NMR, IR, Mössbauer and quantum chemical investigations of metalloporphyrins and metalloproteins

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 (ν_co) 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.