Correlations Between the Heme Structure and the Electronic Absorption and Resonance Raman Spectral Features in Heme Proteins

Abstract

A number of studies on the electronic absorption and resonance Raman (RR) spectra of heme proteins, their models, and their site-directed mutants revealed that the frequencies of some RR bands as well as the absorption wavelength of the π→π* transitions are indicative of the porphyrin core size. In particular, it is possible to establish the coordination and the spin states of the Fe central atom. In addition, the vibrations of the two vinyl substituents in position 2 and 4 of the porphyrin ring, in particular the v(C=C) stretching modes, of heme proteins and models are RR active and have been identified via deuterium substitution [1]. In several RR spectra of heme proteins, including all the cases in which the high resolution X-ray crystal structure was known [2], only a single band due to the two v(C=C) stretching mode occured. It has been found that the modes of both the vinyl groups are superimposed as proved by the RR spectra of mono- and di-vinylhemins [3], and by selective deuteration of the two vinyl groups for cytochrome c peroxidase (CCP) [4] and myoglobin (Mb) [5]. These results excluded the occurrence of vibrational coupling between the two v(C=C) stretching modes of protoheme and are consistent with the results of INDO/CI calculations which indicate that the torsional potential of a given vinyl group is independent of the torsional angle of the other [6]. It has been shown that the appearance of two v(C=C) stretching modes at about 1620-1630 cm−1 in models indicate that two or more conformers in equilibrium exist and that the band at 1630 cm−1 is due to the less perturbed vinyl substituent by site specific interactions. As a consequence it has been hypothesized that the occurrence of two v(C=C) stretching modes in several heme proteins could be due to different site specific interactions by the protein matrix on the two vinyl group substituents in position 2 and 4.