Large-scale QM/MM calculations of the CaMn 4 O 5 cluster in the S 3 state of the oxygen evolving complex of photosystem II. Comparison between water-inserted and no water-inserted structures.

Large-scale QM/MM calculations were performed to elucidate an optimized geometrical structure of a CaMn4 O5 cluster with and without water insertion in the S3 state of the oxygen evolving complex (OEC) of photosystem II (PSII). The left (L)-opened structure was found to be stable under the assumption of no hydroxide anion insertion in the S3 state, whereas the right (R)-opened structure became more stable if one water molecule is inserted to the Mn4 Ca cluster. The optimized Mna(4) -Mnd(1) distance determined by QM/MM was about 5.0 Å for the S3 structure without an inserted hydroxide anion, but this is elongated by 0.2-0.3 Å after insertion. These computational results are discussed in relation to the possible mechanisms of O-O bond formation in water oxidation by the OEC of PSII.