Electron paramagnetic resonance studies of NO-heme-nitrogen base. An interpretation of electron paramagnetic resonance spectra of NO-hemoproteins.

In order to characterize the structure of the heme environment of hemoproteins, electron paramagnetic resonance (EPR) spectra for the NO complex of the iron(II) porphyrin nitrogen bases (pyridine and imidazole derivatives) were measured. The coupling constants of the nitrogen atom of NO (AN1) and the fifth ligand (AN2) and g values were determined from the 9-lined hyperfine using second-derivative display. These EPR parameters varied with changes in trans ligand (trans effect) and heme substitution at positions 2 and 4 (cis effect) as follows. (1) Both AN1 and AN2 increased as the basicity of the nitrogen atom of the fifth ligand increased, while AN1 increased concomitant with the decrease of AN2 by steric hindrance of the fifth ligand. (2) Both AN1 and AN2 increased as the basicity of the porphyrin nitrogen atom decreased. (3) In both cases, the anisotropy of g values (gx and gy) decreased concomitant with the increase of AN2. From the analysis of the EPR spectra of model systems, the substantial difference in the EPR spectra of NO-hemoproteins is discussed in relation to iron-proximal histidine binding and heme-apoprotein interactions.