Spin Crossover in Nitrito-Myoglobin as Revealed by Resonance Raman Spectroscopy.

The myoglobin (Mb) heme Fe-O-N=O and heme Fe-O-N=O/2-nitrovinyl species have been characterized by resonance Raman spectroscopy. In the heme Fe-O-N=O species, the bound nitrite ligand is removed by solvent exchange, thus reforming metmyoglobin (metMb). The high-spin heme Fe-O-N=O unit is converted into a low-spin heme Fe-O-N=O/2-nitrovinyl species that can be reversibly switched between a low- and a high-spin state without removing the bound nitrite ligand, as observed in the case of the heme Fe-O-N=O species. This spin-state change is likely to be accompanied by a general structural rearrangement in the protein-binding pocket. This example is the first of a globin protein that can reversibly change its metal spin state through an internal perturbation. These findings provide a basis for understanding the structure-function relationship of the spin cross found in other metalloenzymes and Fe(III) -porphyrin complexes.