Kara BREN

Kara BREN

University of Rochester, USA

A Metallopeptide Functional Mimic of Redox Metalloenzymes

Abstract:

Metalloenzymes that catalyze multi-electron, multi-proton reactions often have complex structures and multiple redox-active metal ion cofactors. These metal ions facilitate bond-breaking and bond-making steps as well as substrate binding and electron transfer. Metalloenzymes also feature pathways for delivering protons to the active site at the appropriate pKa. In this work, we are mimicking the activity of complex redox metalloenzymes with a surprisingly simple metallopeptide, the water-soluble cobalt complex of Gly-Gly-His (CoGGH) [1]. CoGGH features a redox-active metal ion and a peptide ligand with protonatable sites (Figure 1). Upon subjecting CoGGH in aqueous buffer near neutral pH to bulk electrolysis at potentials below -1.0 V vs. SHE, hydrogen (H2) is evolved with a faradaic yield over 90% and a turnover number >1000. Remarkably, the faradaic yield is not significantly impacted by the presence of air, indicating selectivity for proton reduction over oxygen reduction. Cyclic voltammetry (CV) of CoGGH reveals a catalytic wave with a half-wave potential of -1.1 V vs. SHE, indicating a ~600 mV overpotential at pH 8 [2]. Further electrochemical studies have been performed to determine the observed pseudo-first-order rate constant (kobs) for H2 production catalyzed by CoGGH in the presence of a range of different buffers and at different pH values. The data reveal that buffer has a surprisingly large impact on kobs, with consequences for mechanism. Toward developing this catalyst for use in artificial photosynthesis, the ability of CoGGH to facilitate the storage of light energy as H2 is illustrated by pairing the metallopeptide with [Ru(bpy)3]2+ as a photosensitizer to yield a turnover number of ~1000 for H2 evolution with respect to catalyst. Finally, progress toward employing CoGGH as a catalyst for other multielectron, multiproton reactions of interest will be presented.

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