Water Oxidation in Photosystem II: Energetics and Kinetics of Intermediates Formation in the S₂→S₃ and S₃→S₀ Transitions Monitored by Delayed Chlorophyll Fluorescence

Abstract

Water oxidation by Photosystem II (PSII) is a process of fundamental importance for atmosphere (O₂ production) and biosphere (primary biomass formation). In order to understand this basic biological process and to promote the rationale designs of artificial systems that mimic photosynthetic water oxidation, it is important to understand the energetic and kinetic parameters of intermediates formed in the course of the reaction cycle (S-state cycle). In the present study, we use time-resolved measurements of the delayed chlorophyll fluorescence to estimate rate constants, activation energies, free energy differences, and to discriminate between the enthalpic and the entropic contributions. Using a novel joint-fit simulation approach, kinetic parameters are determined for intermediates in the S₂→S₃ and in the S₃→S₀ + O₂ transitions. The estimated parameters provide evidence for intermediate formation by deprotonation processes that take place already before the electron transfer from the tetra-manganese complex to the light-oxidized Tyr_Z, in both of the above S-state transitions.