Theoretical Research on Primary Chemical Reactions in Photosynthesis.

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1. A molecular orbital theory for many electron system was applied to formulate and classify possible mechanisms in elementary intermolecular electron-transfer(ET) process. The ET matrix element was found to contain four terms which represent an elastic electron-tunneling process( <T_1> ), a doubleexchange process via occupied molecular orbitals( <T_2> ), an inelastic electron-tunneling process( <T_3> ) and a nuclear nonadiabaticity process( <T_4> ). A significant finding is that the <T_3> , which arises from the Coulombic interaction between electron and promoting modes of molecular vibration, can be greatly enhanced far beyond <T_1> when the vibrating charges exist as close as the ET pathway because of its quasi-one dimensionarity.2. A theoretical formula for the Franck-Condon factor in the ET rate taking account of the anharmonicity (3rd and 4th) of molecular vibration was derived with use of the Green's function technique in many-body problems. The effect of anharmonicity was estim … More ated to change the rate constant at most by one order of magnitude. It is likely to be responsible for negative activation processes such as the (Bchl) <2^+> I <Q(_A^-)> <-!>> <(BChl)_2> I <Q_A> reaction in bacteria.3. The enhancement of the ET rate over the classical Marcus theory in the inverted region where the energy gap exceeds the reorganization energy was ascribed to the "phonon side-bands" of high frequency modes of molecular vibrations such as the C-C and C-H bond Stretchings.4. The observed <H_2> O/ <D_2> O isotope effect in cytochrome oxidation reactions seems to indicate the predominance of the <T_3> term, i.e. the promoting mode phonon-induced electron-tunneling mechanism, in biological systems.5. As regards the charge separation process in photosynthetic bacteria, we estimated the <T_3> for each predicted shortest ET pathway which includes hydrogen-bonds and obtained the calculated rate constants and quantum yield in good agreement with experimental results.6. In order to establish the molecular mechanism of photosynthetic water-splitting reaction based on our "microsurface model of catalytic binuclear manganese complex, we optimized the geometries of the <S_0> and <S_1> states with use of ab initio molecualr orbital method. The resultant Mn-Mn atomic distances agree well with those obtained from the EXAFS analysis. Further, the <S_1> state was found to be more stable than the <S_0> state in agreement with the experimental fact. Less