MODELS OF THE PHOTOSYNTHETIC WATER OXIDATION CENTER

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DESCRIPTION: Water oxidation within the photosynthetic apparatus of plants and cyanobacteria represents a source of protons and electrons for CO2 fixation/carbohydrate synthesis for healthy growth. In addition, the evolved oxygen (O2) by-product represents the source of this gas in the atmosphere, and thus this reaction also impacts the health and maintenance of all respiring life on this planet. The water oxidation complex (WOC) is a metallo-unit containing four manganese atoms bridged by oxide (O2-) ions and held to the host polypeptides primarily by carboxylate functionalities of amino acid side-chains. The elucidation of the nature and mechanism of action of the WOC represents one of the most important and toughest current challenges within Metallobiochemistry. To facilitate the attainment of this objective, the proposed work involves the synthesis of the WOC Mn/O/RCO2 cluster in the laboratory rather than its isolation from plants. This Synthetic Analogue Approach has the advantage that larger amounts can be obtained than are normally available from plant sources, and peripheral ligation can be small organic carboxylates rather than polypeptides. Thus, a wide variety of spectroscopic and physicochemical techniques can be employed providing a greater amount of data in a shorter time frame and to a higher degree of precision than is possible for the native site. Previous work has provided synthetic entry into the desired type of tetranuclear Mn/O/RCO2 complexes, and the proposed work will extend the study of structural and other properties, followed by comparisons and contrasts with the multiple oxidation states of the native WOC; this is designed, inter alia, to facilitate interpretation of several unusual properties of the WOC. Further, synthetic analogues in high oxidation states corresponding to those of the WOC states preceding O2 evolution will be electrochemically generated and investigated for their ability to evolve O2, successful attainment of the latter reaction will provide a system that would be amenable to subsequent detailed kinetic and mechanistic study, providing fundamental information on the mechanism of oxidative coupling of two H2O molecules to O2 and the identify of intermediates, e.g., metal-bound peroxide groups.