An electrostatic model for the frequency shifts in the carbonmonoxy stretching band of myoglobin: correlation of hydrogen bonding and the stark tuning rate.

The effect of internal and applied external electric fields on the vibrational stretching frequency for bound CO (nu(CO)) in myoglobin mutants was studied using density functional theory. Geometry optimization and frequency calculations were carried out for an imidazole-iron-porphine-carbonmonoxy adduct with various small molecule hydrogen-bonding groups. Over 70 vibrational frequency calculations of different model geometries and hydrogen-bonding groups were compared to derive overall trends in the C-O stretching frequency (nu(CO)) in terms of the C-O bond length and Mulliken charge. Simple linear functions were derived to predict the Stark tuning rate using an approach analogous to the vibronic theory of activation.(1) Potential energy calculations show that the strongest interaction occurs for C-H or N-H hydrogen bonding nearly perpendicular to the Fe-C-O bond axis. The calculated frequencies are compared to the structural data available from 18 myoglobin crystal structures, supporting the hypothesis that the vast majority of hydrogen-bonding interactions with CO occur from the side, rather than the end, of the bound CO ligand. The nu(CO) frequency shifts agree well with experimental frequency shifts for multiple bands, known as A states, and site-directed mutations in the distal pocket of myoglobin. The model calculations quantitatively explain electrostatic effects in terms of specific hydrogen-bonding interactions with bound CO in heme proteins.