Low-frequency vibrations in resonance Raman spectra of horse heart myoglobin. Iron-ligand and iron-nitrogen vibrational modes

Abstract

The low-frequency regions (150-700 cm-1) of resonance Raman (RR) spectra of various complexes of oxidized and reduced horse heart myoglobin were examined by 441.6 nm excitation. In this frequency range, RR spectra show 10 bands common to all myoglobin derivatives (numbered here for convenience from I-X). Relative intensities of bands, IV, V and X constitute good indicators of the doming state of the heme and of the spin state of the Fe atom. An additional band is present for several complexes (fluorometmyoglobin, hydroxymetmyoglobin, azidometmyoglobin and oxymyoglobin). Isotopic substitutions on the exogenous ligands and of the iron atom (56Fe .fwdarw. 54Fe) allow these additional lines to be assigned to the stretching vibrations of the Fe-6th ligand bond. Bands II are assigned to stretching vibrations of the Fe-N-(pyrrole) bonds. An assignment of bands VI to stretching vibrations of the Fe-N.epsilon. (proximal histidine) bonds is also proposed. Mechanisms for the resonance enhancement of the main low-frequency bands are discussed on the basis of the excitation profiles and of the dispersion curves for depolarization ratios obtained for fluorometmyoglobin and hydroxymetmyoglobin.