The recognition of manganese as an important element in biological systems has increased markedly in the past 10 years. Manganese is especially important in biological oxygen metabolism as demonstrated by its occurrence in a superoxide dismutase, """"""""peudo"""""""" catalase and the photosynthetic oxygen evolving complex (OEC). Available spectroscopic data on the manganese centers in the oxygen evolving complex of photosystem II and the catalase isolated from L. plantarum are growing at a rapid rate. In contrast, structural and mechanistic synthetic models useful toward characterizing manganoenzyme active site structure are rare. The goal of this proposal is to synthesize novel mono- and multinuclear maganese complexes that may be used to elucidate the active site structure and electronic properties of manganoenzymes. These complexes will also be used to test mechanistic proposals for enzymatic catalysis. Specific emphasis will be placed on enzymes which may contain multinuclear center (OEC and """"""""pseudo"""""""" catalase). The following information will be used as a basis for model studies. First, the stoichiometry must range between one and four for the OEC and either one or two for the pseudocatalase. Particular emphasis will be placed on binuclear complexes, although tri- and tetranuclear materials may still be explored. Second, moderately high oxidation state clusters (Mn(III) or Mn(IV)) will be synthesized as both enzymes may contain these oxidation levels. Third, ligands that mimic biologically occurring functional groups such as phenolates, carboxylates and alkoxides shall be employed. Fourth, binucleating agents which not only increase complex stability through the chelate effect, but also present a constant ligand environment with which to study progressive metal centered oxidations will be examined. Thus, a single complex may begin to mimic progressions through sequential enzyme oxidation states. Each compound will be characterized by physical techniques such as EPR, IR and UV-VIS spectroscopies, variable temperature magnetic susceptibilities, x-ray absorption spectroscopy, cyclic voltammetry and, when possible, x-ray crystallography. All Mn(III) and Mn(IV) complexes will be evaluated for their ability to produce dioxygen when subjected to broad band tungsten lamp illumination. Those complexes which show activity will then be subjected to kinetic analysis to establish a chemical mechanisms for the process. This chemistry will then be compared to mechanistic proposals for biological water splitting reactions.
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