Models of the photosynthetic oxygen-evolving centerand electronic structure of Mn clusters

It is known that the oxidation of water to O$_2$ in green plants is associated with a tetramanganese complex of the photosystem II (PSII) protein-cofactor complex but the exact structure of the oxygen-evolving center (OEC) remains unknown. The recent X-ray spectroscopic studies suggest that the OEC contains a cubane-like Mn$_3$CaO$_4$ center linked to a forth Mn by an oxo-bridge. Using ab-initio methods we carry out geometry optimizations for a few models of the OEC and study their electronic properties. We consider the cubane-like Mn$_3$CaO$_4$ center, a funnel-like Mn4 core with Ca ligand and a synthetic Mn$_4$O$_6$ core structure. Possibilities for binding sites and eventual reaction paths of the water splitting are explored. Manganese clusters are recognized also as single-molecule magnets. Therefore we investigate spin properties of ground and excited states of these clusters. High spin ground states of Mn$_2$O$_2$ and Mn$_4$ compared to a low spin ground state of Mn$_4$O$_4$ illustrate a competition between ferromagnetic and antiferromagnetic ordering.