Stepwise isotope editing of [FeFe]-hydrogenases exposes cofactor dynamics

The six-iron cofactor of [FeFe]-hydrogenases (H-cluster) is the most efficient H₂-forming catalyst in nature. It comprises a diiron active site with three carbonmonoxide (CO) and two cyanide (CN⁻) ligands in the active oxidized state (Hₒₓ) and one additional CO ligand in the inhibited state (Hₒₓ-CO). The diatomic ligands are sensitive reporter groups for structural changes of the cofactor. Their vibrational dynamics were monitored by real-time attenuated total reflection Fourier-transform infrared spectroscopy. Combination of 13CO gas exposure, blue or red light irradiation, and controlled hydration of three different [FeFe]-hydrogenase proteins produced 8 Hₒₓ and 16 Hₒₓ-CO species with all possible isotopic exchange patterns. Extensive density functional theory calculations revealed the vibrational mode couplings of the carbonyl ligands and uniquely assigned each infrared spectrum to a specific labeling pattern. For Hₒₓ-CO, agreement between experimental and calculated infrared frequencies improved by up to one order of magnitude for an apical CN⁻ at the distal iron ion of the cofactor as opposed to an apical CO. For Hₒₓ, two equally probable isomers with partially rotated ligands were suggested. Interconversion between these structures implies dynamic ligand reorientation at the H-cluster. Our experimental protocol for site-selective 13CO isotope editing combined with computational species assignment opens new perspectives for characterization of functional intermediates in the catalytic cycle.