Resolving the CO/CN ligand arrangement in CO-inactivated [FeFe] hydrogenase by first principles density functional theory calculations.

The currently presumed assignment of CO/CN ligands in the structure of the active cluster in CO-inactivated [FeFe] hydrogenase is shown to be inconsistent with the available IR data in the enzyme from Clostridium pasteurianum I. A different arrangement has the correct qualitative and quantitative features, reproducing the observed line spacing and intensities and the observed line shift consequent to inactivation with labeled 13CO instead of 12CO. The new assignment is also consistent with the observed change from rhombic to axial symmetry of the electron paramagnetic resonance g tensor upon inactivation.