On the mechanism of photosynthetic oxidation of water in a dimeric oxygen-evolving complex of chloroplasts photosystem II

Based on the analysis of the molecular organization and properties of an is olated oxygen-evolving complex of photosystem II of plant chloroplasts, a m echanism of water oxidation and oxygen release during photosynthesis was pr oposed. It is suggested that the photolysis of water occurs in a dimeric ox ygen-evolving complex consisting of two core complexes. In the region of co ntact of these complexes, a hydrophobic <<boiler>> is formed where the cond itions for screening and stabilization of Z-linanded manganese cations accu mulating positive charges for the oxidation of water molecules are created. A prerequisite to the photolysis of water is the formation of a binuclear [Mn3+-OH ... HO-Mn3+] hydroxyl-manganese associate, which appears in the di meric oxygen-evolving complex after the first two light flashes as a result of photohydrolysis of photochemically oxidized Z-liganded manganese cation s, The process is accompanied by the release of the first water protons to the medium. The photosynthetic oxidation of water hydroxyls occurs at the n ext stage and is considered as synchronous detachment of four electrons fro m two bound OH-groups of the associate upon photooxidation of Mn3+ cations to Mn4+ cations after two subsequent light flashes, This process is accompa nied by the disproportionation of electron density and the formation of a b ond between oxygen atoms of hydroxyls followed by the evolution of molecula r oxygen and protons, and regeneration of two starting Mn2+ cations and the primary state of the system.