MODELS OF THE PHOTOSYNTHETIC WATER OXIDATION ENZYME

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The most important biological role yet recognized for the metal manganese (Mn) is the Mn enzyme which catalyzes water oxidation to dioxygen (O2) within the photosynthetic apparatus of higher plants and algae. Elucidating the structure and mechanism of action of this system represents an area of intense current activity within the biochemical community. To allow detailed information on the Mn site to be obtained at an atomic level, the availability would be highly desirable of small molecular weight Mn complexes to act as models of the native unit. Such models would be invaluable to assist in epr and EXAFS studies of the enzyme, and to allow mechanistic detail to be obtained, such as possible modes of substrate binding and mechanism of subsequent substrate oxidation. No satisfactory models have been available, however, and their synthesis and study represents the primary focus of this research program. Preliminary results described how the first Mn complexes of significant relevance to the enzyme have now been synthesized. These species already display several features in accord with known properties and structural parameters available on the enzyme. EXAFS, epr and solid-state magnetic studies are proposed to further define the correspondence between available synthetic species and the native Mn site. Synthetic extensions to the program will seek to modify the structures, ligation type and oxidation levels of attainable materials to allow broader and more detailed comparisons. Availability of such materials also allows the possibility of investigating substrate binding, reproducing the water oxidation reaction under laboratory conditions, and elucidating the mechanistic detail of and intermediates during the catalytic cycle. The proposed program thus represents an essential and hitherto unavailable component of the ongoing multi-disciplinary investigation of this important biological system.