S0-State Model of the Oxygen-Evolving Complex of Photosystem II

The S-0 -> S-1 transition of the oxygen-evolving complex (OEC) of photosystem II is one of the least understood steps in the Kok cycle of water splitting. We introduce a quantum mechanics/molecular mechanics (QM/MM) model of the S-0 state that is consistent with extended X-ray absorption fine structure spectroscopy and X-ray diffraction data. In conjunction with the QM/MM model of the S-1 state, we address the proton-coupled electron-transfer (PCET) process that occurs during the S-0 -> S-1 transition, where oxidation of a Mn center and deprotonation of a mu-oxo bridge lead to a significant rearrangement in the OEC. A hydrogen bonding network, linking the D1-D61 residue to a Mn-bound water molecule, is proposed to facilitate the PCET mechanism.