Detecting rotational disorder in heme proteins: A comparison between resonance Raman spectroscopy, nuclear magnetic resonance, and circular dichroism

In heme proteins, the canonical and reversed conformations result from the rotation of the heme group by 180° about the α,γ‐meso axis in the protein pocket. The coexistence of the two different heme orientations has been observed both in proteins reconstituted with hemin and in some native proteins. The reversal of the heme orientation can also change certain functional properties of heme proteins. Complementing the results from other experimental techniques, like circular dichroism and nuclear magnetic resonance, resonance Raman spectroscopy provides detailed information on the structure of the reversed heme. This allows one to elucidate the effects of the heme rotation on the vibrational spectra of the peripheral substituents, especially the vinyl groups. Furthermore, the combination of resonance Raman spectroscopy on single crystals and solution samples of heme proteins is proposed to be a sensitive tool to detect heme orientational disorder, even in the absence of structural data.