Mössbauer 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, [OEP⁵⁷Fe(NO)(N-MeIm)]⁺C1^- has been prepared in dimethylacetamide solution and studied by low-temperature Mössbauer 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)]⁺C1^- complex is EPR silent and behaves as a diamagnetic species, with quadrupole splitting ΔE_Q = 1.64 mm s^-1, asymmetry parameter η = 0.4, isomer shift δ = 0.02 mm s^-1, and linewidth Γ= 0.3 mm s^-1. Two electron configurations, Fe(III)-NO (low-spin d⁵, strongly antiferromagnetically coupled to NO), or Fe(II)-NO⁺ (low-spin d⁶, 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.