| Previous studies about the behavior of the frequencies in different hemoproteins bounded with ligands cyanide and carbon monoxide show that Fe(III)-CN and Fe(II)-CO have different structural characteristics, even though CN and CO are isoelectronic. It has been observed that the behavior of stretching frequencies for the CO-heme-protein complexes have an inverse relation. This correlation of stretching frequency of Fe-C and C≡O can be explained in terms of π-back bonding (Kerr, et al., 1985). When back donation increases, the Fe-C stretching vibration increases while the CO stretching energy decreases. On the other hand, the stretching frequency of Fe-C and C≡N has a direct correlation that suggests a different stabilization mode for the cyanide ligand. L. pectinata HbI has a lower C≡N stretching frequency than other heme-proteins. Resonance Raman data strongly suggest stabilization of HbI-CN complex by multipole interactions of the phenylalanine residues and HbI-CN moiety. Density functional calculations (DFT) were carried out in small cyanide systems to study the behavior of C≡N stretching frequency when the nitrogen lone pair interacts with a proton or metal. Also, natural bond orbital analysis was performed which suggest a rehybridization mechanism of the nitrogen orbital. These results give an insight about the stabilization mechanism in hemoglobin I ligand complexes from Lucina pectinata where no hydrogen bonding exists. |
