Stereoelectronic factors in CO, NO and O-2 binding to heme from vibrational spectroscopy and DFT analysis

CO, NO and O-2 are physiologically important molecules whose biochemistry i s largely mediated by transient binding to transition metal ions, especiall y to Fe(II) in the heme prosthetic croup. The mechanism of action is determ ined 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 vibrat ional frequencies, which are available from infrared (IR) and resonance Ram an (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 F eXO unit and modulates backbonding from Fe to XO, resulting in anticorrelat ed shifts in the XO and FeX stretching frequencies. Empirically derived tre nds in the vibrational frequencies are supported by ab initio calculations using the density functional theory (DFT). DFT analysis also provides the c orrect geometries of the unconstrained FeXO units, and has been used to inv estigate the energy required for distortion from the equilibrium geometry. This energy is modest, even for the linear FeCO unit, indicating that steri c forces from distal protein residues are not major determinants of the lig and binding energy. DFT analysis has also illuminated the contentious issue of the extent of FeCO distortion in myoglobin. While X-ray crystallographi c analyses have yielded a range of geometries, the vibrational data are all consistent with ail undistorted upright structure. However, DFT analysis o f the sensitivity of IR polarization measurements, and of the vCO/vFeC back bonding correlation, indicates that the data allow up to 0.5 Angstrom off-a xis displacement of the O atom in the FeCO unit. The energy cost of this mu ch displacement is small, ca. 0.8 kcal mol(-1), a value that is consistent with binding energy measurements on myoglobin mutants. (C) 2001 Elsevier Sc ience B.V. All rights reserved.