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 pept ide-deuteroheme sandwich involving two nonapeptides bearing a histidine res idue in a central position, was studied by UV-visible, EPR, and resonance R aman spectroscopy. The ferric and ferrous states of this new iron species m ainly exist, at pH 7, in a low-spin hexacoordinate form with two axial hist idine ligands coming from the peptide chains. A minor amount of high-spin f orm for the ferric state is also present at pH 7. However, it is mainly hig h-spin at pH 2 or in DMSO. Fe(II)-mimochrome I binds CO with an affinity co mparable to that of myoglobin and hemoglobin. Fe(III)-mimochrome I reacts w ith alkylhydroxylamine and arylhydrazines, leading to the corresponding Fe( II)-nitrosoalkyl and Fe(III)-sigma-aryl complexes, respectively. These reac tions were greatly dependent on the solvent used and on the pH, and were mu ch slower than the corresponding reactions performed by deuterohemin in the presence of excess imidazole. All these results indicate that the reactivi ty of iron-mimochrome I is controlled by the binding of the peptide chains to the iron. The reactivity shown by this complex at neutral pH is intermed iate between that observed for iron porphyrins in the presence of excess im idazole and that of hemoproteins characterized by a strong bis-histidine ax ial coordination, such as cytochrome b(5). Fe(III)-mimochrome I is able to catalyze styrene epoxidation by using a [Fe(III)-mimochrome I]/[H2O2]/[stry rene] 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 oxyg en, at 0 degrees C.