New structure model of oxygen-evolving center and mechanism for oxygen evolution in photosynthesis

Abstract

So far, many important questions and problems concerning the structure and mechanism of photosynthetic oxygen evolution are still unsolved. On the basis of recent achievements in this field, a new structure model is proposed whereby two H2O molecules bind asymmetrically to two manganese ions (Mn 1 II and Mn 4 III ) at the open end of “C” shaped cluster and keep rather large distance. Two histidine residues coordinate to the other two manganese ions in higher oxidation state (Mn 2 IV and Mn 3 IV ) through their nitrogen atoms of the imidazole. CI bound as terminal ligand to Mn 4 III is connected to Ca, and the latter is needed to maintain the special configuration of two Mn2O2 units by bridged-oxo and bridged-carboxylate ligands. The whole structure of oxygen evolution center is asymmetry. A new mechanism for oxygen evolution invokes predictions of asymmetric oxidation of two H2O molecules, dynamic structural changes of oxygen evolving center and indirect proton transport, etc. Only in S2 state, could Mn 1 IV = O, intermediate with high oxidation potential be formed. The S2→ S3 process occurs with significant structural changes, as well as intramolecular and intermolecular hydrogen transfer. The S3 state corresponds to intermediate of Mn 1 IV -O … H … O-Mn 4 IV . During S3 → [S4] → S0, the 0-0 bond is formed only in S4 state. The change of nucleophilic interaction between Cl and manganese ions different oxidation states has consequence for the significant structural changes in H2O oxidation process.