S 1-state model of the O 2-evolving complex of photosystem II

Abstract

We introduce a quantum mechanics/molecular mechanics model of the oxygen-evolving complex of photosystem II in the S 1 Mn 4(IV,III,IV,III) state, where Ca 2+ is bridged to manganese centers by the carboxylate moieties of D170 and A344 on the basis of the new X-ray diffraction (XRD) model recently reported at 1.9 Å resolution. The model is also consistent with high-resolution spectroscopic data, including polarized extended X-ray absorption fine structure data of oriented single crystals. Our results provide refined intermetallic distances within the Mn cluster and suggest that the XRD model most likely corresponds to a mixture of oxidation states, including species more reduced than those observed in the catalytic cycle of water splitting. © 2011 American Chemical Society.