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

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Abstract

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 t42g low-spin configuration. The ironpyrrole-nitrogen (FeNp) and ironimidazole-nitrogen (FeNim) bond lengths in the ‘myoglobin hydrogen-peroxide’ complex have been estimated from the wavenumbers of the ironpyrrole and ironimidazole stretching modes around 345 and 375 cm1− using the FeNp and FeNim 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 (CtNp) was estimated from the position of the anomalously polarized band around 1560–1590 cm1− by comparison with data in the literature on model metalloporphyrins. The FeNim', FeNp and CtNp 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 t42g ‘myoglobin hydrogen-peroxide’ complex apparently undergoes a two-electron photoreduction, with a rate constant of ca. 2.5 × 104− s1− at ca. 80 K, to high spin t42ge2g Fe(II) myoglobin, since there is no evidence for Fe(III) myoglobin.

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    Present address: Biozentrum der Universität Basel, abt. für Biophysikalische Chemie, Basle, Switzerland.

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