Protein influence on the heme in cytochrome c: evidence from Raman difference spectroscopy

Raman difference spectra have been obtained for the cytochromes c of a number of species by simultaneous data acquisition from two samples. Frequency differences as small as 0.1 cm-1 can be measured reproducibly by the technique we have developed. In comparisons between cytochromes c isolated from two different species, the frequency differences in the heme vibrational modes range from 0 to 6 cm-1. The vibrational frequencies of the heme are sensitive to the electronic charge density on the porphyrin macrocycle. The frequency differences are interpreted in terms of the influence of the heme-packed aromatic and highly electronegative amino acid side chains on the pi* charge density and distribution on the heme. Such a control of the electronic properties of the heme by the protein may be important for the function of cytochrome c.