The oxygen-evolving complex (OEC) of Photosystem II (PSII) is a unique Mn4O5Ca2+ cluster that catalyzes the photoactivated water-splitting reaction. It is a model for bio-inspired systems that use solar energy to extract electrons from water and produce fuel. The OEC is oxidized 4 times via electron transfer to P680+, the primary electron donor in PSII, to generate a high valence state that oxidizes water.  The influence of the surrounding protein on the redox and protonation chemistry of the OEC remains an open question.  Here, we present a classical valence model of the OEC that allows the redox state of each Mn and protonation state of bridging and terminal waters to remain in equilibrium with the entire PSII in each S-state. The protons released in response to OEC oxidation are identified.  The last bridging oxygen loses its proton forming S1.  In S2, a proton lost from a terminal water is trapped by the nearby D61, diminishing the proton release to the lumen in agreement with experiment.  In S3, a proton is lost from H337 or D61. The S0/S1 Em is 0.9 V while it is 1.3 V for S1/S2 (vs. SHE), showing that these are within range to be oxidized by P680+.  The trapped proton in the S2 state contributes to the higher S1/S2, Em.  The S3/S2 Em is calculated to be 1.6 V, indicating that an additional ligand is needed to complete the coordination shell of Mn1 before it is oxidized. The S-state cycle is also calculated in chloride-depleted PSII to investigate the requirement for this ion in O2 production.

Proton-coupled electron transfer during the S-state transitions of the oxygen-evolving complex of photosystem II

248th National Meeting (2014)

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