Calculated vibrational frequencies for FeMo-co, the active site of nitrogenase, bearing hydrogen atoms and carbon monoxide

The intramolecular hydrogenation paradigm for the reducing actions of the enzyme nitrogenase postulates that the iron-molybdenum cofactor (FeMo-co, Fe7MoS9N(homocitrate)) as active site contains H atoms bound to Fe and S during the catalytic cycle, and that these H atoms are the reducing agents. The reduction of N-2 and of all other non-physiological substrates is strongly inhibited by carbon monoxide, except for the formation of H-2 from protons. It has been recently reported that vanadium nitrogenase and modified molybdenum nitrogenase reduce CO to hydrocarbons. Therefore many questions now arise about relationships between CO and H on the nitrogenase cofactors. In order to assist the interpretation of kinetic infrared spectral data, vibrational frequencies and modes have been calculated for a variety of possible structures in which FeMo-co bears H atoms, or CO ligands, or both. Fe-H stretching frequencies occur in the same spectral window as the C-O stretching frequencies, with lesser intensity, and both stretches are strongly coupled in some structures. Symmetrical bridging of CO between two Fe atoms of FeMo-co is destabilised by the presence of other ligands on Fe, and the reason for this is evident. Two results for bound formyl, HCO, are reported. These calculations of reference structures allow some interpretation of existing experimental spectra, but, more significantly, they suggest further kinetic infrared experiments to elucidate the chemical mechanism of catalysis by nitrogenase under normal turnover conditions.