Conformational substates of ferricytochrome c revealed by combined optical absorption and electronic circular dichroism spectroscopy at cryogenic temperature

We have investigated the heterogeneity of the Fe(III)–Met80 linkage of horse heart ferricytochrome c by probing the 695 nm charge transfer band with absorption and electronic circular dichroism (ECD) spectroscopy. In order to verify the connection between conformational substates of the Fe(III)–Met80 linkage and the 695 nm band spectral heterogeneity, we have performed experiments as a function of pH (neutral and acidic) and temperature (room and 20 K). At room temperature, the ECD spectrum is blue shifted with respect to the absorption one; the shift is more pronounced at acidic pH and is compatible with the presence of sub-bands. ECD measurements at 20 K highlighted the heterogeneous nature of the 695 nm band and provided direct experimental evidence for the presence of sub-bands. Indeed, while the absorption spectra remained deceivingly unstructured, the ECD spectra showed well resolved peaks and shoulders. A consistent fit of the 20 K absorption and ECD spectra showed that five Gaussians (each centered at the same frequency in the absorption and ECD spectrum) are able to reproduce the observed lineshapes. A careful analysis of frequency shifts and intensity ratios of these sub-bands enabled us to identify at least three distinct sub-bands arising from taxonomic conformational substates of the Fe(III)–Met80 linkage. In view of the major influence of the Fe(III)–Met80 linkage on the redox potential of ferricytochrome c, we speculate that these spectrally distinguishable substates may have different functional roles.