NRVS Spectroscopy Resolves Distinct Bridging Hydride Intermediates in [NiFe]-Hydrogenase

The active-site iron of an H₂-sensing [NiFe]-hydrogenase was selectively labeled with ⁵⁷Fe, allowing the probing of all catalytic intermediates using synchrotron-based nuclear resonance vibrational spectroscopy. Diagnostic metal-hydride vibrations were detected for both the Ni_a-C and Ni_a-SR intermediates, with their assignments being confirmed through H/D isotope substitution and in situ hydride photolysis experiments. Interestingly, these Fe-hydride bands are separated by a large energy gap, reflecting distinct bonding interactions at the metal-hydride site. Despite these differences, vibrational analyses across all catalytically active species reveal a conserved structural rigidity of the [NiFe] center, which appears crucial for sustaining efficient and rapid electron transfer in [NiFe]-hydrogenases.