Magnetic circular dichroism evidence for a weakly coupled heme-radical pair at the active site of cytochrome cd1, a nitrite reductase

In nitrite-treated cytochrome cd1 nitrite reductase, heme d1 is electron paramagnetic resonance silent but paramagnetic. Analysis of the unusual temperature dependence of the magnetic circular dichroism spectra unambiguously demonstrates that the heme d1 is not in the oxoferryl (FeIV=O) state but is low-spin FeIII weakly coupled to a radical species. This species could be either a protein-bound radical generated by a nitrite ion reacting with a heme group resulting in a one-electron oxidation of an amino acid residue, possibly tyrosine or tryptophan, adjacent to heme d1, or a heme d1 FeIIINO complex.