Biochemie und Chemie
Refine
Year of publication
Document Type
- Article (1112)
- Doctoral Thesis (728)
- Book (46)
- Preprint (32)
- Contribution to a Periodical (14)
- Conference Proceeding (11)
- Report (11)
- Review (9)
- diplomthesis (3)
- Part of a Book (2)
Has Fulltext
- yes (1974)
Is part of the Bibliography
- no (1974)
Keywords
- crystal structure (37)
- Crystal Structure (25)
- Synthesis (15)
- ESR Spectra (14)
- RNA (14)
- NMR-Spektroskopie (12)
- hydrogen bonding (11)
- IR Spectra (10)
- NMR spectroscopy (10)
- RNS (9)
Institute
- Biochemie und Chemie (1974)
- Medizin (83)
- Exzellenzcluster Makromolekulare Komplexe (68)
- Biowissenschaften (64)
- Präsidium (64)
- Zentrum für Biomolekulare Magnetische Resonanz (BMRZ) (63)
- Pharmazie (52)
- MPI für Biophysik (49)
- Sonderforschungsbereiche / Forschungskollegs (48)
- Georg-Speyer-Haus (22)
Phasentrennung als Folge der Konkurrenz zwischen "statistischer" und "chemischer" Vermischung
(1977)
The fact that common thermodynamic conditions are valid for all known types of critical phases (liquid-liquid, liquid-gas, and "gas-gas") suggests that a common principle for the interpretation of material phase instability from a molecular point of view must exist. In this paper we show that the principle of competition between "statistical mixing" (i. e. random mixing) and "chemical mixing" (i. e. mixing effected under the influence of chemical interactions) can give this common inter pretation. If the equilibrium states resulting from both types of mixing are sufficiently different, phase separation occurs. We refer to our earlier papers (since 1972) in which we have applied this principle to describe liquid-liquid phase equilibria by "chemical" models, using the equilibrium constants of exchange equilibria between nearest-neighbour complexes as a measure of "chemical" mixing. In this paper we show that the well-known reduced gas-liquid coexistence curve, T/Tc =f(q/qc), can accurately be fitted by a very simple "mixture" model of molecules A with "vacan cies", provided that the contributions of both statistical and chemical mixing are incorporated into the formula for GE. From a discussion of the application to "gas-gas" phase equilibria in the hyper critical region it results that the weight factor r, by which the contribution of statistical mixing enters into GE, must depend on the density of the gas mixture. Phase separation can only occur if, by increasing pressure, the contributions to GE of statistical and chemical mixing have reached the same order of magnitude. From an attempt to apply the same principle to solid-liquid equilibria it is shown under which external conditions a critical point for this type of phase transition can be expected.
A thermodynamic theory of liquid mixtures based on a simple molecular model is developed which describes the equilibrium state as the result of a coupling between a "chemical" and a "statistical" equilibrium. The intermolecular interactions are taken into account by considering "complexes" formed between a given molecule and its z nearest neighbours. The equilibrium mole fractions of these complexes are calculated by application of the ideal law of mass action to an appropriate set of "exchange equilibria". Formulae for the excess functions GE and HE and for the activities of the components are derived for the cases z=1 and z=4. GE depends on an equilibrium constant K describing the deviation from random distribution of the equilibrium mole fractions of the complexes. HE depends on K and on an energy parameter w which is related to differences of pair interactions. K and w are independent parameters, and there is no limitation in respect to amount and sign of the excess functions. The conditions for the existence of a critical solution point are formulated; at this point GE has a value of about 0.56 R T. If a model with two equilibrium constants is used allowing for instance competition between "self-association" and "complex-formation", the existence of closed miscibility gaps becomes possible. Closed miscibility curves are calculated and the conditions for their appearance are discussed. The relations between this theory and Guggenheim's statistical lattice theory of symmetrical mixtures are pointed out.
This paper contains further applications on symmetrical liquid mixtures of the molecular thermodynamic theory which has been developped in part I of this series. The essential feature of this theory is the superposition of "chemical" and “random” exchange equilibria between “complexes” formed by a given molecule and its z nearest neighbours, thus allowing a unified treatment of the thermodynamic phenomena in binary liquid mixtures using the equilibrium constant K of the ideal law of mass action and the energy w of pair interactions as parameters.
The temperature and pressure dependences of K and the evaluation of experimental excess enthalpy and excess volume data are treated. Formulas and examples for the calculation of K and w from isothermal and non-isothermal vapour-liquid equilibrium data are given. The conditions for azeotropy with minimum or maximum vapour pressure, resp., are derived. Melting curves for a symmetric eutectic system with superposed miscibility gap are discussed. Further models for partially miscible liquids with competing self-association and complex-formation are treated showing the phenomenon of two separated miscibility gaps.
The enthalpies of mixing at 25° of diethyl ether, di-n-propyl ether, di-n-butyl ether, di-isopropyl ether, propylene oxide, tetrahydrofuran, and tetrahydropyran with chloroform are determined by an isothermal titration method. As a result, the functions HM-f(N CHCl3) are obtained with a step width of 0.025 of the mole fraction and a relative accuracy of 1 per cent or better. Evaluation of the heat of mixing data by means of equilibrium models ("ideal associated mixture") shows that the systems of aliphatic ethers with chloroform behave rather precisely as one-step equilibria of the type A + B = AB (A = ether; B = chloroform). In the systems of cyclic ethers with chloroform, a second equilibrium step, AB + B = AB2 , must be considered, the importance of which decreases with increasing ring size of the ether. The equilibrium data calculated for the seven ether-diloroform systems are discussed.
As we have shown in a recent paper, the principle of competition between "statistical" and "chemical" mixing represents a molecular thermodynamic approach to all known types of phase separation. This principle is effective if the contributions of two independent spontaneous processes enter into the thermodynamic potential by which the resulting equilibrium state of the system is determined. This is equivalent with the statement that two different forms of entropy exist which are not interchangeable, and for which the law of increasing entropy independently must be valid. As "cooperativity" is introduced by this principle, critical phenomena may be described by simple equilibrium models in which only nearest-neighbour interactions are considered.
Starting from the molar Gibbs free energy GM of the most simple binary equilibrium model z = 1 with nearest-neighbour pairs, nonclassical critical-point exponents α = 0.33 of the molar heat capacity, β = 0.33 of the coexistence curve, γ = 1.33 of the isothermal compressibility, and δ = 4.33 of the critical isotherm, are derived, which are consistent with the well-known exponent in equalities. These non-classical critical-point exponents are independent of the chemical nature of the particles because they are obtained by applying thermodynamic arguments on the coupling constant τ, by which the contribution of "statistical mixing" to GM is weighted.
A generic drug product (World Health Organization (WHO) terminology: multisource product) is usually marketed and manufactured after the expiry date of the innovator’s patent. Generic drugs are less expensive than the innovator products because generic manufacturers do not have to amortize the investment costs of research, development, marketing, and promotion. Multisource products must contain the same active pharmaceutical ingredients (APIs) as the original formulation and have to be shown to be interchangeable with the original formulation. Multisource products have to be shown bioequivalent to the innovator counterpart with respect to pharmacokinetic and pharmacodynamic properties. Multisource products are therefore identical in dose, strength, route of administration, safety, efficacy, and intended use. Bioequivalence can be demonstrated by in vitro dissolution, pharmacokinetic, pharmacodynamic or clinical studies. Since 2000, the U.S. Food and Drug Administration (FDA) allows the approval of certain multisource products solely on the basis of in vitro studies, i.e. by waiving in vivo studies in humans (“Biowaiver”), based on the Biopharmaceutics Classification Scheme (BCS). The BCS characterizes APIs by their solubility and permeability in the gastrointestinal tract (GIT). The different BCS Classes I-IV (Class I: high solubility, high permeability; Class II: low solubility, high permeability; Class III: high solubility, low permeability and Class IV: low solubility, low permeability) result from all possible combinations of high and low solubility with high and low permeability. Since the adoption of the BCS by the FDA in 1995, the BCS criteria have been under continuous development. In 2006, the WHO has released the most recent bioequivalence guidance including relaxed criteria for bioequivalence studies based on modified BCS criteria. According to this guidance, APIs belonging to the BCS classes I – and under defined conditions - II and III – are eligible for a biowaiver-based approval. The principal objective of this work was to characterize the first-line anti tuberculosis APIs, isoniazid, pyrazinamide, ethambutol dihydrochloride and rifampicin, according to their physicochemical, biopharmaceutical, pharmacokinetic and pharmacological properties and to classify them according to the BCS. Ethambutol dihydrochloride and isoniazid were classified as borderline BCS class I/III APIs. Pyrazinamide was classified as a BCS class III and rifampicin as a BCS class II API. Based on the BCS classification and the additional criteria defined in the WHO bioequivalence guidance, the possibility of biowaiver-based approval for immediate release (immediate release) solid oral dosage forms containing the first-line antituberculosis drugs was evaluated. A biowaiver-based approval with defined constraints was recommended for immediate release solid oral dosage forms containing isoniazid (interaction with reducing sugars), pyrazinamide and ethambutol dihydrochloride (relative narrow therapeutic index). Rifampicin was classified as a BCS class II API, and it was concluded that rifampicin containing solid oral immediate release drug products as well as Scale-Up and Post-Approval Changes (SUPAC) changes should not be approved by a biowaiver on the following basis: (i) its solubility and dissolution are highly variable due to polymorphism and instability, (ii) concomitant intake of food and antacids reduces its absorption and bioavailability, (iii) no in vitro predictive dissolution test has been found which correlates to in vivo absorption and (iv) several publications reporting cases of non-bioequivalent and bioinequivalent rifampicin products have been located in the literature. Thus, it is recommended that bioequivalence of rifampicin containing solid oral immediate release drug products should be established by in vivo pharmacokinetic studies in humans. This risk-benefit benefit assessment of a biowaiver-based approval was presented as a poster at the American Association of Pharmaceutical Scientists (AAPS) 2005 and subsequently published as “Biowaiver Monographs” in the Journal of Pharmaceutical Sciences. Based on the assessment of the dissolution properties of the antituberculosis drugs for a biowaiver approval, quality control dissolution methodologies for the International Pharmacopoeia (Pharm. Int.) were developed, presented at the WHO expert meeting and adopted in the Pharm. Int. (http://www.who.int/medicines/publications/pharmprep/OMS_TRS_948.pdf). Additionally, preliminary biowaiver recommendations were also developed for four firstline antimalarial drugs listed on the WHO Essential Medicines List (EML): Quinine, as both the hydrochloride and sulphate, and proguanil hydrochloride were classified as borderline BCS class I/III APIs. Since quinine is a narrow therapeutic index drug and many cases of non-bioequivalence have been reported in the literature, a biowaiverbased approval was not recommended. For solid oral immediate release dosage forms containing proguanil a biowaiver-based approval was recommended under the condition that they dissolve very rapidly. Primaquine phosphate was classified as a BCS class I API. Therefore, a biowaiver-based approval was recommended for immediate release solid oral dosage forms containing primaquine phosphate. Mefloquine hydrochloride was classified as a basic, BCS class IV/II API, making it ineligible for the biowaiver. Additionally, reports of non-bioequivalence and a narrow therapeutic index were found in the scientific literature. Consequently, bioequivalence of solid oral immediate release dosage forms containing mefloquine hydrochloride should be established by in vivo pharmacokinetic studies. The results for quinine hydrochloride and sulphate, proguanil hydrochloride, primaquine diphosphate and mefloquine hydrochloride were presented as a poster at the Pharmaceutical Sciences World Congress (PSWC) 2007 and published as a WHO Collaborating Center Report in June 2006. The aim of this project was to collect, evaluate, generate and publish relevant information for a biowaiver-based approval of essential medicines in order to provide a summary to local regulatory authorities. This information complements the selected list of essential medicines by providing information about the biopharmaceutical properties and pharmaceutical quality of solid oral immediate release dosage forms containing these APIs. The aim of the biowaiver project, inspired by the WHO and brought in life by the International Pharmaceutical Federation (FIP), is to enable access to essential medicines in standardized quality at an affordable price. In this work, a significant contribution to this aim in the form of four biowaiver monographs for the antituberculosis drugs and several reports on the antimalarials has been achieved.
Determining the structure and mechanisms of all individual functional modules of cells at high molecular detail has often been seen as equal to understanding how cells work. Recent technical advances have led to a flush of high-resolution structures of various macromolecular machines, but despite this wealth of detailed information, our understanding of cellular function remains incomplete. Here, we discuss present-day limitations of structural biology and highlight novel technologies that may enable us to analyze molecular functions directly inside cells. We predict that the progression toward structural cell biology will involve a shift toward conceptualizing a 4D virtual reality of cells using digital twins. These will capture cellular segments in a highly enriched molecular detail, include dynamic changes, and facilitate simulations of molecular processes, leading to novel and experimentally testable predictions. Transferring biological questions into algorithms that learn from the existing wealth of data and explore novel solutions may ultimately unveil how cells work.
Bacterial sugar symporters in the Major Facilitator Superfamily (MFS) use the H+ (and in a few cases Na+) electrochemical gradients to achieve active transport of sugar into the cell. Because a number of structures of MFS sugar symporters have been solved recently, molecular insight into the transport mechanism is possible from detailed functional analysis. We present here a comparative electrophysiological study of the lactose permease (LacY), the fucose permease (FucP) and the xylose permease (XylE), which reveals common mechanistic principles and differences. In all three symporters energetically downhill electrogenic sugar/H+ symport is observed. Comparison of the pH dependence of symport at symmetrical pH exhibits broad bell-shaped pH profiles extending over 3 to 6 pH units and a decrease at extremely alkaline pH ≥ 9.4 and at acidic to neutral pH = 4.6–7.5. The pH dependence can be described by an acidic to neutral apparent pK (pKapp) and an alkaline pKapp. Experimental evidence suggests that the alkaline pKapp is due to H+ depletion at the protonation site, while the acidic pKapp is due to inhibition of deprotonation. Since previous studies suggest that a single carboxyl group in LacY (Glu325) may be the only side chain directly involved in H+ translocation and a carboxyl side chain with similar properties has been identified in FucP (Asp46) and XylE (Asp27), the present results imply that the pK of this residue is switched during H+/sugar symport in all three symporters.
Glioblastoma multiforme (GBM) is a deadly primary brain malignancy. Glioblastoma stem cells (GSC), which have the ability to self-renew and differentiate into tumor lineages, are believed to cause tumor recurrence due to their resistance to current therapies. A subset of GSCs is marked by cell surface expression of CD133, a glycosylated pentaspan transmembrane protein. The study of CD133-expressing GSCs has been limited by the relative paucity of genetic tools that specifically target them. Here, we present CD133-LV, a lentiviral vector presenting a single chain antibody against CD133 on its envelope, as a vehicle for the selective transduction of CD133-expressing GSCs. We show that CD133-LV selectively transduces CD133+ human GSCs in dose-dependent manner and that transduced cells maintain their stem-like properties. The transduction efficiency of CD133-LV is reduced by an antibody that recognizes the same epitope on CD133 as the viral envelope and by shRNA-mediated knockdown of CD133. Conversely, the rate of transduction by CD133-LV is augmented by overexpression of CD133 in primary human GBM cultures. CD133-LV selectively transduces CD133-expressing cells in intracranial human GBM xenografts in NOD.SCID mice, but spares normal mouse brain tissue, neurons derived from human embryonic stem cells and primary human astrocytes. Our findings indicate that CD133-LV represents a novel tool for the selective genetic manipulation of CD133-expressing GSCs, and can be used to answer important questions about how these cells contribute to tumor biology and therapy resistance.
In fungi, the mitochondrial respiratory chain complexes (complexes I–IV) are responsible for oxidative phosphorylation, as in higher eukaryotes. Cryo-EM was used to identify a 200 kDa membrane protein from Neurospora crassa in lipid nanodiscs as cytochrome c oxidase (complex IV) and its structure was determined at 5.5 Å resolution. The map closely resembles the cryo-EM structure of complex IV from Saccharomyces cerevisiae. Its ten subunits are conserved in S. cerevisiae and Bos taurus, but other transmembrane subunits are missing. The different structure of the Cox5a subunit is typical for fungal complex IV and may affect the interaction with complex III in a respiratory supercomplex. Additional density was found between the matrix domains of the Cox4 and Cox5a subunits that appears to be specific to N. crassa.