Chapter 4 - CO, NO, and O 2 as Vibrational Probes of Heme Protein Active Sites

Abstract

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 negatively correlated, and the v FeC/ v CO 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 v FeC/ v CO diagram are reviewed for proteins bearing histidine and thiolate axial ligands. Steric crowding has not been found to affect the v FeC/ v CO correlations significantly, except in the special case of cytochrome oxidase, where the heme-bound CO may interact with the nearby Cu B center. NO adducts of Fe(II) heme proteins also show variations in the v FeN and v NO frequencies, but the data show considerable scatter, in contrast to the CO adduct data. However, protein-free Fe(II)porphyrin NO adducts give well-behaved backbonding correlations; new data show this to be true of 6-coordinate (6-c) as well as 5-coordinate (5-c) adducts. The scatter in the protein data is suggested to reflect changes in the FeNO angle induced by distal polar groups, especially histidine. The few data available for NO adducts of Fe(III) heme proteins suggest a weak negative v FeN/ v NO correlation when the proximal ligand is histidine, but a positive correlation when the proximal ligand is thiolate. This behavior is markedly different from that of the isoelectronic Fe(II)CO. DFT modeling indicates that the altered response reflects a change in frontier orbitals resulting from the lowered d z2 energy in Fe(III). A similar pattern is suggested for Fe(II)O 2 adducts from the limited available data.