The major objective of this research is to elucidate the chemical and physical properties of ligand-bridged bimetallic centers in proteins and related model compounds. Specific attention will focus on the design, synthesis, and characterization of small molecule analogs of the bimetallic cores in iron and copper containing proteins. Three major classes of complexes will be studied. The first are diiron (Fe2) compounds having a Mu-oxo and two bridging carboxylate ligands, congruent with the known structure of the dioxygen (O2) carrying marine invertebrate respiratory protein hemerythrin. Diiron centers are also known or postulated to exist in at least three other important proteins including 3-deoxy-D-arabino-heptulosonate 7-phosphate (DAHP) synthase, the first enzyme involved in the common aromatic biosynthetic pathway in bacteria and plants, ribonucleotide reductase, the enzyme responsible for catalyzing the first unique step in DNA synthesis, and the purple acid phosphatases. The second class are dicopper (Cu2) compounds incorporated into binucleating macrocyclic ligands and designed to promote reversible binding of dioxygen analogous to the binuclear copper centers in the arthropod and mollusc respiratory protein hemocyanin. Binuclear copper centers are ubiquitous in biology, occurring in tyrosinase, laccase, ceruloplasmin, and a variety of other enzymes that utilize dioxygen. Iron porphyrins with an attached chelating arm to coordinate copper comprise the third class of complex to be studied. These heterobimetallic complexes will serve as models for the ligand-bridged Fe/Cu center in cytochrome c oxidase, the terminal enzyme in the respiratory redox chain. All compounds will be characterized structurally by X-ray crystallography and ESR or NMR magnetic resonance methods. The magnetic, electronic, and vibrational spectroscopic properties will be measured and compared with those of the protein bimetallic cores to refine and calibrate existing structural assignments. The principles involved in the reversible binding of dioxygen by these ligand-bridged bimetallic centers will be investigated. Selected studies of the metalloproteins will include bulk magnetic susceptibility and, if technically feasible, 63Cu/65Cu NMR spectroscopic measurements. The project will provide fundamental knowledge about several bimetallic centers in biology responsible for the transport and utilization of dioxygen and for essential enzymatic processes.
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