Hemoprotein Models Based on a Covalent Helix-Heme-Helix Sandwich. 3. Coordination Properties, Reactivity and Catalytic Application of Fe(III)- and Fe(II)-Mimochrome I

The coordination state of Fe(III)- and Fe(II)-mimochrome I, a covalent peptide-deuteroheme sandwich involving two nonapeptides bearing a histidine residue in a central position, was studied by UV-visible, EPR, and resonance Raman spectroscopy. The ferric and ferrous states of this new iron species mainly exist, at pH 7, in a low-spin hexacoordinate form with two axial histidine ligands coming from the peptide chains. A minor amount of high-spin form for the ferric state is also present at pH 7. However, it is mainly high-spin at pH 2 or in DMSO. Fe(II)-mimochrome I binds CO with an affinity comparable to that of myoglobin and hemoglobin. Fe(III)-mimochrome I reacts with alkylhydroxylamine and arylhydrazines, leading to the corresponding Fe(II)-nitrosoalkyl and Fe(III)-sigma-aryl complexes, respectively. These reactions were greatly dependent on the solvent used and on the pH, and were much slower than the corresponding reactions performed by deuterohemin in the presence of excess imidazole. All these results indicate that the reactivity of iron-mimochrome I is controlled by the binding of the peptide chainsto the iron. The reactivity shown by this complex at neutral pH is intermediate between that observed for iron porphyrins in the presence of excess imidazole and that of hemoproteins characterized by a strong bis-histidine axial coordination, such as cytochrome b(5). Fe(III)-mimochrome I is able tocatalyze styrene epoxidation by using a [Fe(III)-mimochrome I]/[H2O2]/[stryrene] ratio of 1:10:2000 in phosphate buffer solution (pH 7.2) containing 2% CTAB both under strictly anaerobic conditions and in the presence of oxygen, at 0 degrees C.