Molecular Models for H2-Activating Metalloenzymes

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This award in the Inorganic, Bioinorganic, and Organometallic Chemistry Program supports research on metalloenzymes by Dr. Marcetta Darensbourg of the Chemistry Department, Texas A&M University. The focus of the investigation is the reaction chemistry of (NiFe)hydrogenase, an enzyme that promotes the formation, fixation, and sensing of dihydrogen. The active site of the Desulfovibrio gigas hydrogenase is heterobinuclear (Fe, Ni) and the iron center appears to be associated with cyano and carbonyl groups. In this work, the known organometallic anions CpFe(CN)2(CO) where Fe = Fe(I), Fe(II) will be studied for their ability to signal electron density changes at Cp substitutents caused by hydrogen-bonding or solvent interaction. Vibrational spectroscopy will be used to monitor the electron density changes accompanying redox and enzyme activity levels. In addition, newly designed dithiolate ligands will be used to coordinate nickel complexes to the iron unit which may serve as a structural model for the enzyme. The hypothesis concerning the catalysis process is that either the iron unit provides stabilization via an electron-withdrawing iron site that mediates charge through the thiolates, thus easing accessibility to catalytically active Ni(I); or the dihydrogen production catalysis involves the Fe(II) site, with the nickel dithiolate serving as a bidentate ligand that is electronically tunable by changes in redox state or ligation. Hydrogenase enzymes perform important biological functions that provide energy to cells. These enzymes have been found to contain two metals, iron and nickel, at the active reaction site, but how these metals cooperate in the reaction is unknown. Many past studies have investigated the role of nickel, but this research will elucidate the role of iron, and of the iron-nickel combination. The results may also be relevant to the functioning of mixed metal industrial catalysts whose functioning is not well understood.