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-directed 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 × 10^-4 for sperm whale and horse myoglobin which decreases to -8.0 × 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 × 10^-4), while elephant myoglobin (distal histidine substituted Gin E7) shows an anisotropy ratio of -6.4 × 10^-4. The azide ligand ionically bound to a high-spin iron shows a vanishingly small VCD intensity.