Theoretical study on OH− site and electronic spin state of oxygen-evolving complex in photosystem II at the dark S1 state

Possible protonation and electronic-spin states of oxygen-evolving complex (OEC) in photosystem II have been investigated by using QM(DFT-UB3LYP)/MM(Amber) calculation, in order to elucidate which OEC state satisfies the known experimental results at the dark stable state (S1), i.e. OEC involves Mn4(III2,IV4)-cluster and a S=0 state as the lowest energy electronic-spin state at S1. The configuration of Mn oxidation numbers and the lowest energy spin state within the Mn4-cluster depend on the protonation state of one oxo-anion bridging three Mn ions. When all water-ligands and oxo-bridges form H2O and O2−, respectively, the resulting OEC model involved Mn4(III2,IV2)-cluster and one S=0 state as the lowest energy spin state. To rationalize the O2−-bridge model at S1 state, a new H+-release scheme during the H2O-splitting reaction is proposed