The effect of iron displacement out of the porphyrin plane on the resonance Raman spectra of heme proteins and iron porphyrins

The causes of the strong coupling of the iron-histidine vibration to the Soret resonance in the resonance Raman spectra of deoxyhemoglobin, myoglobin, and peroxidase are explored, using the vibronic theory. It is shown that the extent of the iron displacement out of the plane of the porphyrin nitrogens is the main structural parameter controlling the Fe-NHis band features, such as the dependence of its frequency and intensity on the protein conformation and number of the axial ligands, time evolution after the photolysis of the diatomic complexes of the proteins under consideration, and inverse relationship between the changes Fe-NHis and v4 porphyrin breathing mode frequencies.