Nitrite coordination in myoglobin.

The coordination of nitrite in myoglobin (Mb) has been characterized by resonance Raman spectroscopy and the frequencies of the nitrite bound to the heme Fe as well to the 2-vinyl have been computed by density functional theory (DFT) calculations. The DFT Natural Bond Orbital (NBO) analysis and the extensive isotope-labeling in the resonance Raman experiments indicate that NO2- (O1NO2) is bound to the heme Fe via O1. Based on the vibrational characterization of the reversible transition between low and high spin FeONO/2-nitrovinyl species, we suggest that the key step that triggers the spin-change is the increase of the proximal FeNHis93 bond length. The frequencies of the O and N sensitive bands of the FeONO/2-nitrovinyl species remained largely unchanged in the low- to high-spin transition. Therefore the "greening" process in the reaction of ferric Mb with NO2- proceeds through the FeONO/2-nitrovinyl species, which can exist in either the high or low-spin state.