Relative stability of the S isomers of the oxygen evolving complex of photosystem II.

The oxidation of water to O is catalyzed by the Oxygen Evolving Complex (OEC), a MnCaO complex in Photosystem II (PSII). The OEC is sequentially oxidized from state S to S. The S state, (Mn)(Mn), coexists in two redox isomers: S, where Mn4 is Mn and S, where Mn1 is Mn.

Proton-Coupled Electron Transfer During the S-State Transitions of the Oxygen-Evolving Complex of Photosystem II.

The oxygen-evolving complex (OEC) of photosystem II (PSII) is a unique Mn4O5Ca cluster that catalyzes water oxidation via four photoactivated electron transfer steps. As the protein influence on the redox and protonation chemistry of the OEC remains an open question, we present a classical valence model of the OEC that allows the redox state of each Mn and the protonation state of bridging μ-oxos and terminal waters to remain in equilibrium with the PSII protein throughout the redox cycle. We find that the last bridging oxygen loses its proton during the transition from S0 to S1.