The Mechanism and Quantum-Chemical Modeling of the Photosynthetic Water Oxidation and Oxygen Formation Reaction

The oxygen-evolving complex (OEC) of chloroplast membranes consists of two monomeric pigment-lipoprotein complexes of photosystem 2 (PLPC PS-2) associated according to the mirror symmetry rule. The mechanism of photosynthetic water molecule oxidation and oxygen formation has been developed based on the study on the regularities of its functioning [1]. It is based on the concept that the two-anode reactor is formed in the structure of the water-oxidizing center (WC) of the dimeric complex, which is determined by the symmetry in the location of unidentant ligands (Tyr(Z)) coordinated to Mn cations performing water molecule oxidation. The conditions necessary for the reaction to take place are assigned by the identity of the molecular environment of Mn cations and determined by the structure of analogous D1 proteins of the PLPC PS-2 and by the stabilization of electron vacancies on the photo-oxidized Mn cations in the structure of the hydrophobic boiler formed simultaneously. Taking these data into consideration, we have conducted the quantum-chemical modeling of the reaction process that has confirmed the developed mechanism of water oxidation and molecular oxygen formation. The conditions for the formation of the two-anode reactor of the water oxidation system that exist in the monomeric complex PS2, the structure of which is determined by D1 and D2 proteins homologous to each other, are the same as in the dimeric OEC. However, obviously, the need to increase the ability to absorb light energy for survival arose in plant organisms in the evolutional process with the appearance of the biosphere. This determined the appearance and predominant functioning of the dimeric oxygen-evolving complex of chloroplast membranes, for which the more efficient use of light energy is typical.