Efficient oxygen consumption by hydroxo (protoporphyrinato)iron(III) adsorbed on magnesium oxide powder in the presence of cysteine

The oxygen reduction ability of hydroxo(protoporphyrinato)iron(III) (HEM) a dsorbed on powdery supports in the presence of cysteine was investigated us ing a Clark-type oxygen electrode. All of the oxygen consumption rates for HEM adsorbed on powdery supports having different surface basicity values ( CaO > MgO > TiO2) were larger than the cases of HEM in aqueous solution and of MgO suspension without HEM in the presence of cysteine. Maximum oxygen consumption rate was obtained in the case of the HEM/MgO system. This resul t suggests that the optimum surface basicity of powdery support is necessar y to achieve efficient oxygen consumption. From the measurements for ESR an d Raman spectra of the HEM/MgO system, we confirmed the change for the iron valence of HEM from III to II in the presence of cysteine. The production of cystine through the dimerization of cysteinyl radicals in the reaction m ixture was also confirmed by the measurements of ESR spectra and capillary electrophoresis (CE). A possible mechanism for accelerating the oxygen cons umption rate of the HEM/MgO system in the presence of cysteine is discussed , concerning the effect of the catalytic ability of MgO on the turnovers of iron valence.