TY  - JOUR
A1  - Vierke, Gerhard
A1  - Müller, Manfred
T1  - On the kinetics and mechanism of spontaneous intramolecular reduction of the central metal ion in K2[Mn(IV)-2-α-hydroxyethyl isochlorin e4] acetate in aqueous alkaline solutions and its relation to the binding sites of manganese in photosynthesis
T2  - Zeitschrift für Naturforschung, C
N2  - Spectrophotometric investigation of the kinetics of the spontaneous reduction of the central metal ion in K2[Mn (IV)-2-α-hydroxyethyl-isochlorine e4] acetate in aqueous alkaline solution in the absence of any reducing agent reveals that it is a pseudo-first order reaction which is specifically hydroxide ion catalyzed. The pKα-value of the acid-base equilibrium has been estimated to be 14.4.
Electron transfer to the central metal ion is the rate limiting step. The measurements of its temperature dependence yields an activation enthalpy of ∆H‡ = 12 kcal/mol and an entropy of activation ∆S‡ = - 30 e.u. thus indicating that the electron transfer step is a bimolecular reaction. The most likely reactant is water. The reduction reaction does not take place with appreciable reaction rates at physiological pH. Thus, when bound to a suitable ligand of the chlorin type, Mn (IV)-compounds are sufficiently stable with respect to autoxidation to play some role in biological redox reactions as postulated recently for the photoreactivation process of the water splitting system in photosynthesis.
KW  - Manganese Binding Sites
KW  - Photosynthesis
KW  - Spontaneous Reduction of Mn(IV)-chlorin
KW  - Base Catalysis
Y1  - 2014
UR  - http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/72281
UR  - https://nbn-resolving.org/urn:nbn:de:hebis:30:3-722811
SN  - 0939-5075
SN  - 1865-7125
VL  - 30.1975
IS  - 5-6
SP  - 327
EP  - 332
PB  - Verlag der Zeitschrift für Naturforschung
CY  - Tübingen
ER  -