We review contributions made towards the elucidation of CO and O2 binding geometries in respiratory proteins. Nuclear magnetic resonance, infrared spectroscopy, Mössbauer spectroscopy, X-ray crystallography and quantum chemistry have all been used to investigate the Fe-ligand interactions. Early experimental results showed linear correlations between 17O chemical shifts and the infrared stretching frequency (νco) of the CO ligand in carbonmonoxyheme proteins and between the 17O chemical shift and the 13CO shift. These correlations led to early theoretical investigations of the vibrational frequency of carbon monoxide and of the 13C and 17O NMR chemical shifts in the presence of uniform and non-uniform electric fields. Early success in modeling these spectroscopic observables then led to the use of computational methods, in conjunction with experiment, to evaluate ligand-binding geometries in heme proteins. Density functional theory results are described which predict 57Fe chemical shifts and Mössbauer electric field gradient tensors, 17O NMR isotropic chemical shifts, chemical shift tensors and nuclear quadrupole coupling constants (e2qQ/h) as well as 13C isotropic chemical shifts and chemical shift tensors in organometallic clusters, heme model metalloporphyrins and in metalloproteins. A principal result is that CO in most heme proteins has an essentially linear and untilted geometry (τ = 4°, β = 7°) which is in extremely good agreement with a recently published X-ray synchrotron structure. CO/O2 discrimination is thus attributable to polar interactions with the distal histidine residue, rather than major Fe-C-O geometric distortions.
- Density functional theory (DFT)
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