Spectroscopic Analysis of the Redox Reactions of π-Conjugated Cofactors in Photosynthetic Reaction Center

Abstract

Photosynthesis is a biological process in which solar energy is converted into a storable form of chemical energy. Light-driven electron transfer in a photosynthetic reaction center is performed by various types of redox cofactors with π-electron conjugation. In this chapter, spectroscopic investigations of the electron transfer reactions of π-conjugated cofactors in photosystem II (PSII), which has a function of water oxidation to obtain electrons necessary for CO2 fixation, are introduced. Light-induced Fourier transform infrared difference spectroscopy was used to detect the structures and reactions of π-conjugated cofactors in PSII, i.e., tyrosine YZ, chlorophyll dimer P680, pheophytin electron acceptor PheoD1, and plastoquinone electron acceptors QA and QB. Detailed spectral analyses and estimations of redox potentials were further performed by quantum chemical calculations using the density functional theory and quantum mechanics/molecular mechanics methods. Studies of the regulation mechanisms of forward and backward electron transfer relevant to photoprotection of PSII are also described.