Theoretical investigation on peripheral ligands of oxygen-evolving center in photosystem II

The interaction between the Mn-cluster and its peripheral ligands in oxygen-evolving center is still unclear. Theoretical investigation on the coordination of histidine, H 2 O, and CI to Mn2O2 units in OEC is conducted. The following conclusions are obtained: (i) both histidine and H 2 O molecule, bound to the two Mn ions, respectively, are vertical to the Mn2O2 plane, and maintain a large distance; (ii) the two H 2 O molecules cannot bind to the same Mn2O2 unit. Based on Mn-cluster structure in OEC, we theoretically predict that two H 2 O molecules bind to the two Mn ions at the “C”-shaped open end in S 0 state, while two His residues at the closed end. Cl ion can only terminally ligate at the open end. Individual valence for the four Mn ions in S 0 state is assigned.