Synthesis and Characterization of a Model Nickel Superoxide Dismutase Metallopeptide Functionalized for Hydrogen Production

Abstract

Hydrogenases (H2ase) are naturally occuring enzymes that reversibly catalyze the oxidation and production of H2 from protons at low overpotential with very high catalytic activity. As a result these enzymes have gained increased attention over recent years as potential models for technological materials in the industrial production of hydrogen as a stock fuel. Despite the many attempts to replicate the properties of these enzymes synthetically, very few synthetic models have shown catalytic activity comparable to that of the native enzyme. This project seeks to utilize key features from the coordination environment selected by nature for the seemingly unrelated NiSOD (nickel-superoxide dismutase) metalloenzyme to produce a biological mimic of [NiFe]H2ase. A metallopeptide consisting of the 12 N-terminal residues from the NiSOD primary sequence, which has been extensively studied by the Shearer group, was used as the basis for constructing this mimic because it is a structurally and mechanistically welldefined system. Careful consideration of the requirements necessary for nickel coordination to the NiSOD apopeptide and the influence of structure on the reactivity of the nickel containing metallopeptide provided a guide for its modification resulting in properties that may be useful in the construction of an H2ase active metallopeptide. Substitution of the N-terminal histidine residue with the phosphine PTA (1,3,5-triaza-7-phosphatricyclo[3.3.1.1]decane) is proposed to optimize the active site of this enzyme toward H2 production while maintaining redox activity and stability. The synthesis and characterization by nuclear magnetic resonance, electronic absorption and X-ray absorption spectroscopy of this modified maquette are reported herein. It will be shown that this novel metallopeptide [Ni(H2aseM1)] contains NiII in square pyramidal environment with ligands derived from the PTA (P atom), Cys(2) and Cys(6) (S atoms), the Cys(2) amidate (N atom) and water (H2O). Further studies of this system will be aimed at understanding the functional properties of the metallopeptide as a hydrogenase.