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3727562 
Journal Article 
Water Deprotonation via Oxo-Bridge Hydroxylation and O-18-Exchange in Free Tetra-Manganese Oxide Clusters 
Lang, SM; Fleischer, I; Bernhardt, TM; Barnett, RN; Landman, Uzi 
2015 
Yes 
Journal of Physical Chemistry C
ISSN: 1932-7447
EISSN: 1932-7455 
119 
20 
10881-10887 
WOS:000355158400012 
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 (13511) 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 (D2O)-O-16 and (H2O)-O-18 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 O-2 in photosystem II, thus supporting the involvement of bridging oxygen atoms in this process. First-principles spin density functional theory calculations reveal the molecular mechanism of the water deprotonation and oxo-bridge exchange.