Abstract

One of the fundamental biological reactions, the catalytically activated water-splitting, takes place at an inorganic tetra-manganese monocalcium penta-oxygen (Mn4CaO5) cluster which together with its protein ligands forms the oxygen evolution complex (OEC) of the membrane-bound pigment–protein photosystem II (PSII) of plants, algae, and cyanobacteria. In the first step of a new hierarchical approach to probe fundamental concepts of the water-splitting reactions, we present the gas-phase preparation of an isolated tetra-manganese oxide cluster ion, Mn4O4+, as a simplified model of the OEC. Reactivity studies with D216O and H218O in a gas-phase ion trap experiment reveal the exchange of the oxygen atoms of the cluster with water oxygen atoms. This provides direct experimental evidence for the ability of Mn4O4+ to dissociate water via hydroxylation of the oxo-bridges. The rate of oxygen exchange in the free cluster agrees well with the conversion rate of substrate water to O2 in photosystem II, thus support...

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