IR Signature of NO Binding to a Ferrous Heme Center

The bare, five-coordinate iron(II) heme nitrosyl complex [Fe(II)hemeH-(NO)](+), namely, an iron(II) complex of the dianion of protoporphyrin IX holding a proton on one vinyl group, has been obtained in the gas phase, trapped in the cell of a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer, and assayed by IR multiple photon dissociation (IRMPD) spectroscopy in the 950-1950 cm(-1) fingerprint range. The observed nu(NO) signature at 1720 cm(-1), displaying sensitivity to NO isotope labeling, is significantly lower than the corresponding vibrational frequencies of ferric heme NO adducts, and suggests high pi* electron density at the ferrous nitrosyl group. Quantum chemical calculations, which allow a complete assignment of the vibrational transitions, support a doublet spin electronic configuration, a bent geometry of the Fe-N-O unit, and a cis-conformation of the peripheral propionyl groups for this five-coordinate ferrous heme nitrosyl complex.