Coupling of electron and proton movement in photosynthetic water oxidation

Abstract

The mode of coupling of electron and proton transfer in the reaction pattern of photosynthetic water oxidation is a key issue for deeper understanding of the mechanism of this fundamental process. Model considerations are presented that are based on experimental data gathered from kinetic measurements of individual redox steps as function of i) temperature, ii) pH, iii) replacement of exchangeable protons by deuterons and iv) addition of Ca2+. The basic conclusion emerging from the experimental results is a hypothesis on the central role of hydrogen bond network(s) as fine tuning regulators of reaction coordinates. It is inferred: a) the multiphasic P680+ reduction by YZ comprises two types of reactions, i. e. a fast (t <100ns) electron transfer from YZ with proton shift from its specifically bridged OH group to its acceptor and subsequent slower relaxation processes of the protein matrix within different time domains and local extensions, b) the elementary oxidation steps of the water oxidizing complex (WOC) by YZox are triggered reactions that are limited by reactions other than nonadiabetic electron transfer and c) the redox state S3 is a rapidly exchanging multistate equilibrium of at least two components with different electronic configuration and nuclear geometry. The component formally corresponding to a complexed peroxide is the entatic state for oxidation by YZox under formation of complexed dioxygen and its subsequent release via substrate/product exchange.