Better H2 processing catalysts from earth-abundant materials are a necessary component for the effort to free us from carbon-based fuels. [NiFe] H2ases have attracted attention in part because these enzymes are less sensitive to O2 than the [FeFe] H2ases, while still having good catalytic activity. They contain an active site with Ni and Fe bridged by cysteine ligands. Our initial soft x-ray studies of NiFe H2ases suggested that the reduced Ni-R species contained high-spin Ni(II). Over the years, this result has variously agreed or been in conflict with reported DFT calculations. We are currently working to record new soft x-ray XMCD data to check the reproducibility of this result.In the mean time, we have obtained direct evidence for a hydride bridge in the Ni-R form of H2ase using nuclear resonance vibrational spectroscopy (NRVS). We observe mixing of Fe-H motion with Fe-CO & Fe-CN modes as well as a unique 'wagging' mode involving H motion perpendicular to the initial NiHFe plane. The DFT calculations that best match the NRVS data assume a high-spin Ni(II). Hydride binding to the H2ase NiFe active site is asymmetric, with the stronger interaction with the Fe side of the metal pair. The predicted asymmetry is strongly dependent on the spin state of the Ni(II) ion.The issue of Ni spin state in Ni enzymes has received relatively little attention. A spin state transition at Ni(II) moves the hydride by ~0.2 �. Although seemingly trivial, this distance change could have a profound effect on the proton transfer rate. Since the high-spin and low-spin Ni(II) are close in energy and modulated by ligand coordination geometry, this provides a mechanism for slight changes in protein conformation to have dramatically modulate the catalytic activity of the NiFe site.

Influence of Ni spin state on the properties of NiFe hydrogenase: Exploration via soft X-ray absorption, NRVS, and DFT calculations

248th National Meeting (2014)

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