Formation of a hydroperoxy complex of heme during the reaction of ferric myoglobin with H2O2

Reaction of ferric myoglobin with hydrogen peroxide (H2O2) was studied by following the changes in absorption spectra of myoglobin during its conversion to a ferryl form. Analyses of the data by global analysis revealed that an intermediate species, which has not been hitherto described, occurred prior to the formation of ferryl myoglobin. At a pHs below 7, the new species exhibits the Soret absorption maximum at 408 nm, which shifts to 414 nm on raising the pH to above 8. By an analogy with the pH-dependent transition between the acidic and alkaline forms of ferric myoglobin, we suggest that the intermediate species is a mixture of a high-spin Fe3+(H2O2) form and a Fe3+(HO2)(-) form, the latter of which is in a thermal equilibrium between the high- and low-spin forms.