Spectroscopic Definition of Electronic Structure and Contributions to Reactivity in Binuclear Non-Heme Iron and Manganese Enzymes

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SolomonMCB 98160511. TechnicalBinuclear non-heme iron active sites are present in a wide variety of proteins and enzymes, which perform different biological functions.These include oxygen binding, activation for hydroxylation, oxidation, desaturation and phosphate ester hydrolysis. All react with dioxygen from the binuclear ferrous state except the purple acid phosphatases, which are active at the mixed valent level. Crystal structures are now available for at least one oxidation state of each enzyme and define structural differences, which could relate to differences in reactivity. Peroxo and high valent oxo intermediates have been trapped for several of these enzymes and a number of relevant model systems are now available. Binuclear manganese sites are present in manganese catalase and manganese ribonucleotide reductase which are thought to have similar m-oxo, m-carboxylato bridged structures to the binuclear non-heme iron enzymes, and a Mn202 unit appears to be present in the tetra manganese cluster of the OEC. An excited state spectroscopic approach employing absorption, circular dichroism, magnetic circular dichroism and resonance Raman spectroscopies to define the excited states, variable temperature variable field magnetic circular dichroism analyzed using an exchange plus zero field splitting Hamiltonian to probe the ground state and electronic structure calculations is being utilized to study these proteins and relevant model complexes. These studies are directed toward obtaining detailed insight into: electronic and geometric structural contributions to the reactivity of these sites, geometric and electronic structural differences which relate to differences in function, the electronic structure of oxygen intermediates which promote the formation of high valent bridging oxo intermediates and the properties of these intermediates which contribute to reactivity, the relation of the binuclear manganese enzymes to the binuclear non-heme iron enzymes, and the properties of the Mn202 unit and interactions between these units which promote 0-0 bond formation.2 Non-TechnicalBinuclear non-heme iron active sites are present in a wide variety of proteins and enzymes, which perform different biological functions. These include oxygen binding, activation for hydroxylation, oxidation, desaturation and phosphate ester hydrolysis. Binuclear mangnese sites are present in enzymes which are involved in peroxide disproportionation and ribonucleotide reduction, and are thought to have similar structures to the binuclear non-heme iron enzymes. A related tetranuclear mangnese cluster is present in the oxygen-evolving center in photosynthesis. Detailed chemical and spectroscopic studies of these protein active sites, intermediates in their reactions with dioxygen and related small molecule inorganic complexes which model these binuclear sites are presented. These studies should contribute significantly toward defining general structure/function correlations for dioxygen binding and activation by binuclear non-heme iron enzymes, their relation to binuclear manganese enzymes and factors which contribute to dioxygen activation versus 0-O bond formation. The enzymes studied are important in catalytic processes (alkane oxidation technology, etc.), biotechnology (oxidoreductases, etc.), environmental regulation (CH4, etc.), oxygen evolution, detoxification, and DNA and fatty acid biosynthesis. Further, the spectroscopic methodology developed as part of this study has applications in understanding the structures and reactivities of a variety of important classes of metal sites in biology and catalysis.