An Infrared Study of NO Bonding to Heme B and Hemoglobin A. Evidence for Inositol Hexaphosphate Induced Cleavage of Proximal Histidine to Iron Bonds

Five- and six-coordinate nitrosyl hemes have been prepared and their infrared, electron paramagnetic resonance (EPR), and visible-Soret spectra compared with the corresponding spectra for nitrosyl hemoglobin A (Hba-NO) determined both in the presence and the absence of inositol hexaphosphate (IHP). The five- and six-coordinate NO complexes prepared from either dipyridine or pyridine carbonyl protoheme dimethyl ester had N-O stretch bands (nuno) near 1675 and 1625 cm-1, respectively. These frequencies are sensitive to change in solvent (nuno decreased as the dipole moment of the solvent increased) and, with six-coordinate species, to changes in trans ligand. However, these solvent and trans ligand effects were small compared with the difference (ca. 50 cm-11) between five- and six -coordinate species. The nature of the trans ligand affected the relative proportions of the two...