Resonance Raman spectroscopy on [NiFe] hydrogenase provides structural insights into catalytic intermediates and reactions.

[NiFe] hydrogenases catalyze the reversible cleavage of hydrogen and, thus, represent model systems for the investigation and exploitation of emission-free energy conversion processes. Valuable information on the underlying molecular mechanisms can be obtained by spectroscopic techniques that monitor individual catalytic intermediates. Here, we employed resonance Raman spectroscopy and extended it to the entire binuclear active site of an oxygen-tolerant [NiFe] hydrogenase by probing the metal-ligand modes of both the Fe and, for the first time, the Ni ion. Supported by theoretical methods, this approach allowed for monitoring H-transfer from the active site and revealed novel insights into the so far unknown structure and electronic configuration of the hydrogen-binding intermediate of the catalytic cycle, thereby providing key information about catalytic intermediates and reactions of biological hydrogen activation.