The long-term research objective is to synthesize and fully characterize new types of metal-thiolate complexes for the purpose of obtaining fundamental information about the structural, spectroscopic and reactivity properties of metal-cysteine interactions in proteins. It is proposed that synthetic analogs be acquired and studied as models for the nickel-sulfur and iron-sulfur centers in the hydrogenase (H2-ase) enzymes which catalyze the reversible formation and consumption of dihydrogen (H2) via the redox reaction: 2 H+ + 2 e-H2. Beyond being significant in terms of microbial metabolism, the area of H2 production and consumption from renewable resources is important in the realm of the global energy supply. Although substantial discoveries have been made regarding the biochemistry of the H2-ase enzymes, the exact nature o the active site is not understood. This is an area where fundamental inorganic coordination chemistry can contribute to the understanding of how metal-sulfur centers can catalyze the 2 H+ + 2 e- H2 transformation. It proposed that polydentate chelate ligands be used to prepare metal complexes that replicate the physical and reactivity properties of the metal centers in the H2-ase enzymes. Attention will be given to the synthesis and characterization of Ni complexes which are analogs for the detected forms of the Ni center (Ni-A; Ni-B; Ni-silent; Ni-C) as well as for Ni-hydrogen species that might occur as intermediates in the catalytic cycle. Plans are given which will allow for the synthesis of metal centers in a variety of biologically viable coordination structures and oxidation states [Ni(+n); n = 1,2,3,4]. Chemico-physical studies of the model compounds will be used to obtain a fundamental understanding of the relationship between the spectroscopic and structural and electronic properties of the metal centers. These techniques will include: X-ray crystallography, electrochemical, magnetic susceptibility, electronic, ESR, vibrational, NMR, EXAFS and Mossbauer measurements. Special consideration will be given to understanding the reactivity properties of the H2-ase enzymes. An understanding of the mechanism by which the metal center effects the oxidation of H2 and the reduction of H+ will be sought. It is proposed that the reactivity properties of the metal center can be tuned by modifying the electronic and structural characteristics of the metal-coordination environment. Substantial results have been acquired which suggest that these goals can be achieved. It is anticipated that this work will contribute to understanding the fundamental chemistry and redox-active processes in H2- ase as well as to the chemistry of other biologically relevant metal- cysteine centers.
