| Résumé | CO, NO and O2 are physiologically important molecules whose biochemistry is largely mediated by transient binding to transition metal ions, especially to Fe(II) in the heme prosthetic group. The mechanism of action is determined by interactions of the FeXO unit (X=C, N or O) with protein residues in the heme pocket. These interactions can be assessed via the FeXO vibrational frequencies, which are available from infrared (IR) and resonance Raman (RR) spectra. The vibrational frequencies are largely determined by the donor properties of the proximal ligand, and by the electrostatic field of the residues on the distal side of the bound XO. This field polarizes the FeXO unit and modulates backbonding from Fe to XO, resulting in anticorrelated shifts in the XO and FeX stretching frequencies. Empirically derived trends in the vibrational frequencies are supported by ab initio calculations using the density functional theory (DFT). DFT analysis also provides the correct geometries of the unconstrained FeXO units, and has been used to investigate the energy required for distortion from the equilibrium geometry. This energy is modest, even for the linear FeCO unit, indicating that steric forces from distal protein residues are not major determinants of the ligand binding energy. DFT analysis has also illuminated the contentious issue of the extent of FeCO distortion in myoglobin. While X-ray crystallographic analyses have yielded a range of geometries, the vibrational data are all consistent with an undistorted upright structure. However, DFT analysis of the sensitivity of IR polarization measurements, and of the νCO/νFeC backbonding correlation, indicates that the data allow up to 0.5 Å off-axis displacement of the O atom in the FeCO unit. The energy cost of this much displacement is small, ca. 0.8 kcal mol−1, a value that is consistent with binding energy measurements on myoglobin mutants. |
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