CO as a vibrational probe of heme protein active sites

Carbon monoxide is a useful vibrational probe of heme binding sites in proteins, because FeCO backbonding is modulated by polar interactions with protein residues, and by variations in the donor strength of the trans ligand. This modulation is sensitively monitored by the CO and FeC stretching frequencies, which are readily detectable in infrared and resonance Raman spectra. The two frequencies are anticorrelated, and the νFeC/νCO position along the correlation line reflects the type and strength of distal polar interactions. Changes in the trans ligand donor strength shift the correlation to higher or lower positions. Illustrative applications of the νFeC/νCO diagram are reviewed for proteins bearing histidine and thiolate axial ligands. Steric crowding has not been found to affect the νFeC/νCO correlations significantly, except in the special case of cytochrome oxidase, where the heme-bound CO may interact with the nearby CuB center.