Mechanism for photosynthetic O2 evolution.

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Abstract
We present a mechanism for photosynthetic O2 evolution based on a structural conversion of a Mn4O6 adamantane -like complex to a Mn4O4 cubane -like complex. EPR spectral data obtained from the S2 state of the O2-evolving complex are characteristic of a Mn4O4 cubane-like structure. Based on this structure for the manganese complex in the S2 state as well as a consideration of the other evidence available on the natural system and the coordination chemistry of manganese, structures are proposed for the five intermediate oxidation states of the manganese complex. A molecular mechanism for the formation of an O.sbd.O bond and the displacement of O2 from the S4 state is easily accommodated by the proposed model. The model is discussed in terms of recent EPR, x-ray, and UV spectral data obtained from the manganese site in the photosynthetic O2-evolving complex.