Paramagnetic Resonance Study of Nitric Oxide Hemoglobin

Abstract

Abstract The electron paramagnetic resonance absorption of nitric oxide hemoglobin was observed as a possible model system for oxyhemoglobin. The electron paramagnetic resonance spectrum showed three g factors indicating a rhombic symmetry around the paramagnetic center. By use of 15NO, the unpaired electron was found very little associated with NO nitrogen, in contrast to what has been customarily assumed. Modifying NO-hemoglobin with sodium dodecyl sulfate (SDS) shifted the whole spectrum toward a lower magnetic field retaining the rhombic symmetry. The 14N hyperfine structure was now resolved in one of the peaks with the splitting of 15 gauss. The number of SDS molecules to complete this transformation was found equal to the number of the basic amino acid residues. At high SDS concentrations (g0.2 m) further transformation of the spectrum took place which indicated the randomization of the structure and the change of symmetry type around the paramagnetic center from rhombic to axial. NO-hemoglobin powdered specimen after dehydration gave a spectral pattern indistinguishable from that of the SDS-modified NO-hemoglobin. This spectral conversion to dehydrated type could be repeated completely reversibly by evacuating and replenishing H2O vapor. The interaction with 0.03 to 0.5 m sodium salicylate resulted in a decrease of absorption with concomitant appearance of four new peaks, tentatively interpreted as due to a low spin Fe(III) type electronic configuration. The possible implication of the rearrangement of the electronic state was discussed in connection with the known oxidase-like activity of oxyhemoglobin under similar conditions.