Understanding the structure and dynamics of hydrogenases by ultrafast and two-dimensional infrared spectroscopy.

Hydrogenases are valuable model enzymes for sustainable energy conversion approaches using H, but rational utilization of these base-metal biocatalysts requires a detailed understanding of the structure and dynamics of their complex active sites. The intrinsic CO and CN ligands of these metalloenzymes represent ideal chromophores for infrared (IR) spectroscopy, but structural and dynamic insight from conventional IR absorption experiments is limited. Here, we apply ultrafast and two-dimensional (2D) IR spectroscopic techniques, for the first time, to study hydrogenases in detail. Using an O-tolerant [NiFe] hydrogenase as a model system, we demonstrate that IR pump-probe spectroscopy can explore catalytically relevant ligand bonding by accessing high-lying vibrational states. This ultrafast technique also shows that the protein matrix is influential in vibrational relaxation, which may be relevant for energy dissipation from the active site during fast reaction steps. Further insights into the relevance of the active site environment are provided by 2D-IR spectroscopy, which reveals equilibrium dynamics and structural constraints imposed on the H-accepting intermediate of [NiFe] hydrogenases. Both techniques offer new strategies for uniquely identifying redox-structural states in complex catalytic mixtures vibrational quantum beats and 2D-IR off-diagonal peaks. Together, these findings considerably expand the scope of IR spectroscopy in hydrogenase research, and new perspectives for the characterization of these enzymes and other (bio-)organometallic targets are presented.