Electronic spectra for nitrosyl(protoporphyrin IX dimethyl ester)iron(II) and its complexes with nitrogenous bases as model systems for nitrosylhemoproteins

Soret and visible absorption spectra for nitrosyl(protoporphyrin IX dimethyl ester)iron(II) (Fe(PPIXDME)(NO] and its complexes with nitrogenous bases (imidazoles, pyridines, aliphatic amines, and cyclic secondary amines) as model systems for nitrosylhemoproteins have been measured in various solvents. As the solvent polarity increases, the Soret and visible absorption bands for the five-coordinate Fe(PPIXDME) (NO) were shifted to shorter wavelengths. Accompanying the coordination of a nitrogenous base to the vacant axial position of Fe(PPIXDME)(NO), the Soret band becomes sharp and the band maximum is shifted to longer wavelengths. The band positions for the six-coordinate Fe(PPIXDME)(NO)(Base) complex are not sensitive to the pi-bonding ability of the axial ligand trans to NO group. The electronic spectra of five-coordinate Fe(PPIXDME)(NO) and six-coordinate Fe(PPIXDME)(NO)(Base) complexes are interpreted in relation to the structural information. The comparison of the spectra for model systems with those for nitrosylhemoproteins is discussed.