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 <i>Clostridium pasteurianum</i> 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 <sup>13</sup>CO instead of <sup>12</sup>CO. The new assignment is also consistent with the observed change from rhombic to axial symmetry of the electron paramagnetic resonance <b>g</b> tensor upon inactivation.