The interaction between manganese(IV)-hematoporphyrin IX and water

Mn(IV)-hematoporphyrin IX, prepared from Mn(III)-hematoporphyrin IX by addition of 1 equiv. of strong oxidant in alkaline solution, reverts to Mn(III)-hematoprophyrin IX in the absence of added reducing agents and at a rate which is strongly pH dependent. A study of the kinetics reveled a first-order dependence upon the concentration of Mn(IV)-hematoporphyrin IX at pH 13 and higher-order dependence at values of pH below 12. ΔH{double barred pipe} = 20 kcal/mole and ΔS{double barred pipe} = -25 e.u. were determined at pH 13. Evidence for the formation of a percaboxylic acid was provided by infrared spectra. Free O₂ or H₂O₂ were not found under the conditions of the reactions. A scheme is presented which is consistent with the obervations.