwand des 'peak-picking' in NOESY-Spektren ist hauptsächlich bedingt durch spektrale Überlagerung, Rauschsignale und Artefakte in NOESY-Spektren. Daher werden für das effizientere automatische 'Peak-picking' zuverlässige Filter benötigt, um die relevanten Signale auszuwählen. In der vorliegenden Arbeit wird ein neuer Algorithmus für die automatische Proteinstrukturrechnung beschrieben, der automatisches 'Peak-picking' von NOESY-Spektren beinhaltet, die mit Hilfe von Wavelets entrauscht wurden. Der kritische Punkt dieses Algorithmus ist die Erzeugung inkrementeller Peaklisten aus NOESY-Spektren, die mit verschiedenen auf Wavelets basierenden Entrauschungsprozeduren prozessiert wurden. Mit Hilfe entrauschter NOESY-Spektren erhält man Signallisten mit verschiedenen Konfidenzbereichen, die in unterschiedlichen Schritten der kombinierten NOE-Zuordnung/Strukturrechnung eingesetzt werden. Das erste Strukturmodell beruht auf stark entrauschten Spektren, die die konservativste Signalliste mit als weitgehend sicher anzunehmenden Signalen ergeben. In späteren Stadien werden Signallisten aus weniger stark entrauschten Spektren mit einer größeren Anzahl von Signalen verwendet. Die Auswirkung der verschiedenen Entrauschungsprozeduren auf Vollständigkeit und Richtigkeit der NOESY Peaklisten wurde im Detail untersucht. Durch die Kombination von Wavelet-Entrauschung mit einem neuen Algorithmus zur Integration der Signale in Verbindung mit zusätzlichen Filtern, die die Konsistenz der Peakliste prüfen ('Network-anchoring' der Spinsysteme und Symmetrisierung der Peakliste), wird eine schnelle Konvergenz der automatischen Strukturrechnung erreicht. Der neue Algorithmus wurde in ARIA integriert, einem weit verbreiteten Computerprogramm für die automatische NOE-Zuordnung und Strukturrechnung. Der Algorithmus wurde an der Monomereinheit der Polysulfid-Schwefel-Transferase (Sud) aus Wolinella succinogenes verifiziert, deren hochaufgelöste Lösungsstruktur vorher auf konventionelle Weise bestimmt wurde. Neben der Möglichkeit zur Bestimmung von Proteinlösungsstrukturen bietet sich die NMR-Spektroskopie auch als wirkungsvolles Werkzeug zur Untersuchung von Protein-Ligand- und Protein-Protein-Wechselwirkungen an. Sowohl NMR Spektren von isotopenmarkierten Proteinen, als auch die Spektren von Liganden können für das 'Screening' nach Inhibitoren benutzt werden. Im ersten Fall wird die Sensitivität der 1H- und 15N-chemischen Verschiebungen des Proteinrückgrats auf kleine geometrische oder elektrostatische Veränderungen bei der Ligandbindung als Indikator benutzt. Als 'Screening'-Verfahren, bei denen Ligandensignale beobachtet werden, stehen verschiedene Methoden zur Verfügung: Transfer-NOEs, Sättigungstransferdifferenzexperimente (STD, 'saturation transfer difference'), ePHOGSY, diffusionseditierte und NOE-basierende Methoden. Die meisten dieser Techniken können zum rationalen Design von inhibitorischen Verbindungen verwendet werden. Für die Evaluierung von Untersuchungen mit einer großen Anzahl von Inhibitoren werden effiziente Verfahren zur Mustererkennung wie etwa die PCA ('Principal Component Analysis') verwendet. Sie eignet sich zur Visualisierung von Ähnlichkeiten bzw. Unterschieden von Spektren, die mit verschiedenen Inhibitoren aufgenommen wurden. Die experimentellen Daten werden zuvor mit einer Serie von Filtern bearbeitet, die u.a. Artefakte reduzieren, die auf nur kleinen Änderungen der chemischen Verschiebungen beruhen. Der am weitesten verbreitete Filter ist das sogenannte 'bucketing', bei welchem benachbarte Punkte zu einen 'bucket' aufsummiert werden. Um typische Nachteile der 'bucketing'-Prozedur zu vermeiden, wurde in der vorliegenden Arbeit der Effekt der Wavelet-Entrauschung zur Vorbereitung der NMR-Daten für PCA am Beispiel vorhandener Serien von HSQC-Spektren von Proteinen mit verschiedenen Liganden untersucht. Die Kombination von Wavelet-Entrauschung und PCA ist am effizientesten, wenn PCA direkt auf die Wavelet-Koeffizienten angewandt wird. Durch die Abgrenzung ('thresholding') der Wavelet-Koeffizienten in einer Multiskalenanalyse wird eine komprimierte Darstellung der Daten erreicht, welche Rauschartefakte minimiert. Die Kompression ist anders als beim 'bucketing' keine 'blinde' Kompression, sondern an die Eigenschaften der Daten angepasst. Der neue Algorithmus kombiniert die Vorteile einer Datenrepresentation im Wavelet-Raum mit einer Datenvisualisierung durch PCA. In der vorliegenden Arbeit wird gezeigt, dass PCA im Wavelet- Raum ein optimiertes 'clustering' erlaubt und dabei typische Artefakte eliminiert werden. Darüberhinaus beschreibt die vorliegende Arbeit eine de novo Strukturbestimmung der periplasmatischen Polysulfid-Schwefel-Transferase (Sud) aus dem anaeroben gram-negativen Bakterium Wolinella succinogenes. Das Sud-Protein ist ein polysulfidbindendes und transferierendes Enzym, das bei niedriger Polysulfidkonzentration eine schnelle Polysulfid-Schwefel-Reduktion katalysiert. Sud ist ein 30 kDa schweres Homodimer, welches keine prosthetischen Gruppen oder schwere Metallionen enthält. Jedes Monomer enhält ein Cystein, welches kovalent bis zu zehn Polysulfid-Schwefel (Sn 2-) Ionen bindet. Es wird vermutet, dass Sud die Polysulfidkette auf ein katalytischen Molybdän-Ion transferiert, welches sich im aktiven Zentrum des membranständigen Enzyms Polysulfid-Reduktase (Psr) auf dessen dem Periplasma zugewandten Seite befindet. Dabei wird eine reduktive Spaltung der Kette katalysiert. Die Lösungsstruktur des Homodimeres Sud wurde mit Hilfe heteronuklearer, mehrdimensionaler NMR-Techniken bestimmt. Die Struktur beruht auf von NOESY-Spektren abgeleiteten Distanzbeschränkungen, Rückgratwasserstoffbindungen und Torsionswinkeln, sowie auf residuellen dipolaren Kopplungen, die für die Verfeinerung der Struktur und für die relative Orientierung der Monomereinheiten wichtig waren. In den NMR Spektren der Homodimere haben alle symmetrieverwandte Kerne äquivalente magnetische Umgebungen, weshalb ihre chemischen Verschiebungen entartet sind. Die symmetrische Entartung vereinfacht das Problem der Resonanzzuordnung, da nur die Hälfte der Kerne zugeordnet werden müssen. Die NOESY-Zuordnung und die Strukturrechnung werden dadurch erschwert, dass es nicht möglich ist, zwischen den Intra-Monomer-, Inter-Monomer- und Co-Monomer- (gemischten) NOESY-Signalen zu unterscheiden. Um das Problem der Symmetrie-Entartung der NOESY-Daten zu lösen, stehen zwei Möglichkeiten zur Verfügung: (I) asymmetrische Markierungs-Experimente, um die intra- von den intermolekularen NOESY-Signalen zu unterscheiden, (II) spezielle Methoden der Strukturrechnung, die mit mehrdeutigen Distanzbeschränkungen arbeiten können. Die in dieser Arbeit vorgestellte Struktur wurde mit Hilfe der Symmetrie-ADR- ('Ambigous Distance Restraints') Methode in Kombination mit Daten von asymetrisch isotopenmarkierten Dimeren berechnet. Die Koordinaten des Sud-Dimers zusammen mit den NMR-basierten Strukturdaten wur- den in der RCSB-Proteindatenbank unter der PDB-Nummer 1QXN abgelegt. Das Sud-Protein zeigt nur wenig Homologie zur Primärsequenz anderer Proteine mit ähnlicher Funktion und bekannter dreidimensionaler Struktur. Bekannte Proteine sind die Schwefeltransferase oder das Rhodanese-Enzym, welche beide den Transfer von einem Schwefelatom eines passenden Donors auf den nukleophilen Akzeptor (z.B von Thiosulfat auf Cyanid) katalysieren. Die dreidimensionalen Strukturen dieser Proteine zeigen eine typische a=b Topologie und haben eine ähnliche Umgebung im aktiven Zentrum bezüglich der Konformation des Proteinrückgrades. Die Schleife im aktiven Zentrum umgibt das katalytische Cystein, welches in allen Rhodanese-Enzymen vorhanden ist, und scheint im Sud-Protein flexibel zu sein (fehlende Resonanzzuordnung der Aminosäuren 89-94). Das Polysulfidende ragt aus einer positiv geladenen Bindungstasche heraus (Reste: R46, R67, K90, R94), wo Sud wahrscheinlich in Kontakt mit der Polysulfidreduktase tritt. Das strukturelle Ergebnis wurde durch Mutageneseexperimente bestätigt. In diesen Experimenten konnte gezeigt werden, dass alle Aminosäurereste im aktiven Zentrum essentiell für die Schwefeltransferase-Aktivität des Sud-Proteins sind. Die Substratbindung wurde früher durch den Vergleich von [15N,1H]-TROSY-HSQC-Spektren des Sud-Proteins in An- und Abwesenheit des Polysulfidliganden untersucht. Bei der Substratbindung scheint sich die lokale Geometrie der Polysulfidbindungsstelle und der Dimerschnittstelle zu verändern. Die konformationellen Änderungen und die langsame Dynamik, hervorgerufen durch die Ligandbindung können die weitere Polysulfid-Schwefel-Aktivität auslösen. Ein zweites Polysulfid-Schwefeltransferaseprotein (Str, 40 kDa) mit einer fünffach höheren nativen Konzentration im Vergleich zu Sud wurde im Bakterienperiplasma von Wolinella succinogenes entdeckt. Es wird angenommen, dass beide Protein einen Polysulfid-Schwefel-Komplex bilden, wobei Str wässriges Polysulfid sammelt und an Sud abgibt, welches den Schwefeltransfer zum katalytischen Molybdän-Ion auf das aktive Zentrum der dem Periplasma zugewandten Seite der Polysulfidreduktase durchführt. Änderungen chemischer Verschiebungen in [15N,1H]-TROSY-HSQC-Spektren zeigen, dass ein Polysulfid-Schwefeltransfer zwischen Str und Sud stattfindet. Eine mögliche Protein-Protein-Wechselwirkungsfläche konnte bestimmt werden. In der Abwesenheit des Polysulfidsubstrates wurden keine Wechselwirkungen zwischen Sud und Str beobachtet, was die Vermutung bestätigt, dass beide Proteine nur dann miteinander wechselwirken und den Polysulfid-Schwefeltransfer ermöglichen, wenn als treibende Kraft Polysulfid präsent ist.
We investigate transverse hadron spectra from relativistic nucleus-nucleus collisions which reflect important aspects of the dynamics - such as the generation of pressure - in the hot and dense zone formed in the early phase of the reaction. Our analysis is performed within two independent transport approaches (HSD and UrQMD) that are based on quark, diquark, string and hadronic degrees of freedom. Both transport models show their reliability for elementary pp as well as light-ion (C+C, Si+Si) reactions. However, for central Au+Au (Pb+Pb) collisions at bombarding energies above ~ 5 A.GeV the measured K+- transverse mass spectra have a larger inverse slope parameter than expected from the calculation. Thus the pressure generated by hadronic interactions in the transport models above ~ 5 A.GeV is lower than observed in the experimental data. This finding shows that the additional pressure - as expected from lattice QCD calculations at finite quark chemical potential and temperature - is generated by strong partonic interactions in the early phase of central Au+Au (Pb+Pb) collisions.
We investigate hadron production as well as transverse hadron spectra in nucleus-nucleus collisions from 2 A.GeV to 21.3 A.TeV within two independent transport approaches (UrQMD and HSD) that are based on quark, diquark, string and hadronic degrees of freedom. The comparison to experimental data demonstrates that both approaches agree quite well with each other and with the experimental data on hadron production. The enhancement of pion production in central Au+Au (Pb+Pb) collisions relative to scaled pp collisions (the 'kink') is well described by both approaches without involving any phase transition. However, the maximum in the K+/Pi+ ratio at 20 to 30 A.GeV (the 'horn') is missed by ~ 40%. A comparison to the transverse mass spectra from pp and C+C (or Si+Si) reactions shows the reliability of the transport models for light systems. For central Au+Au (Pb+Pb) collisions at bombarding energies above ~ 5 A.GeV, however, the measured K +/- m-theta-spectra have a larger inverse slope parameter than expected from the calculations. The approximately constant slope of K+/-spectra at SPS (the 'step') is not reproduced either. Thus the pressure generated by hadronic interactions in the transport models above ~ 5 A.GeV is lower than observed in the experimental data. This finding suggests that the additional pressure - as expected from lattice QCD calculations at finite quark chemical potential and temperature - might be generated by strong interactions in the early pre-hadronic/partonic phase of central Au+Au (Pb+Pb) collisions.
To be published in J. Phys. G - Proceedings of SQM 2004 : We review the results from the various hydrodynamical and transport models on the collective flow observables from AGS to RHIC energies. A critical discussion of the present status of the CERN experiments on hadron collective flow is given. We emphasize the importance of the flow excitation function from 1 to 50 A.GeV: here the hydrodynamic model has predicted the collapse of the v2-flow ~ 10 A.GeV; at 40 A.GeV it has been recently observed by the NA49 collaboration. Since hadronic rescattering models predict much larger flow than observed at this energy we interpret this observation as evidence for a first order phase transition at high baryon density r b. Moreover, the connection of the elliptic flow v2 to jet suppression is examined. It is proven experimentally that the collective flow is not faked by minijet fragmentation. Additionally, detailed transport studies show that the away-side jet suppression can only partially (< 50%) be due to hadronic rescattering. Furthermore, the change in sign of v1, v2 closer to beam rapidity is related to the occurence of a high density first order phase transition in the RHIC data at 62.5, 130 and 200 A.GeV.
We investigate hadron production and transverse hadron spectra in nucleus-nucleus collisions from 2 A·GeV to 21.3 A·TeV within two independent transport approaches (UrQMD and HSD) based on quark, diquark, string and hadronic degrees of freedom. The enhancement of pion production in central Au+Au (Pb+Pb) collisions relative to scaled pp collisions (the ’kink’) is described well by both approaches without involving a phase transition. However, the maximum in the K+ p+ ratio at 20 to 30 A·GeV (the ’horn’) is missed by ~ 40%. Also, at energies above ~5 A·GeV, the measured K± mT-spectra have a larger inverse slope than expected from the models. Thus the pressure generated by hadronic interactions in the transport models at high energies is too low. This finding suggests that the additional pressure - as expected from lattice QCD at finite quark chemical potential and temperature - might be generated by strong interactions in the early pre-hadronic/partonic phase of central heavy-ion collisions. Finally, we discuss the emergence of density perturbations in a first-order phase transition and why they might affect relative hadron multiplicities, collective flow, and hadron mean-free paths at decoupling. A minimum in the collective flow v2 excitation function was discovered experimentally at 40 A·GeV - such a behavior has been predicted long ago as signature for a first order phase transition.
We investigate hadron production as well as transverse hadron spectra from proton-proton, proton-nucleus and nucleus-nucleus collisions from 2 A·GeV to 21.3 A·TeV within two independent transport approaches (HSD and UrQMD) that are based on quark, diquark, string and hadronic degrees of freedom. The comparison to experimental data on transverse mass spectra from pp, pA and C+C (or Si+Si) reactions shows the reliability of the transport models for light systems. For central Au+Au (Pb+Pb) collisions at bombarding energies above ~5 A·GeV, furthermore, the measured K± transverse mass spectra have a larger inverse slope parameter than expected from the default calculations. We investigate various scenarios to explore their potential effects on the K± spectra. In particular the initial state Cronin effect is found to play a substantial role at top SPS and RHIC energies. However, the maximum in the K+/..+ ratio at 20 to 30 A·GeV is missed by 40% and the approximately constant slope of the K± spectra at SPS energies is not reproduced either. Our systematic analysis suggests that the additional pressure - as expected from lattice QCD calculations at finite quark chemical potential µq and temperature T- should be generated by strong interactions in the early pre-hadronic/partonic phase of central Au+Au (Pb+Pb) collisions.
We investigate the effects of strong color fields and of the associated enhanced intrinsic transverse momenta on the phi-meson production in ultrarelativistic heavy ion collisions at RHIC. The observed consequences include a change of the spectral slopes, varying particle ratios, and also modified mean transverse momenta. In particular, the composition of the production processes of phi-mesons, that is, direct production vs. coalescence-like production, depends strongly on the strength of the color fields and intrinsic transverse momenta and thus represents a sensitive probe for their measurement.
Im Rahmen dieser Dissertation wurde die Photophysik und die elektronische Struktur einer Klasse neuartiger Donator-Akzeptor-Ladungstransfer-Komplexe untersucht. Im Wesentlichen bestehen diese Verbindungen aus einem Ferrocen-Donator (Fc) und organischen Akzeptoren, die über B-N-Bindungen verbrückt sind, welche sich bei dieser Art von makromolekularen Systemen spontan bilden. Zentraler Gegenstand dieser Arbeit war die spektroskopische Untersuchung des Metall-zu-Ligand-Ladungstransfers (engl. Abkürzung: MLCT) im elektronischen Anregungszustand dieser kationischen Komplexverbindungen, die im Weiteren als „Fc-B-bpy“-Verbindungen bezeichnet werden. Die vorliegende Arbeit analysiert eine Vielzahl miteinander verwandter Fc-B-bpy-Derivate. Die Arbeit ist gegliedert in 1.) die Analyse der Absorptionsspektren vom UV- bis zum nahen Infrarot-Spektralbereich (250-1000 nm) von Lösungen, dotierten Polymer-Dünnfilmen und Einkristallen, 2.) die zeitaufgelöste optische Spektroskopie des angeregten Zustands auf der Pikosekunden-Zeitskala, 3.) die Analyse elektrochemischer Messungen an Lösungen, und 4.) die Auswertung quantenchemischer Berechnungen. Für die zeitaufgelösten Messungen wurde ein komplexes optisches Spektroskopie-System mit breitbandigen Femtosekunden-Pulsen sowie den entsprechenden zeitaufgelösten Detektionsmethoden (spektral gefilterte Weißlicht-Detektion) aufgebaut. Die Ergebnisse dieser Arbeit beweisen die Existenz eines MLCT-Übergangs mit fast vollständigem Übergang eines Fc-Donator-Elektrons zum B-bpy-Akzeptor bei optischer Anregung. Die vergleichenden Untersuchungen der spektroskopischen Eigenschaften verschiedener Derivate liefern wichtige Information für die Entwicklung neuartiger Derivate, einschließlich verwandter Polymere, mit verbesserten spektroskopischen Eigenschaften. Es wurden transiente Absorptionsmessungen bestimmter Fc-B-bpy-Derivate in Lösung nach gepulster Anregung der MLCT-Bande (bei 500 nm) über einen Zeitbereich von 0,1-1000 ps und einen Wellenlängenbereich von 460-760 nm vorgenommen. Aus den Messergebnissen geht hervor, dass die Relaxation aus dem angeregten MLCT-Zustand in den Grundzustand auf verschiedenen Zeitskalen geschehen kann, welche im Bereich zwischen ~18 und 900 ps liegen. Ein Vergleich verschiedener Derivate mit unterschiedlicher Flexibilität in der Konformation zeigt, dass die Starrheit der Bindungen zwischen Donatoren und Akzeptoren ein wesentlicher Faktor für die Lebensdauer des angeregten Zustands ist. Wenn die Akzeptorgruppen relativ frei rotieren können, ist es der Verbindung möglich, eine Geometrie einzunehmen, von der aus ein effizienter, strahlungsfreier Übergang in den Grundzustand erfolgen kann. Dieser Befund zeigt einen Weg auf, wie neuartige, verwandte Verbindungen mit größerer Lebensdauer das angeregten Zustands synthetisiert werden können, indem darauf geachtet wird, daß eine starre molekulare Architektur zwischen Donator und Akzeptor verwirklicht wird.
The paper analyses the financial structure of German inward FDI. From a tax perspective, intra-company loans granted by the parent should be all the more strongly preferred over equity the lower the tax rate of the parent and the higher the tax rate of the German affiliate. From our study of a panel of more than 8,000 non-financial affiliates in Germany, we find only small effects of the tax rate of the foreign parent. However, our empirical results show that subsidiaries that on average are profitable react more strongly to changes in the German corporate tax rate than this is the case for less profitable firms. This gives support to the frequent concern that high German taxes are partly responsible for the high levels of intracompany loans. Taxation, however, does not fully explain the high levels of intra-company borrowing. Roughly 60% of the cross-border intra-company loans turn out to be held by firms that are running losses. JEL - Klassifikation H25 , F23 .
This paper evaluates the effects of Public Sponsored Training in East Germany in the context of reiterated treatments. Selection bias based on observed characteristics is corrected for by applying kernel matching based on the propensity score. We control for further selection and the presence of Ashenfelter's Dip before the program with conditional difference-in-differences estimators. Training as a first treatment shows insignificant effects on the transition rates. The effect of program sequences and the incremental effect of a second program on the reemployment probability are insignificant. However, the incremental effect on the probability to remain employed is slightly positive. JEL - Klassifikation: H43 , C23 , J6 , J64 , C14
The Box-Cox quantile regression model using the two stage method introduced by Chamberlain (1994) and Buchinsky (1995) provides an attractive extension of linear quantile regression techniques. However, a major numerical problem exists when implementing this method which has not been addressed so far in the literature. We suggest a simple solution modifying the estimator slightly. This modification is easy to implement. The modified estimator is still [square root] n-consistent and its asymptotic distribution can easily be derived. A simulation study confirms that the modified estimator works well.
This paper investigates the magnitude and the main determinants of share price reactions to buy-back announcements of German corporations. For our comprehensive sample of 224 announcements that took place between May 1998 and April 2003 we find average cumulative abnormal returns around -7.5% for the thirty days preceding the announcement and around +7.0 % for the ten days following the announcement. We regress post-announcement abnormal returns with multiple firm characteristics and provide evidence which supports the undervaluation signaling hypothesis but not the excess cash hypothesis or the tax-efficiency hypothesis. In extending prior empirical work, we also analyze price effects from initial statements of firms that they intend to seek shareholder approval for a buy-back plan. Observed cumulative abnormal returns on this initial date are in excess of 5% implying a total average price effect between 12% and 15% from implementing a buy-back plan. We conjecture that the German regulatory environment is the main reason why market variations to buy-back announcements are much stronger in Germany than in other countries and conclude that initial statements by managers to seek shareholders’ approval for a buy-back plan should also be subject to legal ad-hoc disclosure requirements.
This paper analyzes empirically the distribution of unemployment durations in West- Germany before and after the changes during the mid 1980s in the maximum entitlement periods for unemployment benefits for elderly unemployed. The analysis is based on the comprehensive IAB employment subsample containing register panel data for about 500.000 individuals in West Germany. We analyze two proxies for unemployment since the data do not precisely measure unemployment in an economic sense. We provide a theoretical analysis of the link between the durations of nonemployment and of unemployment between jobs. Our empirical analysis finds significant changes in the distributions of nonemployment durations for older unemployed individuals. At the same time, the distribution of unemployment durations between jobs did not change in response to the reforms. Our findings are consistent with an interpretation that many firms and workers used the more bene cial laws as a part of early retirement packages but those workers who were still looking for a job did not reduce their search effort in response to the extension of the maximum entitlement periods. This interpretation is consistent with our theoretical model under plausible assumptions. JEL: C24, J64, J65
We show that multi-bank loan pools improve the risk-return profile of banks’ loan business. Banks write simple contracts on the proceeds from pooled loan portfolios, taking into account the free-rider problems in joint loan production. Thus, banks benefit greatly from diversifying credit risk while limiting the efficiency loss due to adverse incentives. We present calibration results that the formation of loan pools reduce the volatility in default rates, proxying for credit risk, of participating banks’ loan portfolios by roughly 70% in our sample. Under reasonable assumptions, the gain in return on equity (in certainty equivalent terms) is around 20 basis points annually.
Global reserves of coal, oil and natural gas are diminishing; global energy requirements however are dramatically increasing. Renewable energy sources lower the threat to the earth’s climate but are not able to meet the energy consumption in major urban areas. The opinion of many experts is that the future will be dominated by hydrogen. However, this gas is essentially totally manufactured from fossil fuels and is hence of limited abundance – not to mention the hazards involved in its utilisation. - A novel energy concept involving solar and thus carbon-independent hydrogen-based technology necessitates an intermediate storage vehicle for renewable energy. This future energy carrier should be simple to manufacture, be available to an unlimited degree or at least be suitable for recycling, be able to store and transport the energy without hazards, demonstrate a high energy density and release no carbon dioxide or other climatically detrimental substances. - Silicon successfully functions as a tailor-made intermediate linking decentrally operating renewable energy-generation technology with equally decentrally organised hydrogen-based infrastructure at any location of choice. In contrast to oil and in particular hydrogen, the transport and storage of silicon are free from potential hazards and require a simple infrastructure similar to that needed for coal.
This paper compares the accuracy of credit ratings of Moody s and Standard&Poors. Based on 11,428 issuer ratings and 350 defaults in several datasets from 1999 to 2003 a slight advantage for the rating system of Moody s is detected. Compared to former research the robustness of the results is increased by using nonparametric bootstrap approaches. Furthermore, robustness checks are made to control for the impact of Watchlist entries, staleness of ratings and the effect of unsolicited ratings on the results.
National borders in Europe have been opening since 1992 and the Union is expanding to embrace more countries prompting enterprises to consider alternative and more attractive locations outside their home country to handle part of their activities (Van Dijk and Pellenbarg, 2000; Cantwell and Iammarino, 2002). International relocation is becoming more and more popular even for small and medium-sized firms that are involved in a growing internationalisation process, mirroring the path of multinational enterprises. Italy, like other industrialised countries, is experiencing a fragmentation of the production chain: firms tend to shift high labour-intensive manufacturing activities to areas characterised by an abundance of low-cost labour (i.e. Central Eastern Europe, India, South East Asia, Latin America, Russia and Central Asia). The internationalisation process by Italian district SMEs has assumed significant dimensions. It has become a relevant topic in recent economic debate because of its consequences for the local context and, in particular, the implication for the survival of the Italian district model (see, among others, Becattini, 2002; Rullani, 1998 and Cor, 2000). The purpose of the paper is twofold: it aims at (i) identifying the managerial approaches to the internationalisation process adopted by the Italian district SMEs and by the Industrial District (ID) itself and (ii) at investigating whether the international delocalisation to the South Eastern European countries (SEECs) constitutes a threat or an opportunity for the Italian district model. The paper is organised as follows. The general introduction is followed by a description of the evolution of the internationalisation processes in Italy over the last three decades. Section three presents a discussion of the internationalisation strategies adopted by Italian SMEs. Section four focuses on the internationalisation process of the Italian industrial districts SMEs. A review of the studies on the subject is offered in section five. Section six presents a qualitative study on the internationalisation process as undergone by sports shoes manufacturers in the Montebelluna district, in north-east Italy. This study shows different managerial strategies to the internationalisation process and emphasises that the motivations can evolve over time, from originally cost-saving to increasingly market-oriented or global strategies. On the basis of a literature review, section seven investigates whether internationalisation constitutes a threat (i.e. loss of jobs and knowledge) or an opportunity (i.e. enlargement of the ID, update district s competitiveness) for the district model. Finally, some summarising remarks in section eight conclude the paper.
The hypothesis that oxidative stress plays a role in the pathogenesis of Alzheimer’s disease (AD) was tested by studying oxidative damage, acitvities of antioxidant enzymes and levels of reactive oxygen species (ROS) in several models. To this end, mouse models transgenic for mutant presenilin (PS1M146L) as well as mutant amyloid precursor protein (APP) and human post mortem brain tissue from sporadic AD patients and age-matched controls were studied. Aging leads to an upregulation of antioxidant enzyme activities of Cu/Zn-superoxide dismutase (Cu/Zn-SOD), glutathione peroxidase (GPx) and glutathione reductase (GR) in brains from C57BL/6J mice. Simultaneously, levels of lipid peroxidation products malondialdehyde MDA and 4-hydroxynonenal HNE were reduced. Additionally, pronounced gender effects were observed, as female mice display better protection against oxidative damage due to higher activity of GPx. Hence, antioxidant enzymes provide an important contribution to the protection against oxidative damage. In PS1M146L transgenic mice oxidative damage was only detectable in 19-22 months old mice, arguing for an additive effect of aging and the PS1 mutation. Both HNE levels in brain tissue as well as mitochondrial and cytosolic levels of ROS in splenic lymphocytes were increased in PS1M146L mice. Antioxidant defences were unaltered. In PDGF-APP and PDGF-APP/PS1 trangenic mice no changes in any of the parameters studied were observed in any age group. In contrast, Thy1-APP transgenic mice display oxidative damage as assessed by increased HNE levels. Reduced activity of Cu/Zn-SOD may explain this observation. Additionally, gender modified this effect, as female APP transgenic mice display higher b-secretase cleavage of APP and simultaneously increased HNE levels and reduced Cu/Zn-SOD activity earlier than male mice, i.e. from an age of 3 months and before the formation of Ab plaques. Reduced Cu/Zn-SOD activity was also found in another APP transgenic mouse model, in APP23 mice. In post mortem brain tissue from sporadic AD patients activities of Cu/Zn-SOD and GPx were however increased, and changes were most pronounced in temporal cortex. Simultaneously, levels of HNE but not MDA were elevated. Additionally, in vitro stimulation of lipid peroxidation led to increased MDA formation in samples from AD patients, indicating that increased activity of Cu/Zn-SOD and GPx are insufficient to protect against oxidative damage. Furthermore, the observed changes were subject to a gender effect, as samples from female AD patients showed increased activities of Cu/Zn-SOD and GPx as well as increased HNE levels, indicating that brain tissue from females is more sensitive towards oxidative damage. Levels of soluble Ab1-40 were positively correlated with with MDA levels and activities of Cu/Zn-SOD and GPx. Additionally, levels of lipid peroxidation products MDA and HNE are gene-dose-dependently modulated by the Apolipoprotein E4 allele, the most important genetic risk factor for AD known so far. While MDA levels were negatively correlated with MMSE scores, a measure for cognitive function, HNE levels were highest in AD patients with moderate cognitive impairment. Hence, increased HNE levels may play an important role in neurodegenerative events at an early disease stage. In summary, oxidative damage, as assessed by increased HNE levels, could be detected in sporadic AD patients and in different transgenic mouse models. The results of this thesis therefore support the further research of pharmacological targets aiming at augmentation of antioxidant defences for therapy or prophylaxis of Alzheimer’s disease.
The cytochrome bc1 complex is a cornerstone in bioenergetic electron transfer chains, where it carries out tasks as diverse as respiration, photosynthesis, and nitrogen fixation. This homodimeric multisubunit membrane protein has been studied extensively for several decades and the enzyme mechanism is described with the modified protonmotive Q cycle. Still, the molecular and kinetic description of the catalytic cycle is not complete and questions remain regarding the bifurcation of electron transfer at the quinol oxidation (Qo) site, substrate occupancy, pathways of proton conduction, and the nature of the Rieske protein domain movement. We used competitive inhibitors to study the molecular architecture at the Qo site with X-ray crystallography. The structure of the enzyme with the substrate analog 5-n-heptyl-6-hydroxy-4,7-dioxobenzothiazole (HHDBT) bound at the Qo site was determined at 2.5 Å resolution. Spectroscopic studies showed that HHDBT is negatively charged when bound at the active site. Mechanistic interpretations from inhibitor binding are in line with single occupancy model for quinol oxidation and structural analysis supports the proposed proton transfer pathway. For functional insight into the enzyme mechanism, redox-sensitive protonation changes were studied by Fourier transform infrared spectroscopy. The protein purification procedure was optimized for less delipidation and the isolated enzyme was more active. Furthermore, two new phospholipids were identified in the X-ray structures, including a cardiolipin. Strikingly, conserved lipid binding cavities were observed in structural comparison with homologous enzymes. The functional role of tightly bound phospholipids will be discussed. Finally, the Qo site is a target for various compounds of agricultural and pharmaceutical importance. Importantly, the X-ray structures permit detailed analysis of the molecular reasons for acquired resistance to and treatment failure of Qo site inhibitors, such as atovaquone, that is used to treat malaria and pneumonia, as discussed herein.
The cytochrome bc1 complex or ubiquinol:cytochrome c oxidoreductase (QCR) catalyses electron transfer from ubiquinol to cytochrome c in respiration and photosynthesis coupled to a vectorial proton transport across the membrane, in which the enzyme resides. In both bacteria and eukaryotic organisms, QCR participates in supramolecular assembly of membrane proteins that comprise the respiratory or photosynthetic chain. In the present work, proton transfer pathways, substrate binding and the supramolecular assembly of the respiratory chain in yeast were probed by structure-based site-directed mutagenesis and characterization of the variants. Both active sites centre P, the place of quinol oxidation, and centre N, where quinone reduction takes place, lack direct access to the bulk solvent necessary for proton release and uptake. Based on the X-ray structure, proton transfer pathways were postulated. Analysis at centre P showed, that E272 and Y132 of cytochrome b are important for QCR catalysis as indicated by increased superoxide production and lowered Cyc1p reductase activity in these variants. Pre-steady state heme reduction kinetics in combination with stigmatellin resistance indicated that charge and length of the side chain at position 272 are crucial for efficient docking of the ISP to form the enzyme substrate complex and for electron bifurcation at centre P. Variants of Y312 and F129, both residues of cytochrome b, showed an increased Km indicating participation of these residues in coordination of ubiquinol or the possible intermediate semiquinone anion radical. F129 proved to be crucial for a functional Q-cycle as indicated by respiratory negative growth phenotype and a lowered H+/e- stoichiometry of F129 variants. At centre N, the postulated CL/K and E/R proton transfer pathways are located at opposite sites of the bound ubiquinone. Variants in the surface residues R218 (cytochrome b) and E52 (Qcr7) of the E/R pathway and E82 (Qcr7) of the CL/K pathway showed instability upon purification indicating an important role of these residues for QCR integrity. The slowed down centre N reduction kinetics in H85 (CL/K), R218 and N208 (both E/R) variant was attributed to a destabilised semiquinone anion consistent with the observed decreased sensitivity towards the site-specific inhibitor antimycin and an increased Km. Variants of residues of both pathway, E82Q and R218M, exhibited a decreased H+/e- stoichiometry indicating a crucial role of both residue for maintaining a working Q-cycle and supporting the proposed protonation of the substrate via the Cl/K and the E/R pathway. Long-range interaction between centre N and centre P were observed by altered reduction kinetics of the high potential chain and increased superoxide production in the centre N variants. The role of the cation-pi-interaction between F230 of Cyt1p and R19 of cytochrome c in binding of the redox carrier to QCR was analysed. In F230L hydrophobic interaction were partially lost as was deduced from the ionic strength dependence of Cyc1p reductase activity and Cycp1 binding, as detected by ionic strength sensitive Kd and Km for Cyc1p. The decreased enzymatic rate of F230W could be explained by a disturbed binding of Cyc1p to the variant enzyme. F230 may influence the heme mid point potential and thereby the electron transfer rate to Cyc1p. Reduction of Cobp via both centre P and centre N was disturbed suggesting an interaction between high and low potential chain. Supramolecular association between QCR and cytochrome c oxidase (COX) in yeast mitochondria was probed by affinity chromatography of a his-tagged QCR in the presence of the mild detergent digitonin. In comparison to purification with laurylmaltoside, the presence of both QCR and COX subunits was detected in the elution fractions by SDS-PAGE, Cyc1p reductase and TMPD oxidase activity assays and immunoblot analysis. The CL-dependent formation of the supercomplex between QCR and COX was analysed by replacement variants in the CL-binding site of QCR in CL containing and CL free environment. With an increasing number of replacements of the three lysines the CL-binding pocket supercomplex formation was not abolished, when CL is present as shown by BN-PAGE analysis. This was supported by the synergetic decrease in enzyme activity for both enzymes upon increased number of replacements. In the CL-free environment, no supracomplex formation was observed for a wildtype CL binding site. By replacements of two lysines in the CL-binding pocket, supercomplex formation could be recovered as revealed by BN-PAGE. This indicates, that CL may serve as a charge neutralizer for the lysines near the presumed interaction domain between complex III and complex IV. The obtained results for centre P provide new information of residues critical for stabilisation of ubiquinol and controlling electron short circuit reactions. The observations for centre N variants clearly support the proposed two proton transfer pathways and the role of the bound phospholipids in centre N kinetics. Variants in the Cyc1p binding site suggest a role for F230 both in Cyc1p binding and electron transfer. Clear interaction between the high and low potential chain in both Cyt1p and centre N variants strongly support long-range interactions in the complex. Studies on the supramolecular association of complex III and complex IV indicate a new role of Cl in stabilising a supracomplex.
The enzyme quinol:fumarate reductase (QFR) from the anaerobic e-proteobacterium Wolinella succinogenes is part of the anaerobic respiratory system of this organism. It couples the reduction of fumarate to succinate to the oxidation of menaquinol to menaquinone. W. succinogenes uses fumarate as terminal electron acceptor and can use various substrates (e.g., formate or molecular hydrogen) as electron donors. The concerted catalytic substrate turnover of either a hydrogenase or a formate dehydrogenase in conjunction with QFR contributes to the generation of an electrochemical potential gradient across the bacterial plasma membrane, which is used for the phosphorylation of ADP with inorganic phosphate, Pi, to ATP. In addition to an FAD (in subunit A) and three iron-sulfur clusters (in subunit B), QFR binds a low- and a high-potential heme b group in its transmembrane subunit C, as was ultimately shown in the crystal structure at 2.2 Å resolution (Lancaster et al., 1999, Nature 402, 377– 385). Both hemes are part of the electron transport chain between the two catalytic sites of this redox enzyme. The midpoint potentials of the hemes are well established but their assignment to the distal and proximal positions in the structure had not yet been determined. Furthermore, QFR from W. succinogenes has been proposed to exhibit a novel coupling mechanism of transmembrane electron and proton transfer, which has been described in the so-called “E-pathway” hypothesis (Lancaster, 2002, Biochim. Biophys. Acta 1565, 215–231). The aim of this project was to characterize the relationship between structure and function of QFR and to investigate the details of the proposed coupling mechanism (“Epathway”) with the help of computer-based electrostatic calculations on the QFR wild-type (WT) coordinates, and electrochemically induced FTIR and VIS difference spectroscopy on the QFR WT and available variant enzymes (in particular enzyme variant E180Q, in which the glutamic acid at position C180 has been replaced by a glutamine). 1.) It was demonstrated in this study that the diheme-containing QFR exhibits stable and reproducible electrochemically induced FTIR difference bands in the midinfrared range from 1800 cm-1 to 1000 cm-1 that reflect transitions from the reduced to the oxidized state of the enzyme. The spectral features that were observed in the FTIR difference spectra are fully reversible when changing from a reductive to an oxidative reference potential at the working electrode and vice versa. This indicates that the underlying redox reactions of the enzyme at the gold grid working electrode are also fully reversible under the applied experimental conditions. The same reversible spectral redox behavior in the visible range could also be ascertained for the Soret- and a-band of the two heme b groups of QFR. This behavior allowed to reliably determine the heme b midpoint potentials of QFR at various pH values. Analysis of the FTIR difference spectra in the amide I range yields evidence for structural reorganizations of the polypeptide backbone upon the electrochemically induced redox reaction. 2.) The redox titrations of the high- and low-potential heme b of QFR as simulated by multiconformation continuum electrostatics (MCCE) calculations showed a very high level of agreement with respect to the experimentally observed midpoint potentials of the heme b groups at pH 7. As determined with the help of the theoretical calculations, prominent features governing the differences in redox potential between the two hemes are the higher loss of reaction field energy for the proximal heme and the stronger destabilization of the oxidized form of the proximal heme due to several buried and ionized Arg and Lys residues. The explicit incorporation of crystallographically identified water molecules in the calculations had a noticeable effect on the absolute values of the determined midpoint potentials, although the relative difference of the two obtained midpoints did not change significantly. The results of the electrostatic calculations clearly showed that the lowpotential heme corresponds to the distal position bD in the structure, and that the high-potential heme is identical to the proximal heme bP. This assignment could previously not be achieved unequivocally with experimental methods. 3.) In addition, the currently discussed mechanism of coupled electron and proton transfer in the QFR of W. succinogenes (i.e., the “E-pathway” hypothesis) is further supported by the results of this study. The simulations of intermediate states of electron transfer via the heme b groups show that the protonation state of the key amino acid residue Glu C180 depends on the redox states of the heme groups as suggested in the “E-pathway” hypothesis. This result yields a possible mechanism for the coupling of transient transmembrane proton transfer via Glu C180 to the electron transfer via the heme b groups, since Glu C180 could be part of a “proton wire” and its redox-dependent protonation state could serve as the regulatory element of the “E-pathway”. Furthermore, the results of simulated heme reduction indicate that the side chain of Glu C180 also changes its conformation with respect to the redox state of the hemes. Both major results concerning the role of Glu C180, the change of protonation as well as the reorientation of the side chain upon reduction of the heme groups, are consistent with the results from electrochemically induced FTIR difference spectroscopy: Of particular interest was the spectral range above 1710 cm-1, where C=O stretching vibrations of protonated COOH carboxyl groups absorb, because those groups can act as proton donors, respectively acceptors, and can be involved in intra-protein proton transfer reactions. It was possible to observe signals of such protonated carboxyl groups originating from QFR enzyme, which either change their protonation state and/or experience an environmental change in the course of the induced redox reaction. This finding was supported by the fact that the relevant FTIR difference signals are sensitive to an isotopic hydrogen/deuterium (1H/2H) exchange via the buffer solution, since they were shifted towards lower wavenumbers in D2O. Furthermore, it could be shown with the help of site-directed mutagenesis that the acidic residue Glu C180, which is located in the membranespanning, diheme-containing subunit C of QFR, is contributing to the redox dependent signal of protonated carboxyl groups. The observed residual signal in the FTIR double-difference spectrum of QFR wild-type and enzyme variant E180Q (Glu C180 has been replaced with a Gln residue) could be interpreted as a protonation/deprotonation event that is superimposed by an environmental effect on the specific C=O vibration. This result strongly supports the proposed “E-pathway” of coupled transmembrane electron and proton transfer in the QFR enzyme, which states that residue Glu C180 is an essential constituent of a transient redox-controlled transmembrane proton transfer pathway. 4.) As a second possible constituent of the suggested “E-pathway”, the ring C propionate of the distal heme was found to be unusually fully protonated in all simulated redox states, indicating a possible role as a transient proton donor/acceptor in the “E-pathway”. Similarly to Glu C180, experimental evidence from FTIR difference spectroscopy on a modified QFR with 13C-labeled heme propionates was obtained, which indicates an involvement of at least one of the two propionates of heme bD in proton transfer. The observed signals can tentatively be interpreted as a redox-coupled (de)protonation of the ring C propionate of bD, which is possibly xiii superimposed by a conformational or environmental change of the specific propionate. 5.) Also the observation of a strong redox Bohr effect for both heme b groups in QFR is in line with the proposed “E-pathway” hypothesis, as this effect yields a possible and well-established mechanism for the coupling of proton transfer and redox changes of the heme groups. The comparison of the observed effect in QFR WT and E180Q together with the results from FTIR spectroscopy and MCCE calculation indicate that the ring C propionate of the distal heme is dominating the pHdependence of the midpoint potential of bD, and that the corresponding group for bP is Glu C180. The origin of the redox Bohr effect for bP in the enzyme variant E180Q (which is dramatically changed with respect to the WT) could not be identified unequivocally, but the observation of this redox Bohr effect in the variant implies the presence of other protolytic groups, which interact with heme bP and which may be necessary for a functional “E-pathway”.
Die vorliegende Arbeit soll einen Beitrag zur Erforschung der Verarbeitungsmechanismen des Gehirns leisten. Die Erregung des komplexen Systems "Hirn" liefert Antworten, deren Analyse zu einem besseren Verständnis dieser Informationsverarbeitung führt. Zu diesem Zweck wurde das Gehirn mit unterschiedlichen visuellen Stimuli angeregt und die hirnelektrischen Signale gemessen, die von Nervenzellgruppen (Multiunits) im visuellen Kortex der Katze ausgesandt wurden.Die verwendeten Stimuli waren ein Streifenmuster sowie eine Zufallspunktverteilung, deren Kohärenz beliebig geändert werden konnte. Darüber hinaus wurden die Antworten auf eine Vielzahl von Stimuli analysiert, die nur aufgrund des Bewegungskontrastes zwischen punktdefiniertem Objekt und Hintergrund zu erkennen sind (Shape-from-Motion- (SFM-) Stimuli). Die aufgenommenen Daten wurden mit Hilfe einer umfangreichen Signalanalyse untersucht. So wurden in Abhängigkeit von der Stimulusbedingung die Anzahl der Nervenimpulse pro Zeiteinheit (Feuerraten), Synchronisation, Frequenzverteilung sowie Kopplung von Aktionspotenzialen und LFPDaten analysiert. Die Experimente im ersten Teil dieser Arbeit untersuchten den Einfluss von Kohärenz auf die Verarbeitung von Bewegungsinformation im primären visuellen Areal (A17) und im posteromedialen lateralen suprasylvischen Sulcus (Area PMLS) der Katze. Es konnte gezeigt werden, dass Multiunits in A17 und PMLS sowohl auf Streifenmuster als auch auf Zufallspunktverteilungen antworten und dass die Stärke der Antwort als eine Funktion der Stimulusrichtung variiert. Die Vorzugsrichtung ist in beiden Arealen weitgehend unabhängig von der Art des verwendeten Stimulus, was darauf hindeutet, dass die Stimulusrichtung für Streifenmuster und Zufallspunktmuster in diesen Arealen durch einen einheitlichen Mechanismus bestimmt wird. Bei einer Abnahme der Stimuluskohärenz zeigen die Multiunits eine Abnahme der Feuerrate, wobei im Vergleich zu PMLS in A17 eine stärkere Abnahme der Kohärenz nötig ist, um die gleiche Abnahme der Feuerrate zu erreichen. Dieses Ergebnis konnte durch die unterschiedlichen Größen der rezeptiven Felder der beiden Areale erklärt werden und ist ein weiterer Hinweis darauf, dass eine wichtige Funktion von PMLS in der Analyse von Bewegung und räumlich verteilter Information liegt. Da beide Areale keine signifikante Änderungen der Feuerrate bei Inkohärenzniveaus von mehr als 50% zeigten, scheinen sie nicht in der Lage zu sein, die Bewegungsrichtung eines inkohärenten Zufallspunktmusters nahe der psychophysischen Detektionsschwelle von 95% auf der Basis von Feuerraten zu erkennen. Die Korrelation der Aktionspotenziale unterschiedlicher Multiunits zeigte bereits bei einer geringen Abnahme der Stimuluskohärenz eine monotone Verbreiterung des zentralen Maximums in den Korrelogrammen beider Areale. Die Stärke der Synchronisation hingegen war kaum beeinflusst. Darüberhinaus kam es zu einer Verschiebung der Leistung im lokalen Feldpotential (LFP) von hohen hin zu niedrigen Frequenzbereichen. Diese Verschiebung wurde auch für die Kopplung zwischen LFP und Akvi tionspotenzialen nachgewiesen. Diese Resultate unterstützen die Theorie, dass präzise Synchronisation und hochfrequente Oszillationen ein Mechanismus für die Bindung kohärenter Objekte sind. Sie zeigen darüber hinaus, dass Synchronisation auch nicht kohärente Stimuli binden kann und dass die Verschiebung im LFP hin zu niedrigeren Frequenzen wichtig für die Integration verteilter Information über einen größeren visuellen Raum sein kann. Da bei hohen Inkohärenzniveaus keine präzise Synchronisation mehr nachgewiesen werden konnte, kann jedoch auch die Synchronisation nicht als alleiniger Mechanismus zum Erkennen einer Bewegungsrichtung eines inkohärenten Zufallspunktmusters herangezogen werden. In den Experimenten im zweiten Teil dieser Arbeit wurde untersucht, wie das Gehirn SFM-Stimuli verarbeitet. Die Auswertungen der Feuerraten haben gezeigt, dass Multiunits in PMLS sowohl auf helligkeitsdefinierte Kontrastbalken als auch auf SFM-Balken reagieren. Die Stärke der Antwort hängt von der Kombination von Stimulus und Hintergrund und von der relativen Bewegungsrichtung zueinander ab. Während ähnliche Feuerraten für Balken mit hohem Kontrast relativ zum Hintergrund und für punktdefinierte Balken gefunden wurde, die sich über einen dunklen Hintergrund bewegten, führte ein statischer Zufallspunkthintergrund zu einer starken Abnahme der von dem SFM-Balken hervorgerufenen Antwort. Ein in die Gegenrichtung bewegter Hintergrund sowie ein reduziertes Kohärenzniveau des Zufallspunkthintergrundes führten dazu, dass die Multiunits auf den SFM- Balken nicht mehr mit einer Zunahme der Feuerraten reagierten. Um die hemmende Wirkung des Hintergrundes aufzuheben, musste der Hintergrund auf einer Fläche des visuellen Feldes, die der Größe des rezeptiven Feldes entsprach, abgedeckt werden. Dieses Ergebnis zeigt, dass die Feuerraten für diese Art Stimulus nicht wesentlich von Arealen außerhalb des rezeptiven Feldes beeinflusst werden. Zur weiteren Analyse der Fähigkeit von PMLS, SFM-Balken nur aufgrund des Bewegungskontrastes zwischen punktdefiniertem Objekt und Hintergrund zu erkennen, wurde mit Hilfe von zwei Tuningkurven-Stimuli, bei denen sich die Bewegungsrichtung der Punkte innerhalb des Balkens um 90° unterschied, die Vorzugsrichtung der Multiunits bestimmt. Die Auswertung ergab, dass sich die gemessene Vorzugsrichtung der Multiunit um 45° drehte, obwohl sich die Bewegungsrichtung des Balkens selbst nicht änderte. Darüber hinaus wurden verschiedene SFM-Stimuli untersucht, die alle dieselbe Bewegungsrichtung des Balkens, jedoch unterschiedliche Bewegungsrichtungen der Punkte innerhalb des Balkens aufwiesen. Wenn PMLS die Bewegung des SFM-Objekts statt der Bewegung der einzelnen Punkte verarbeitet, sollte die Feuerrate für alle diese Bedingungen identisch sein. Die Ergebnisse zeigen jedoch, dass sich die durch die verschiedenen SFM-Stimuli hervorgerufenen Feuerraten verringerten, je weiter sich die Punkte, die den Balken bildeten, von der Bewegungsrichtung des Balkens – und damit von der Vorzugsrichtung der Multiunit – weg bewegten. Durch dieses Ergebnis konnte gezeigt werden, dass Multiunits in PMLS nicht in der Lage sind, die Richtung von kinetisch definierten Balken zu analysieren und statt dessen nur die Bewegung der einzelnen Komponenten erfassen.
Proton-translocating NADH:ubiquinone oxidoreductase (complex I) transports two electrons from NADH to membranal ubiquinone: in this process protons are translocated across the membrane, producing 40% of the total proton gradient between matrix side and intermembrane space. Mitochondrial complex I contains at least 46 subunits in mammals, and has a molecular weight of around 1000 kDa. Electronic microscopy analysis showed that complex I has an L-form, which consists of two domains: a peripheral “arm” (hydrophilic domain) and a membrane “arm” (hydrophobic domain). The peripheral domain, which protrudes into the matrix, contains one non-covalently bound flavin mononucleotide (FMN) and the iron-sulfur clusters N1a, N1b, N2, N3, N4 and N5 as redox active groups. They transport electrons from NADH to ubiquinone. Cluster N2 is supposed to be the immediate electron donor to ubiquinone by virtue of its highest and pH dependent redox midpoint potential (Em,7 –150 mV). The exact location of the tetra-nuclear cluster N2 is still object of discussion. The TYKY and the PSST subunits contain three binding motifs for tetranuclear clusters which are formed by twelve cysteins. In an effort to investigate the “ubiquinone reduction module” of complex I, in the first part of this work site directed mutagenesis of the TYKY and PSST subunits has been carried out. Mutant strains were characterised in terms of complex I content, catalytic activity and EPR signature of cluster N2. The second part of this work was aimed at developing a substrate inducible version of the internal alternative NADH:ubiquinone oxidoreductase (NDH2i). A substrate inducible NDH2i is expected to offer a “switch” between complex I activity dependent (no NDH2i activity) and independent (NDH2i activity) cell growth, by changing between activating and non-activating substrates. This strategy would allow the screening for two types of complex I mutants, which is a prerequisite for realising a random PCR mutagenesis of single subunits of complex I, that allows the production of a high number of point mutations in relatively short time. Y. lipolytica complex I deficiency mutant strains could be easily identified, by virtue of their inability to survive under complex I dependent growth conditions (no NDH2i activity). By this way, amino acids that have an important role for complex I structure or function could be identified by subsequent sequence analysis. Each of the twelve cysteines that form the above mentioned three binding motifs for iron-sulfur cluster have been mutagenised. In mutant mitochondrial membranes, no assembled complex I could be detected. From these data one may conclude that the mutagenised 6 SUMMARY 92 cysteines play an important role for complex I stability, or that are a prerequisite for complex I assembly in Y. lipolytica, but there is not direct evidence indicating that any of the four mutagenised residues acts as a ligand. Two aspartates in the PSST subunit, Asp-99 and Asp-115, were found to be essential for complex I catalytic activity. EPR spectroscopic analysis indicated that the electron transfer to N2 cluster was not blocked and implied that this was not the reason for the loss of catalytic activity. From these data it can be concluded that D99 and D115 play a vital role for complex I NADH:ubiquinone reductase activity, but are not ligands for cluster N2 and that their position is not close enough to the cluster to influence directly its electromagnetic environment. Three mutations, identified in the PSST and TYKY homologous subunits of patients affected with Leigh syndrome (V119M in PSST, P78L and R101H in TYKY) were reconstructed in the obligate aerobic yeast Y. lipolytica. This approach may help to understand the aetiology of the Leigh syndrome, in terms of the ability of complex I to oxidize NADH and to transport electrons. In fact, all three mutations showed effects on electron transport, reducing the VMax by about 50%. Mutant V119M in the PSST subunit, which had a lethal effect in two patients that were homozygous for this mutation, affects a fully conserved residue. Overall, the results from site directed mutagenesis carried out so far support the theory that the “catalytic core ” (N2 cluster and quinone binding site) of complex I has been evolved from the electron transfer module of the [Ni-Fe] hydrogenases. In fact, mutagenesis of residues that are fully conserved between complex I and [Ni-Fe] hydrogenases, showed dramatic effects on complex I in terms of assembly (cysteine mutants) or catalytic activity (D99-D115). Differently, changing aspartate 174 and glutamic acid 185 (not fully conserved, Fig 4.1A) had little or no effect on the Michaelis-Menten parameters and N2 EPR signal. In recent years Y. lipolytica has been developed as a yeast genetic system to study mitochondrial complex I. The present work introduced the promoter for the isocitrate lyase (pICL1) as a useful tool for the substrate selective expression of the internal version of the alternative NADH:ubiquinone oxidoreductase (pICL1-NDH2i). This allows to rescue complex I deficiencies “in vivo” selectively by growth on acetate (or ethanol) medium. The integration of the pICL1-NDH2i construct into the genome of Y. lipolytica and subsequent deletion of nuclear-coded subunits like PSST, TYKY and 49 kDa, would contribute to further develop this organism as a useful genetic model for studying subunits of mitochondrial complex I by site directed mutagenesis.
Stability, unfolding and refolding of the outer membrane protein porin from Paracoccus denitrificans was investigated using genetic and spectroscopic methods. Structural and functional activity studies on wild type and mutant porins: The site-directed mutants were constructed based on conserved residues and evidences on the role of certain amino acids from previous studies with OmpF. Secondary structure analysis of wild type and mutants E81Q, W74C, E81Q/D148N, E81Q/D148N/W74C by FTIR and CD spectroscopy are in line with the fact that porins are predominantly ß-sheet structure. The functional activity studies by black lipid bilayer techniques showed that the wild type and mutants W74C, E81Q/D148N, E81Q/D148N/W74C have a conductance of 3.25 nS. For mutant E81Q conductance of 1.25nS was more predominant over 3.25 nS. The activity of the mutants was observed to be far less than the wild type. This indicates that structural similarities does not implies similar functional activity. Thermal stability analysis of porin in detergent micelles and reconstituted into liposomes: Thermal stability analysis of wild type and mutants in detergent micelles showed changes in secondary and quaternary structure. It was found that wild type porin unfolds into aggregated structure with a high transition temperature of 86.2 °C. For mutants E81Q, W74C, E81Q/D148N the transition temperature was found to be 84.2 °C, 80.3 °C and 80.2 °C respectively. Functional activity assays at high temperatures revealed that the protein tends to loose its activity on heating up to 50 °C. This shows that structural stability does not imply functionality in the case of porins. Thermal stability analysis of porin reconstituted into liposomes showed that there was no change in the secondary and quaternary structure of the protein up to 100 °C, revealing that the protein becomes more thermostable when it is reconstituted into liposomes. Refolding of aggregated porin: This study shows that disaggregation of ß-sheet membrane protein porin is possible by changing its chemical and thermodynamic parameters. An increase of the solution pH to 12 or above results in opening up of the aggregated protein into unordered structure, as observed by FTIR and CD spectroscopy. This unordered structure could be refolded into native-like structure forming trimers. The secondary structure of the refolded protein deviated slightly from the native one. The thermal stability analysis of the native-like refolded proteins showed that the unfolding pattern is entirely different when compared to the native porins. pH dependent unfolding of porin: Thermal stability of porin at different pH values showed that the protein is stable in a pH range of 1-11. At pH 12 and above the protein unfolds into unordered structure instead of aggregating. The high pH unfolding of porin is a reversible process. The secondary structure of the refolded protein varied slightly from the native-one. Whereas thermal stability was entirely different. This shows that even though the unfolding of porin at high pH is reversible, it results in changes in local interaction between the amino acids resulting in a difference in stability. Unfolding in presence of urea and guanidinium hydrochloride (GuHCl): Denaturation of porin in the presence of chemical denaturants like urea and GuHCl showed that porin unfold into unordered structure. The unfolding is a reversible process. Unfolded protein was refolded into detergent micelles and liposomes. Refolding into detergent micelles was faster compared to refolding into liposomes, as seen by kinetic gel shift assays. The refolding into liposomes showed the presence of intermediates similar to those reported for OmpF. This study shows the difference in thermal stability of the outer membrane protein porin from Paracoccus denitrificans in detergent micelles and native-like liposomes. It suggests various unfolding pathways, which can be further investigated for unfolding and refolding kinetics. This report also suggests that it is possible to refold a heat-aggregated protein.
Results were presented from Brownian dynamics simulations for cyt c molecules approximated as spherical particles with diameter 2R ' 3.3 nm interacting with a charged planar membrane surface. Using the well-known Ermak-McCammon algorithm of ref. [36, 37] for solving the Langevin equations (see Chapter 2), a new computer program in C++ was developed. An overview of the way it is implemented is given in Chapter 3. The program in its current state is able to compute the trajectories (translation and rotation) of hundreds of spherical particles in systems with typical dimensions of 103 − 1003 nm3 . As explained in the introductory Chapter 1 the motivation for studying the dynamics of cyt c molecules in such systems came from the progress in the research of photosynthetic bacteria, e.g. While the internal processes of energy transduction (light harvesting, channelling to RC, charge separation) are quite well understood, the dynamics of soluble cyt c as an electron transporter in this context is not yet clear. In many textbooks one can find illustrations where a single cyt c is responsible for the electron transport between two integral membrane proteins (the reaction centre RC and the bc1 complex). But as pointed out in publications like refs. [49], [59], [60], [61] or [62] biological cells are crowded with different molecules. Consequently, one can assume that the electron transport between two integral membrane proteins is not simply taken on by one single cyt c molecule. Instead it is likely that many of these particles are located in a cyt c pool above the membrane and that they perform the electron transport in turns. Thus, it is desirable to have a simulation package that is able to compute the trajectories of many proteins. Note that the detailed processes of electron transfer and binding to membrane proteins are not modelled here. The details of these processes are quite complicated so that we refrained from including them in the coarse-grained simulations. Here, the actual binding is simply defined by a particle distance zb from the membrane which marks the beginning of the attractive potential. ...
Hinreichend kalte und dichte Quarkmaterie ist ein Farbsupraleiter. Ähnlich wie Elektronen in einem gewöhnlichen Supraleiter bilden Quarks Cooper-Paare. Während bei Elektronen der Austausch von Phononen zu einer Anziehung führt, ist im Falle von Quarks der Antitriplett-Kanal der starken Wechselwirkung attraktiv. Arbeiten in den letzten Jahren haben verschiedene Phasen von farbsupraleitender Quarkmaterie untersucht und sich dabei vor allem auf Phasen konzentriert, m denen der Gesamtspin eines Cooper-Paares verschwindet. In der vorliegenden Dissertation habe ich hauptsächlich Farbsupraleiter diskutiert, deren Cooper-Paare im Spin-Triplett-Kanal kondensieren, d.h. die Cooper-Paare haben den Gesamtspin 1. Diese Art von Supraleiter ist möglicherweise relevant für Systeme in der Natur, wie z.B. das Innere von Neutronensternen. Denn bei der Spin-0-Farbsupraleitung wird vorausgesetzt, dass die Fermi-Impulse zweier Quark-Flavor gleich ist oder zumindest hinreichend klein, was für realistische Systeme, also für nicht zu große Dichten, fragwürdig ist. Diese Einschränkung gibt es im Falle von Spin-1-Farbsupraleitern nicht, da hier Quarks des gleichen Flavors Cooper-Paare bilden. Ich habe in meiner Dissertation die verschiedenen möglichen Phasen eines Spin-1-Farbsupraleiters systematisch klassifiziert. Dies wurde mit Hilfe von gruppen-theoretischen Methoden durchgeführt, basierend auf der Tatsache, dass die Farbsupraleitung durch das theoretische Konzept der spontanen Symmetriebrechung beschrieben werden kann. Ähnlich wie bei supraflüssigem Helium-3 gibt es eine Vielzahl theoretisch möglicher Phasen. Ich habe die physikalischen Eigenschaften von vier dieser Phasen untersucht, nämlich der polaren und planaren Phasen sowie der A- und CSL-(color-spin-locked)Phasen. Mit Hilfe der QCD-Lückengleichung wurde die Energielücke sowie die kritische Temperatur bestimmt. Es stellt sich heraus, dass die Energielücke eines Spin-1-Farbsupraleiters um 2-3 Größenordnungen kleiner ist als die eines Spin-0-Farbsupraleiters, d.h. sie liegt im Bereich von 10 - 100 keV. Zwei besondere Eigenschaften der Energielücke werden diskutiert, nämlich eine 2-Lücken-Struktur, die in zwei der untersuchten Fälle auftritt, sowie mögliche Anisotropien, insbesondere Nullstellen der Lückenfunktion. Die Berechnung der kritischen Temperatur zeigt, dass es durchaus farbsupraleitende Materie in einer Spin-1-Phase im Innern von Neutronensternen geben kann, da die Temperatur von alten Neutronensternen im Bereich von einigen keV oder sogar darunter liegt. Darüber hinaus wurde die Frage untersucht, ob ein Farbsupraleiter auch ein gewöhnlicher Supraleiter ist. In diesem Zusammenhang ist die Frage von Interesse, ob ein Spin-1-Farbsupraleiter gewöhnliche Magnetfelder aus seinem Innern verdrängt, was sicherlich Auswirkungen auf die Observablen eines Neutronensterns hätte. Tatsächlich stellt sich heraus, dass ein Spin-1-Farbsupraleiter, im Gegensatz zu einem Spin-0-Farbsupraleiter, einen elektronmagnetischen Meissner-Effekt aufweist. Dieses Ergebnis wurde mit Hilfe von gruppentheoretischen Überlegungen vorausgesagt und mit Hilfe einer detaillierten Berechnung der Photon-Meissner-Massen bestätigt.
A new experimental setup, for pump-probe fs DFWM measurements, which is based on a femtosecond laser system, has been constructed. It allows for the investigation of molecular species in the gas phase at different temperatures, from ~30 K in a seeded supersonic jet up to ~500 K in a heat-pipe oven. In comparison to other RCS methods the employed fs DFWM technique is less complicated and gives much higher signal-to-noise ratio [BFZ86, FeZ95a, CKS89, CCH90, HCF91, WRM02, Rie02]. A general computer code for the simulation of fs DFWM spectra of nonrigid asymmetric top molecules has been developed. This new DFWM code in combination with a non-linear fitting routine allows one to determine rotational and centrifugal distortion constants and obtain information on the polarizability tensor components from the experimental spectra. Fs DFWM spectroscopy was successfully applied to the medium-sized molecules benzene and benzene-d6 in a gas cell and in a supersonic jet. The spectrum from a seeded expansion has been measured up to delay time of 3.9 ns (restricted by the length of the delay stage) with excellent signal-to-noise ratio (102-103). In that way 87 and 72 J-type transients have been recorded for benzene and benzene-d6, correspondingly. A relative accuracy on the order of 10-5 has been achieved for the rotational constant. From the room temperature experiments, precise values of centrifugal distortion constants DJ and DJK have been extracted. The literature data for cyclohexane have been revised and a new precise rotational constant B0 has been obtained, which is (+5.5 MHz) shifted from the one reported in the former Raman investigation by Peters et al. [PWW73]. Additionally, high-level ab initio calculations of cyclohexane have been carried out using a large number of basis sets at several levels of theory. In particular, the vibrational averaging effects have been examined in order to critically compare the experimentally determined and theoretically evaluated rotational constants. The contribution of highly symmetric vibrational modes to vibrational averaging effects was clarified. More structural information could be obtained from fs DFWM measurements of asymmetric top species, since different type of rotational recurrences can appear, and all three rotational constants (A, B, C) can be extracted. On the other hand the analysis of the asymmetric top spectra is no longer trivial. In fact the simple formula for rotational recurrence periods of symmetric top species (Tab. 2.1) can not be applied to asymmetric top molecules. Thus, in order to extract high-resolution data for asymmetric species, a complete fitting of the experimental spectra is necessary. The fs DFWM and (1+2') PPI method have been applied to the asymmetric top molecules pyridine in the ground (S0) and pDFB in the S0 and electronically excited (S1) states. By fitting the measured fs DFWM spectra the rotational and centrifugal distortion constants have been extracted with good precision and the value of the parametrized polarizability angle of pDFB was obtained. In this work, the first application of fs DFWM spectroscopy to a molecular cluster has been reported. Also, the chemical equilibrium between monomeric and dimeric species was studied. In particular formic and acetic acid have been investigated in a gas cell and in a supersonic expansion. Many spectral features have been observed and analized in the fs DFWM spectrum of formic acid vapor in a gas cell at room temperature. Most of them were attributed to rotational recurrences of the formic acid monomer, but also spectral feature originating from the formic acid dimer of O-H×××O/O×××H-O type have been detected and analyzed. From the fitted simulation, the rotational and centrifugal distortion constants, and parametrized polarizability angle were extracted for the dimeric structure of O-H×××O/O×××HO type. With the assumption of unperturbed monomers a center-of-mass distance of R = 2.990 ± 0.001 Å for the monomers within the dimer has been calculated from the spectroscopic results. This distance is 0.028 Å smaller than that reported from electron diffraction [ABM69]. Thus, the centers-of-mass distance was assigned as the main point of disagreement between results of fs DFWM and electron diffraction experiments. In contrast to formic acid, acetic acid shows strong recurrences from dimeric species even in the fs DFWM spectra at room temperature. This is explained in terms of different symmetry of the moments-of-inertia tensor, which modulates the intensity of RRs (the fs DFWM signal is in general stronger from symmetric species). Due to the symmetric nature of the acetic acid dimer, only the sum (B+C) of the rotational constants has been extracted. The changes of geometrical parameters upon dimer formation have been analyzed for, both, formic and acetic acid. A heat-pipe oven has been used in order to overcome the main drawback of fs DFWM spectroscopy – the square dependence of the signal intensity on the sample number density. Two-ring molecules (cyclohexylbenzene, para-cyclohexylaniline and nicotine) with low vapor pressure (<0.1 mbar) at room temperature have been investigated. From the analysis of the experimental and ab-initio results for CHB and pCHA a nearly perpendicular conformation of the aromatic vs. cyclohexane ring for both system is inferred. The enlargement of the benzene ring of CHB in the electronically excited state (S1) has been found to cause the smaller rotational constants in S1. This conclusion has been drawn from the comparison of the ground and electronically excited state experimental rotational constants in combination with ab-initio calculations. The extraction of precise structural information for nicotine was not possible due to it weak fs DFWM signal. However, the fact that fs DFWM technique can be applied to conformational analysis of molecular species in an equilibrium mixture opens other applications for this kind of spectroscopy. In general the results obtained in this work show that the fs DFWM technique, being an experimental implementation of RCS, provides one with an important tool for structural analysis of molecular species in the gas phase in particular for the species to which microwave spectroscopy can not be applied. It gives spectra with excellent signal-to-noise ratio even at low number density samples expanded in a seeded supersonic jet. It provides an alternative and innovative approach towards rotational Raman spectroscopy of large polyatomic molecules applicable under various experimental conditions (broad temperature and pressure range). With the introduction of femtosecond (10-15s) laser pulses (usually picosecond (10-12s) laser pulses were used in RCS) an improvement in time-resolution and therefore in precision for the rotational constants by more than one order of magnitude has been achieved. Molecular systems in the ground electronic state without permanent dipole moment and chromophore can be studied with high precision, providing thereby molecular benchmark systems for the electronic structure theory. As has been shown, the study of molecular clusters by fs DFWM spectroscopy is possible, but its potential are restricted by the square dependence of the fs DFWM signal from the sample number density, which is even more important for supersonic jet expansions. Here, the application of near-resonant and resonant FWM schemes should help in order to compensate for the low sample concentration. With the introduction of the heat pipe oven for fs DFWM experiments, the investigation of large nonvolatile molecules under equilibrium conditions is possible now. In our laboratory the first results on the structural analysis of different conformers of pyrrolidine in the gas phase have been obtained [MaR04]. This method can have even more prospects for the structural investigations of large molecular species in combination with new non-thermal gas phase sources for nonvolatile molecules, like laser desorption [CTL89], laser oblation [MHL83], electrospray [FMM90], laser induced liquid beam ion desorption [KAB96, Sob00] etc. A very recent application of the fs DFWM technique is the investigation of the influence of strong laser fields on molecular gas phase sample, which could range from active alignment [PPB03] over molecular deformation to field ionization [CSD03]. In regards to future development in fs DFWM spectroscopy for more complex molecules one has to take into account, how large amplitude motions such as the van der Waals vibrations or internal rotation (see section 6.4.2) affect the rotational coherences. In any case, femtosecond Degenerate Four-Wave Mixing as experimental implementation of Rotational Coherence Spectroscopy can be considered as an innovative, developing, and powerful method for the structural investigation of the molecular species, which are hard to study by classical frequency-resolved spectroscopy [FeZ95a, Dan01, Rie02].
Nitric oxide (NO) represents a short-lived mediator that pivotally drives keratinocyte movements during cutaneous wound healing. In this study, we have identified p68 DEAD box RNA helicase (p68) from a NO-induced differential keratinocyte cDNA library. Subsequently, we have analyzed regulation of p68 by wound-associated mediators in the human keratinocyte cell line HaCaT. NO, serum, growth factors and pro-inflammatory cytokines were potent inducers of p68 expression in the cells. p68 was constitutively expressed in murine skin, but rapidly down-regulated upon injury. The down-regulation appeared to be transient, as p68 protein expression increased again after the inflammatory phase of repair. However, p68 protein expression did not completely disappear during wound inflammation, as immunohistochemistry and cell fractiona tion analysis revealed a restricted localization of p68 in keratinocyte nuclei of the developing epithelium. In line, cultured human (HaCaT) and murine (PAM 212) keratinocyte cell lines showed a nuclear localization of the helicase. Moreover, confocal microscopy revealed a strong localization of p68 protein within the nucleoli of the keratinocytes. Functional analyses demonstrated that p68 strongly participates in keratinocyte proliferation and gene expression. Keratinocytes that constitutively overexpressed p68 protein were characterized by a marked increase in serum-induced proliferation and vascular endothelial growth factor (VEGF) expression, whereas down-regulation of endogenous p68 using small interfering RNA (siRNA) markedly attenuated serum-induced proliferation and VEGF expression. Altogether, our results suggest a tightly controlled expression and nucleolar localization of p68 in keratinocytes in vitro and during skin repair in vivo that functionally contributes to keratinocyte proliferation and gene expression.
Reliable communication in the central nervous system requires the precise control of the duration and the intensity of neurotransmitter action at specific molecular targets. After their release at the synapse, neurotransmitters activate pre- and/or postsynaptic receptors. To terminate synaptic transmission, neurotransmitters are in turn inactivated by either enzymatic degradation or active uptake into neuronal and/or glial cells by neurotransmitter transporters. In the present study, two types of membrane proteins involved in transcellular signal transduction were investigated, the P2X receptors, which are ATP-gated ion channels and the glutamate transporters of the EAAT family. The first part of this study is concerned with the targeting and anchoring of P2X receptors at specific locations. P2X receptors play a role of fast excitatory neurotransmission to extracellular ATP in both the peripheral and central nervous system. For several ligand-gated ion channel, like glycine receptors or nicotinic acetylcholine receptors, it is known that specific binding proteins exist, which are involved in receptor trafficking and anchoring of the receptors at appropriate sites on the synapse. Within the P2X family, amino acid homology is scattered over the protein sequence excepted of the cytoplasmic C-terminal tails, which do not share significant sequence similarity, indicating that they might provide peculiar properties to the respective receptor isoforms. Using GST fusion proteins containing the C terminal end of the P2X2A, P2X5 and P2X7 subunits as baits, ßIII tubulin was identified by MALDI-TOF mass spectrometry as a direct interacting partner of P2X2A. ßIII tubulin did not interact with P2X5 nor with P2X7. The tubulin binding motif of P2X2A could be confined to a 42 amino acid long region ranging from amino acid 371 to 412 of the complete P2X2A subunit. This domain, which includes a total of six serine residues and twelve proline residues, interestingly overlaps to a significant extent with a 69 amino acid long sequence, which is lacking in P2X2B, a splice variant of P2X2A. P2X2B receptors are known to desensitize - significantly faster than P2X2A receptors. The interaction of the P2X2A receptor with ßIII tubulin may contribute to receptor desensitization as well as tethering of the P2X2A receptor at specialized regions of the cell. In a second part of this work, the oligomeric state of two distantly related glutamate transporters, the human glial glutamate transporter hEAAT2, and the glutamate transporter ecgltP of E.coli was determined. Excitatory amino acid transporters (EAATs) buffer and remove synaptically released L-glutamate and maintain its concentration below neurotoxic levels. Mammalian glutamate transporter subunits are known to form homomultimers, but controversial numbers of subunits per transporter complex have been reported, ranging from 2-5. Both hEAAT2 and ecgltP proteins expressed at high levels in Xenopus laevis oocytes, from which they were purified in a [35S]methionine-labeled form under nondenaturing conditions by metal affinity chromatography. Blue native PAGE analysis revealed that both the hEAAT2 and ecgltP transporters exist exclusively as homogenous populations of homotrimers in Xenopus oocytes. The trimeric structure was corroborated by chemical crosslinking. Also, ecgltP purified as a recombinant protein from its natural host E.coli migrated as a trimeric protein on blue native PAGE gels. The conservation of the quaternary structure from prokaryotes to mammals assigns an important functional role to the trimeric structure. Glutamate transporters are known to exhibit a dual mode of operation by functioning both as glutamate Na+/K+/H+ co-transporters and as anion channels. It is intriguing to speculate that the EAAT monomer is responsible for the secondary active transport of glutamate, whereas a barrel-like arrangement of the three subunits forms a central anion pore mediating anion conductivity.
The detailed mechanism of the 20 S proteasome from Thermoplasma acidophilum is unknown. Substrates are degraded processively to small fragments without the release of intermediates, but the basis for this unique degradation mode remains obscure. The proteasome is a molecular machine, but how the different nanocompartments interplay and whether more than one substrate can be treated simultaneously has not been elucidated yet. To address these questions we had to disable the functionality of one aperture in order to dissect whether the other pore can compensate for the loss. As it is challenging to introduce mutations solely around one pore aperture of the highly symmetrical construct, we chose a novel approach by unique orientation of the proteasome at interfaces. For this purpose we purified recombinant 20 S proteasomes, where hexahistidine tags were fused either around the entrances or at the sides. According to electron microscopic studies we immobilized these constructs uniformly either end-on or side-on at metal-chelating interfaces (lipid vesicles, lipid monolayers and self-assembled thiol monolayers). Degradation of small fluorogenic peptides and large proteins like casein was analyzed. Small substrates were degraded with comparable activity by free and immobilized proteasomes, irrespective of their orientation. Thus it can be assumed that peptides can pass the sealed entrance of the 'dead-end' proteasome. However, larger substrates like fluorescently labeled casein were processed near the temperature optimum by side-on immobilized and soluble proteasomes with threefold activity compared to end-on immobilized proteasomes. Hence it can be concluded that one pore is sufficient for substrate entry and product release. In other words, the pore and antechamber can fulfil a triple function in the import and unwinding of substrates and the egress of products. With means of surface plasmon resonance the exact substrate/proteasome stoichiometry could be determined to ~1 for 'dead-end' proteasomes and ~2 for side-on immobilized (active and inactive) proteasomes. Most importantly, a fit with the Hill equation revealed positive cooperativity for side-on immobilized (Hill coefficient ~2) in contrast to end-on immobilized proteasomes (Hill coefficient ~1). Thus in case of soluble proteasomes two substrates bind presumably in opposite antechambers with positive cooperativity. The off-rate of casein as substrate is twofold for the active side-on immobilized proteasome in comparison to the end-on immobilized proteasome. The exact 2:1 stoichiometry of the off-rates equals the ratio of exit pathways amenable in case of side-on orientated versus 'dead-end' immobilized proteasomes. Thus crevices along the cylindrical body of the 20 S proteasome seem not to participate in the egress of small products. An inactive proteasome mutant displays a concentration-dependent off-kinetic against casein. Accordingly, the off-rate of the bisubstrate:proteasome complex can be attributed around half the value of the monosubstrate:proteasome complex. Consequently, substrates exit the inactive proteasome via the route of access due to obstruction of the trans side with an entering substrate. Hence the active proteasomes have to chop substrates down to small fragments prior to release through both pores. Thus the processive degradation mode might result from positive binding cooperativity. The on-rate constants for casein suggested that substrate association represents a two-step process comprising a rate-limiting translocation step and a fast binding step. As fluorescence cross-correlation revealed that two substrates can be co-localized in the proteasome and bind successively with increasing affinity (KD,1 = 8 µM versus KD,2 = 700 nM), an allosteric transition in the proteasome can be assumed. Combining our results with the data from other research groups led to a mechanistic model for the 20 S proteasome. Accordingly, the first substrate undergoes a slow translocation step, binds in the antechamber and diffuses subsequently to the catalytic centers, where it is degraded. By switching on the catalytic activity, the pores at both termini are dilated via conformational changes. Hence entry of the second substrate into the proteasome is facilitated due to omission of the rate-determining translocation step. The second substrate is either accommodated in the antechamber before it is processed (alternating degradation) or, most probably, is directly threaded into the central cavity (simultaneous degradation). As effusing peptides compete with entering proteins for binding in the antechamber, the pores are kept in an open state. After finishing digestion the pores are closed and a new degradation cycle can be reinitiated. In summary, substrate association with the proteasome underlies an ordered alternating binding mechanism in contrast to the random mode of degradation. Thus the two-stroke engine offers the advantage of speeding up degradation without enhancing complexity.
Transmembrane proteins play crucial roles in biological systems as active or passive channels and receptors. Experimentally only few structures could be determined so far. Gaining structural insights enables besides a general understanding of biological mechanisms also further processing such as in drug design. Due to the lack of experimental data, reliable theoretical predictions would be of high value. However, for the same reason, missing data, the knowledge-based class of prediction methods that is well established for soluble proteins can not be applied. The goal of predicting transmembrane protein structures with ab initio methods demands locating the free energy minimum. Main difficulties here are, first, the computational costs of explicitly calculating all involved interactions and, second, providing an algorithm that is capable of finding the minimum within an extremely complex and rugged energy landscape. We have developed promising energy functions that describe the interactions of amino acids on a residue level, reducing computational costs while still containing most information on the atomistic level. We have also found a way to describe the interaction of the residues with its surrounding in a realistic manner by distinguishing residues exposed to the environment from those buried within helices using a sphere algorithm. The sphere algorithm can also be applied for a different purpose: one can measure how densely sidechains are packed for certain helical conformations, and thereby get an estimate of the sidechain entropy. In addition, overcrowding effects can be identified which are not well-described by the energy functions due to the pairwise calculation. To determine the absolute free energy minimum, we assume the helices to be located on an equidistance grid with slightly larger distances than to be expected. Optimizing the helices on the grid provides a starting point that should enable common minimizing algorithms, gradient-based or not, to find the absolute minimum beyond the grid. To simulate the dynamics of the helices on large time scales, we split them into rigid body dynamics and internal dynamics in terms of the dihedrals. The former one is well-known with its inherent problem of numerical drift and plenty of approaches to it, among which we have chosen the quaternions to represent the rotation of the rigid bodies. The latter one requires a detailed analysis of the torque size exerted on the dihedrals caused by the forces acting on the residues.
Die 5 Lipoxygenase (5 LO) ist das Schlüsselenzym in der Synthese von Leukotrienen. Sie wird auf transkriptioneller und posttranskriptioneller Ebene reguliert. Die Differenzierung myeloider Zelllinien mit 1,25-Dihydroxyvitamin D3 (1,25(OH)2D3) und transformierendem Wachstumsfaktor beta (TGFbeta) führt zu einer Erhöhung der 5 LO mRNA-, Protein-Bildung und der zellulären Enzymaktivität. Hier wurde gezeigt, dass dabei reife, nicht jedoch prä-mRNA der 5 LO im Zytosol und im Zellkern stark angereichert wird und dass beide Agentien in die mRNA-Prozessierung eigreifen. Obwohl die Bindung von VDR-Retinoid-X-Rezeptor (RXR)-Heterodimeren an Bindungsstellen im 5 LO-Promotor mittels DNAseI-Footprinting und EMSAs nachgewiesen wurde, konnten Reportergene unter der Kontrolle des 5 LO-Promotors in transienten und stabilen Transfektionen durch 1,25(OH)2D3/TGFbeta nicht stimuliert werden. Offensichtlich wird die Induktion der Expression der 5 LO durch 1,25(OH)2D3/TGFbeta durch Elemente außerhalb des Promotors vermittelt. In transienten Transfektionen führte der Einbau der kodierenden Sequenz der 5 LO in Luziferase-Plasmide bei Cotransfektion von VDR/RXR zu einer 5 fachen Induktion der Reportergen-Aktivität durch 1,25(OH)2D3/TGFbeta, was durch zusätzlichen Einbau der letzten vier Introns auf eine 13-fache Erhöhung gesteigert wurde. Der VDR zeigte einen Ligand-unabhängigen Effekt. Diese Reportergen-Effekte waren promotorunabhängig und von der kodierenden Sequenz gesteuert. RT-PCR-Analyse wies auf eine Deletion von Teilen der kodierenden Sequenz im Laufe der mRNA-Prozessierung hin, was durch 1,25(OH)2D3/TGFbeta verhindert wird. Auch Cotransfektion der TGFbeta-Effektoren Smads 3/4 führte in Abhängigkeit von der kodierenden Sequenz und in geringerem Maße von der 3'-UTR und den Introns J M, aber unabhängig vom Promotor, zu einer starken Erhöhung der Reportergenaktivität. Die 5 LO-Expression wird in den untersuchten Zellen vermutlich durch posttranskriptionelle Prozesse (Splicing, mRNA-Reifung) herunterreguliert, während 1,25(OH)2D3/TGFbeta die Expression der 5 LO durch eine Gegenregulation zu erhöhen, an der Komplexe beteiligt sind, die vermutlich Smads, VDR-RXR-Dimere, andere Transkriptionsfaktoren, Coaktivatoren, RNA-Polymerase II und Splicing-Faktoren enthalten. Hyperacetylierung des 5 LO-Promoters durch Inkubation mit mit dem Histondeacetylase-Inhibitor TsA führte zu einer transkriptionellen Aktivierung. Die kodierende Sequenz (und die Introns) wirkt diesem Effekt vermutlich durch die Rekrutierung von HDACs an VDR oder Smads, die direkt oder indirekt an die kodierende Region binden, entgegen.
The transporter associated with antigen processing (TAP) plays a pivotal role in the adaptive immune response against virus-infected or malignantly transformed cells. As member of the ABC transporter family, TAP hydrolyzes ATP to energize the transport of antigenic peptides from the cytosol into the lumen of the endoplasmic reticulum. TAP forms a heterodimeric complex composed of TAP1 and TAP2 (ABCB2/3). Both subunits contain a hydrophobic transmembrane domain and a hydrophilic nucleotide-binding domain. The aim of this work was to study the ATP hydrolysis event of the TAP complex and gain further insights into the mechanism of peptide transport process. To analyze ATP hydrolysis of each subunit I developed a method of trapping 8- azido-nucleotides to TAP in the presence of phosphate transition state analogs followed by photocross-linking, immunoprecipitation, and high-resolution SDS-PAGE. Strikingly, trapping of both TAP subunits by beryllium fluoride is peptide-specific. The peptide concentration required for half-maximal trapping is identical for TAP1 and TAP2 and directly correlates with the peptide-binding affinity. Only background levels of trapping were observed for low affinity peptides or in the presence of the herpes simplex viral protein ICP47, which specifically blocks peptide binding to TAP. Importantly, the peptideinduced trapped state is reached after ATP hydrolysis and not in a backward reaction of ADP binding and trapping. In the trapped state, TAP can neither bind nor exchange nucleotides, whereas peptide binding is not affected. In summary, these data support the model that peptide binding induces a conformation that triggers ATP hydrolysis in both subunits of the TAP complex within the catalytic cycle. The role of the ABC signature motif (C-loop) on the functional non-equivalence of the NBDs was investigated. The C-loops of TAP transporter contain a canonical C-loop (LSGGQ) for TAP1 and a degenerated ABC signature motif (LAAGQ) for TAP2. Mutation of the leucine or glycine (LSGGQ) in TAP1 fully abolished peptide transport. TAP complexes with equivalent mutations in TAP2 showed however still residual peptide transport activity. To elucidate the origin of the asymmetry of the NBDs of TAP, we further examined TAP complexes with exchanged C-loops. Strikingly, the chimera with two canonical C-loops showed the highest transport rate whereas the chimera with two degenerated C-loops had the lowest transport rate, demonstrating that the ABC signature motifs control the peptide transport efficiency. All single-site mutants and chimeras showed similar activities in peptide or ATP binding, implying that these mutations affect the ATPase activity of TAP. In addition, these results prove that the serine of the C-loop is not essential for TAP function, but rather coordinates, together with other residues of the C-loop, the ATP hydrolysis in both nucleotide-binding sites. To study the coupling between the ATP binding/hydrolysis and the peptide binding, the putative catalytic bases of the TAP complex were mutated to generate the so-called EQ mutants. The mutations did not influence the peptide-binding ability. Dimerization of the NBDs of EQ mutants upon ATP binding does not alter the peptide binding property. At 27°C, both ATP and ADP could induce the loss of peptide-binding ability (Bmax) only in the variants bearing a mutated TAP2. Further studies are required to deduce at which stage in the catalytic cycle the peptide-binding site is affected. In addition, mutation of the putative catalytic base of both subunits showed a magnesium-dependent peptide transport activity, demonstrating these mutants did not abolish the ATP hydrolysis. Thus, the function of this acidic residue as the catalytic base is not likely to be universe for all ABC transporters.
Ligands of Iron-Sulphur Cluster N2: In this work the ubiquinone reducing catalytic core of NADH:ubiquinone oxidoreductase (complex I) from Y. lipolytica was studied by a series of point mutations replacing conserved histidines or arginines in the 49-kDa subunit. Although the missing 4th ligand of cluster N2 could not be found in the 49-kDa subunit of complex I, it was clearly demonstrated that iron-sulphur cluster N2 resides directly on the interface between the PSST and 49-kDa subunits. The results presented in this work show that residues in the 49-kDa subunit have strong influence on this redox centre and also on catalytic activity. The strong influence of Arg-141 and His-226 residues in 49-kDa subunit on this cluster can be deducted from complete loss of N2 signals in EPR spectra such as in case of mutants H226A and R141A. In the case of mutant H226M the EPR signal from cluster N2 was shifted and cluster N2 even lost the pH dependence of its redox midpoint potential and became more similar to the other so called 'isopotential' clusters. Specifically in the case of mutants R141M and R141K the characteristic signature of cluster N2 became undetectable in EPR spectra. However, specific dNADH:DBQ oxidoreductase activity that could be inhibited with the specific complex I inhibitors DQA and rotenone was not absolutely abolished but rather reduced. These reductions in complex I activity did not correspond to similar reductions in the specific EPR signal of cluster N2 as it was observed in the His-226 mutant series. No indications could be found that these mutations had modified the magnetic properties of cluster N2, resulting in different EPR spectra. From these observations it could be concluded that both mutants R141K and R141M virtually or entirely lack iron-sulphur cluster N2. The rates in complex I activity could be reconciled with electron transfer theory: After removal of a single redox centre in a chain, electron transfer rates are predicted to be still much faster than steady-state turnover of complex I. These results from mutants R141K, R141M and also the result from mutant H226M that protons are being pumped even if the redox midpoint potential of cluster N2 is not pH dependent questions the prominent role in the catalytic mechanism of complex I that has been ascribed to cluster N2. Histidine 91 and 95 were found to be absolutely essential for activity of complex I since in both mutants complex I was fully assembled and artificial NADH:HAR activity was parental whereas complex I specific dNADH:DBQ activity was abolished. The signal from cluster N2 in EPR spectra was parental for all His-91 and -95 mutants. Mutations at the C-terminal arginine 466 affected ubiquinone affinity and inhibitor sensitivity but also destabilised complex I. All these results provide further support for a high degree of structural conservation between the 49-kDa subunit of complex I and the large subunit of water soluble [NiFe] hydrogenases. Remodelling of Human Pathogenic 49-kDa Mutations in Y. lipolytica: Y. lipolytica has been proven a good system for studying complex I properties and thus also for studying defects that occur in humans. In this work pathogenic mutations in the 49-kDa subunit of complex I were recreated and studied. The P232Q mutant showed non-assembly of complex I and this is probably the cause why this mutation was lethal in patients. The mutants R231Q and S416P were parental for the content, artificial and also specific complex I activity, Km for DBQ and IC50 for DQA. From these results we can conclude that these two residues Arg-228 and Ser-413 in mammalian cells have specific structural importance for the 49-kDa subunit even if they are not directly involved in catalytic process.
This dissertation study argues that 'policy advice formation', as a discourse development, is a differentiated hybrid resultant from merger between comparative education and policy studies disciplines. Through discourse analysis based on John Creswell's format, this study identifies revisions, restatements and shifts in emphasis of theories, methodological models and challenge topics of comparative education and policy studies. Findings which display the development of policy advice formation' discourse. In conclusion, this study found differential patterns seemingly formed because of collaborative affects of standardization in education science knowledge expressed within discourse.
In the present study the cryo-immunogold technique was used and optimized for investigating the ultrastructure and immunolabeling of synaptic proteins. It is evidently a suitable method for the localization of membrane proteins since the antigens are not treated with any chemical denaturation before immunolabeling except for the fixation and since the antigens are directly exposed to the surface of the cryo-ultrasections. The v-SNARE VAMP II and the vesicle-associated proteins SV2 and Rab3A were detected extensively at small vesicles in the mossy fiber terminals. The t-SNARE SNAP-25, and N-type and P/Q type Ca2+ channels were allocated to the plasma membrane both at the active zone and outside the active zone. SNAP-25 and N-type Ca2+ channels appeared also at synaptic vesicles. A significantly increased immunolabeling of VAMP II, SV2, Rab3A, SNAP-25 and N-type Ca2+ channels was found at the active zones of fast synapses, indicating a concentration of these proteins at sites of exocytosis. The widespread distribution of the t-SNARE SNAP-25 at the axonal plasma membrane reveals that membrane-targeting specificity cannot be determined solely by v/t-SNARE interactions. Additional control components are required to assure the docking and exocytosis of the synaptic vesicles at active zones. The novel protein Bassoon was only found at active zones of central synapses and showed the highest specific labeling among all proteins investigated. Its labeling pattern implies an association of Bassoon with the presynaptic dense projections, the structural guide for vesicle exocytosis. The involvement of Bassoon in the organization of the neurotransmitter release site suggests that Bassoon may play an important role in determining the specificity of vesicle docking and fusion. In the neurosecretory endings of neurohypophysis the synaptic proteins VAMP II, SNAP- 25, SV2, Rab3A, and the N-type Ca2+ channels showed a preferential labeling over microvesicles. Moreover, the immunolabeling intensity of these proteins over microvesicles corresponded closely to that over synaptic vesicles. This suggests that these synaptic proteins share an identical association with synaptic vesicle and microvesicles. A significant labeling of SNAP-25, the N-type Ca2+ channels and VAMP II was also detected at the plasma membrane near the clustered microvesicles, indicating the competence of microvesicles for docking and exocytosis along the plasma membrane in the absence of active zones. No significant labeling of VAMP II, SNAP-25, SV2 and N-type Ca2+ channel was observed at the membrane of neurosecretory granules. This is in agreement with the notion that synaptic vesicles and microvesicles possess regulatory mechanisms for exocytosis different from those of granules. In contrast, a/ß-SNAP and NSF were found on the granules, and Rab3A and the P/Q-type Ca2+ channels on granules in a subset of terminals. Rab3A is associated specifically with the oxytocin-containing granule population. Interestingly, some plasma membrane proteins, such as SNAP-25 and even N-type Ca2+ channels and P/Q-type Ca2+ channels, were observed not only at the plasma membrane but also at the vesicular organelles. This suggests that these vesicular organelles may be involved in transporting newly synthesized proteins from the soma to the plasma membrane of the terminal. Furthermore, the vesicular pool of the Ca2+ channels may serve in the stimulationinduced translocation into the plasma membrane when required. Using the conventional preembedding method with Epon and the post-embedding method with LR Gold, VAMP II was localized at vesicular organelles of varying size and on horseradish peroxidase filled endocytic organelles in cultured astrocytes, with and without stimulation in the presence of the horseradish peroxidase. This indicates that VAMP II is involved in the cycle of vesicular exocytosis and endocytosis in astrocytes. U373 cells are capable of expressing all three members of the synaptic SNARE complex (v-SNARE VAMP II, t-SNARE syntaxin I and SNAP25). This indicates the competence of U373 to carry out regulated exocytosis by means of the classical SNARE mechanism. In addition, the ubiquitous v-SNARE cellubrevin and the endosome-associated small GTPbinding protein Rab5 could be expressed in U373 cells. All recombinant synaptic proteins investigated in U373 cells revealed a punctuate cellular distribution under the fluorescence microscope, suggesting that they are mainly associated with intracellular compartments. The cryo-electron microscopy provided direct evidence for the association of all expressed proteins with electron-lucent vesicular organelles. It further supports the potential of U373 MG cells to release low molecular weight messengers by a regulated exocytosis mechanism. In addition, myc-VAMP II was found on dispersed granules. Probably, VAMP II also participates in the exocytosis event of granules in U373 cells. Gold labeling for the two presumptive t-SNAREs syntaxin I and SNAP-25 in U373 cells was confined to the vesicular organelles. At the ultrastructural level no significant labeling was identified at the plasma membrane. The high level of colocalization of the two SNARE proteins VAMP II and syntaxin I in the cell body and in cell processes suggests that the two proteins are mostly sorted into identical vesicular organelles. A partial colocalization of VAMP II and cellubrevin as well as of VAMP II and Rab5 was observed under the fluorescence microscope. At the ultrastructural level, a colocalization of VAMP II and cellubrevin as well as of VAMP II and Rab5 was found on some clustered vesicles. The partial colocalization of VAMP II and cellubrevin implies that they similarly function as v-SNAREs. The partial colocalization of Rab5 with VAMP II in U373 cells suggests that the endosomal protein Rab5 is associated with VAMP II-containing organelles during some stages of their life cycle.