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

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.