A Synthetic Model of the Mn3Ca Subsite of the Oxygen-Evolving Complex in Photosystem II

A model compound sheds light on the puzzling role of calcium in the metal cluster that oxidizes water during photosynthesis. Within photosynthetic organisms, the oxygen-evolving complex (OEC) of photosystem II generates dioxygen from water using a catalytic Mn4CaOn cluster (n varies with the mechanism and nature of the intermediate). We report here the rational synthesis of a [Mn3CaO4]6+ cubane that structurally models the trimanganese-calcium–cubane subsite of the OEC. Structural and electrochemical comparison between Mn3CaO4 and a related Mn4O4 cubane alongside characterization of an intermediate calcium-manganese multinuclear complex reveals potential roles of calcium in facilitating high oxidation states at manganese and in the assembly of the biological cluster.

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