Anharmonic Protein Motions and Heme Deformations in Myoglobin Cyanide Probed by Absorption and Resonance Raman Spectroscopy

Abstract

The Soret absorption of myoglobin cyanide in a 65% glycerol/water mixture was measured as a function of temperature between 20 and 300 K. The data were analyzed by using an earlier model relating each transition into the vibronic manifold of the electronic B-state to a Voigtian band profile (Cupane et al. Eur. Biophys. J. 1995, 23, 385, 1995). Its Gaussian part contains a temperature-dependent component due to the coupling of low-frequency modes to the Soret transition. The analysis of the vibronic substructure was facilitated by comparison with vibronic coupling parameters derived from the line intensities in the polarized Raman spectra taken with Soret excitation. From the depolarization ratios of several Raman lines, the existence of asymmetric heme macrocycle distortions was inferred, which lift the degeneracy of the excited B state. Raman intensities and depolarization ratios were then analyzed by a theory that formulates the polarizability tensor in terms of a time-independent perturbation theory. T...