Influence of the heme pocket conformation on the structure and vibrations of the Fe-CO bond in myoglobin: A QM/MM density functional study

The influence of the distal pocket conformation on the structure and vibrations of the heme-CO bond in carbonmonoxy myoglobin (MbCO) is investigated by means of hybrid QM/MM calculations based on density functional theory combined with a classical force field. It is shown that the heme-CO structure (QM treated) is quite rigid and not influenced by the distal pocket conformation (MM treated). This excludes any relation between FeCO distortions and the different CO absorptions observed in the infrared spectra of MbCO (A states). In contrast, both the CO stretch frequency and the strength of the CO . . . His64 interaction are very dependent on the orientation and tautomerization state of His64. Our calculations indicate that the CO . . .N-epsilon type of approach does not contribute to the A states, whereas the CO . . .H-N-epsilon interaction is the origin of the A(1) and A(3) states, the His64 residue being protonated at N-epsilon. The strength of the CO . . . His64 interaction is quantified, in comparison with the analogous O-2. . . His64 interaction and with the observed changes in the CO stretch frequency. Additional aspects of the CO . . . His64 interaction and its biological implications are discussed.