Electrogenic Reactions Associated with Electron Transfer in Photosystem I

Abstract

Photoelectric methods to study charge transfer processes in photosynthetic organisms are reviewed along with comparative analyses of photoelectric reactions in Photosystem I, Photosystem II, and the reaction center of purple bacteria. Particular emphasis is placed on a comparison of dielectrically weighted photoelectric signal amplitudes with local structural parameters obtained from the X-ray diffraction data. This analysis demonstrates that the effective dielectric constant (epsilon) is distributed heterogeneously over the pigment-protein complexes of Photosystem 1, Photosystem II, and the bacterial reaction center. The electrogenicity in the Photosystem I complex and its interaction with natural donors (plastocyanin, cytochrome c(6)), natural acceptors (ferredoxin, flavodoxin), and artificial acceptors and donors (methyl viologen and other redox dyes) were studied. The profile of distribution of the dielectric constant along the stretch of the complexes was calculated. It was found that the epsilon value is minimal at the core of the complexes and gradually increases toward the periphery. To a first approximation, the hierarchy of the dielectric constant values derived from photoelectric experiments qualitatively correlates with the hierarchy of the rate constants of charge transfer reactions along the photosynthetic electron transport chain. The fastest primary reactions of charge separation and stabilization of separated charges occur in the core of the complexes in the picosecond time range, whereas slower secondary reactions take place on the periphery of the complexes in the microsecond to millisecond time range. The correlation between the local dielectric constant and the electron transfer rate constant at a corresponding chain segment is discussed in terms of mechanism. It is suggested that these mechanisms are specified by the dielectric reorganization and relaxation of the medium.