Characterization of the electronic properties and geometric environment of the iron atom in the 'myoglobin hydrogen-peroxide' Complex by soret-excited resonance raman spectroscopy

The low temperature (ca. 80 K) Soret-excited resonance Raman spectrum of the 'myoglobin hydrogen-peroxide' complex has been obtained and compared to those of Fe(II) and Fe(III) myoglobin. The RR spectra suggest that the iron atom in the 'myoglobin hydrogen-peroxide' complex is formally in the Fe(IV) state with a t⁴_2g low-spin configuration. The ironpyrrole-nitrogen (FeN_p) and ironimidazole-nitrogen (FeN_im) bond lengths in the 'myoglobin hydrogen-peroxide' complex have been estimated from the wavenumbers of the ironpyrrole and ironimidazole stretching modes around 345 and 375 cm^1- using the FeN_p and FeN_im distances in Fe(II) myoglobin available from the X-ray crystallographic data in the literature. The distance from the centre of the porphyrin to the pyrrole nitrogen atoms (CtN_p) was estimated from the position of the anomalously polarized band around 1560-1590 cm^1- by comparison with data in the literature on model metalloporphyrins. The FeN_im', FeN_p and CtN_p distances for the 'myoglobin hydrogen-peroxide' complex were found to be 2.09, 2.08 and 2.01 Å; the distance from the iron atom to the plane of the pyrrole nitrogen atoms was determined by triangulation and found to be 0.53^+0.14_-0.18 Å. With laser lilumination of 200 mW power, the low-spin t⁴_2g 'myoglobin hydrogen-peroxide' complex apparently undergoes a two-electron photoreduction, with a rate constant of ca. 2.5 × 10^4- s^1- at ca. 80 K, to high spin t⁴_2ge²_g Fe(II) myoglobin, since there is no evidence for Fe(III) myoglobin.