Infrared analysis of ligand- and oxidation-induced conformational changes in hemoglobins and myoglobins

Effects of the binding of O2 and CO to heme iron (II) of deoxy forms and of the oxidation of deoxy forms to aquoiron (III) complexes on the infrared spectra of hemoglobins and myoglobins have been examined. Spectra were measured for aqueous solutions 3-4 mM in heme of human, bovine, and equine hemoglobins and sperm whale, bovine, and equine myoglobins in 10 mM sodium phosphate buffer, pH 7.4, at 20 degrees C. All ligand binding and oxidation reactions resulted in similar spectral shifts in the region 1665 to 1670 cm-1, a portion of the amide I region assignable to beta-turn structure. There were no other significant changes in the amide I region, a finding consistent with no other alterations in secondary structure. The major bands near 1655 cm-1 associated with alpha-helices were consistently at 2 cm-1 lower wavenumber for myoglobins than for hemoglobins. The changes in solution infrared spectra observed in this study may result at least in part from conformational changes at the FG corner associated with movements of F and E helices that have been noted previously in crystal structures.