Oxygen infrared spectra of oxyhemoglobins and oxymyoglobins. Evidence of two major liganded O2 structures

The dioxygen stretch bands in infrared spectra for solutions of oxy species of human hemoglobin A and its separated subunits, human mutant hemoglobin Zurich (beta 63His to Arg), rabbit hemoglobin, lamprey hemoglobin, sperm whale myoglobin, bovine myoglobin, and a sea worm chlorocruorin are examined. Each protein exhibits multiple isotope-sensitive bands between 1160 and 1060 cm-1 for liganded 16O2, 17O2, and 18O2. The O-O stretch bands for each of the mammalian myoglobins and hemoglobins are similar, with frequencies that differ between proteins by only 3-5 cm-1. The spectra for the lamprey and sea worm hemoglobins exhibit greater diversity. For all proteins an O-O stretch band expected to occur near 1125 cm-1 for 16O2 and 17O2, but not 18O2, appears split by approximately 25 cm-1 due to an unidentified perturbation. The spectrum for each dioxygen isotope, if unperturbed, would contain two strong bands for the mammalian myoglobins (1150 and 1120 cm-1) and hemoglobins (1155 and 1125 cm-1). Two strong bands separated by approximately 30 cm-1 for each oxy heme protein subunit indicate that two major protein conformations (structures) that differ substantially in O2 bonding are present. The two dioxygen structures can result from a combination of dynamic distal and proximal effects upon the O2 ligand bound in a bent-end-on stereochemistry.