Selective examination of heme protein azide ligand-distal globin interactions by vibrational circular dichroism

Vibrational circular dichroism (VCD) spectra of the antisymmetric stretch of azide ligated to the heme of a series of evolutionarily diverse and site-direct mutant hemoglobins and myoglobins are anomalously intense and demonstrate an intriguing sensitivity to subtle protein--ligand interactions. The antisymmetric stretch of the azide ligand covalently bound to the low-spin iron shows an anisotropy ratio of -9.5 x 10(-4) for sperm whale and horse myoglobin which decreases to -8.0 x 10(-4) for human and carp hemoglobin and Chironimus thummi thummi III monomeric hemoglobin. The VCD spectra of these heme-azide complexes depend upon the interactions of the azide ligand with distal heme pocket residues such as the E7 distal histidine and E11 valine. The site-directed mutants of sperm whale (distal histidine substituted Gly E7) and human (distal valine substituted Asn E11) myoglobin have vanishingly small anisotropy ratios (<-0.5 x 10(-4)), while elephant myoglobin (distal histidine substituted Gln E7) shows an anisotropy ratio of -6.4 x 10(-4). The azide ligand ionically bound to a high-spin iron shows a vanishingly small VCD intensity.