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Abstract

Hydrogenase proteins catalyze the reversible conversion of molecular hydrogen to protons and electrons. While many enzymatic states of the [NiFe] hydrogenase have been studied extensively, there are multiple catalytically relevant EPR-silent states that remain poorly characterized. Analysis of model compounds using new spectroscopic techniques can provide a framework for the study of these elusive states within the protein. We obtained optical absorption and resonance Raman (RR) spectra of (dppe)Ni(μ-pdt)Fe(CO)₃ and [(dppe)Ni(μ-pdt)(μ-H)Fe(CO)₃][BF₄], which are structural and functional model compounds for the EPR-silent Ni–SI and Ni–R states of the [NiFe] hydrogenase active site. The studies presented here use RR spectroscopy to probe vibrational modes of the active site, including metal–hydride stretching vibrations along with bridging ligand–metal and Fe–CO bending vibrations, with isotopic substitution used to identify key metal–hydride modes. The metal–hydride vibrations are essentially uncoupled and represent isolated, localized stretching modes; the iron–hydride vibration occurs at 1530 cm^–1, while the nickel–hydride vibration is observed at 945 cm^–1. The significant discrepancy between the metal–hydride vibrational frequencies reflects the slight asymmetry in the metal–hydride bond lengths. Additionally, time-dependent density functional theory (TD-DFT) calculations were carried out to obtain theoretical RR spectra of these compounds. On the basis of the detailed comparison of theory and experiment, the dominant electronic transitions and significant normal modes probed in the RR experiments were assigned; the primary transitions in the visible wavelengths represent metal-to-metal and metal-to-ligand charge transfer bands. Inherent properties of metal–hydride vibrational modes in resonance Raman spectra and DFT calculations are discussed together with the prospects of observing such vibrational modes in metal–hydride-containing proteins. Such a combined theoretical and experimental approach may be valuable for characterization of analogous redox states in the [NiFe] hydrogenases.