Role of Protons in Photosynthetic Water Oxidation: pH Influence on the Rate Constants of the S-state Transitions and Hypotheses on the S₂→S₃ Transition

Abstract

The mode of proton relocation from the water-oxidizing Mn complex of photosystem II (PSII) toward the aqueous phase is of key importance in photosynthetic water oxidation. An adequate description of the interrelation of proton and electron removal from the Mn complex is still lacking. We reinvestigate the influence of the pH on the rate constants of the redox transitions of the Mn complex (S-state transitions). For high-activity PSII membrane particles from spinach, near-UV absorption transients (at 360 nm) induced by trains of ns-Laser flashes were analyzed. To obtain the rate constants of the ‘pure’ S-state transitions, a stringent deconvolution of the raw transients was carried out. The transients of the S₁→S₂ and S₂→S₃ transitions exhibit mono-exponential behavior whereas the transients of the ‘S₃→S₀ + O₂’ transition display a lag-phase behavior that is assignable to formation of a reaction intermediate by deprotonation. The rate constants of the electron transfer (ET) in the S₁→S₂ and S₃→S₀ transitions exhibit a negligible pH-dependence only. The rate constant of the S₂→S₃ transition decreases below pH 6 significantly, but clearly less than those of the lag-phase in the S₃→S₀ transition. Two models for the coupling of electron and proton transfer in the S₂→S₃ transition are discussed.