Possible mechanisms for the O-O bond formation in oxygen evolution reaction at the CaMn4O5(H2O)4 cluster of PSII refined to 1.9Â Ã X-ray resolution

► UB3LYP calculations of CaMn4O5 cluster were performed. ► We proposed possible mechanisms for the O-O bond formation in OEC of PSII. ► The nucleophilic attack of hydroxide to O(56) and/or O(57) sites can be possible.