Light absorption, electron paramagnetic resonance and resonance Raman characteristics of nitridochromium(V) protoporphyrin-IX and its reconstituted hemoproteins

A surprisingly stable complex of the photolyzed product of azidochromium(III)protoporphyrin-IX was prepared examined by light absorption, electron paramagnetic resonance (EPR) and resonance Raman spectroscopies. The characteristic EPR spectrum for this complex was consistent with a nitrichromium(V)-porphyrin complex which was two oxidation equivalents above the resting Cr(III)_complex. The Cr(V)-N stretching mode was observed at 1010 cm^-1 by resonance Raman spectroscopy. A simple diatomic harmonic oscillation model gave a force constant of 6.7 mdyn/Å for the Cr(V)-N bond, in the region characteristic for the metal-nitrogen triple bond. Nitridochromium(V) protoprophyrin-IX reconstituted myoglobin and cytochrome c peroxidase were prepared for the first time. The nitridochromium(V)-porphyrins in these apo-proteins were unstable in contrast with the protein-free chromium(V)porphyrin. Upon irradiation of the azide complexes of the chromium(III) protoporphyrin-IX reconstituted myoglobin and cytochrome c peroxidase with ultraviolet light aerobically at room temperature, the characteristic optical and EPR spectra for nitridochromium(V) derivatives were observed. The optical spectra of these phoro-induced products were different from those of the nitridochromium(V) protoporphyrin-IX reconstituted hemoproteins. The electrochemical structures of the unusual metalloporphyrin seemed to be modulated by the heme surrounding amino acid residues.