Mossbauer spectroscopic studies of the six-coordinate heme-nitric oxide complex of iron(III) octaethylporphyrin N-methylimidazole, the first model of the nitrophorin-NO complexes

The NO, N-methylimidazole complex of isotopically enriched octaethylporphyrinatoiron(III) chloride, [(OEPFe)-Fe-57(NO)(N-MeIm)Cl-+(-) has been prepared in dimethylacetamide solution and studied by low-temperature Mossbauer spectroscopy in the presence and absence of a magnetic field. This complex is a model for the Fe(III)-NO complexes of the nitrophorins of the blood-sucking insect, Rhodnius prolixus, where NO is released from the histidine-coordinated ferrihemin center of each of the proteins upon the insect's injection of the saliva into the victim. The [OEPFe(NO)(N-MeIm)]Cl-+(-) complex is EPR silent and behaves as a diamagnetic species, with quadrupole splitting Delta E-Q = 1.64 mm s(-1), asymmetry parameter eta = 0.4, isomer shift delta = 0.02 mm s(-1), and linewidth Gamma = 0.3 mm s(-1). Two electron configurations, Fe(III)-NO (low-spin d(5), strongly antiferromagnetically coupled to NO), or Fe(II)-NO+ (low-spin d(6), purely diamagnetic), are possible. Which is the actual configuration cannot be determined until detailed molecular calculations are carried out. The low-spin OEPFe(III)-bis-N-methylimidazole complex also present in this sample has EPR g-values, quadrupole splitting, isomer shift, and hyperfine splittings typical of rhombic low-spin ferriheme centers.