Oxidation States of Manganese Hematoporphyrin IX in Aqueous Solution

Manganese hematoporphyrin IX and its dimethyl ester have been prepared, and the most stable form of these complexes in air is shown by elemental analyses and magnetic susceptibility measurements to be one in which manganese is in the 3 + oxidation state. Three other oxidation states are demonstrated to be reversibly obtainable in aqueous alkaline solution, and oxidation-reduction potentials are reported for the system Mn^III/Mn^II hematoporphyrin IX between pH 6.7 and 13.6 and the system Mn^IV/Mn^III hematoporphyrin IX between pH 9.9 and 13.6. Both systems show evidence for a redox-linked proton function in Mn^III hematoporphyrin IX with a pK′ value of 12, which was verified by spectrophotometric determination, and it is suggested that the equilibrium reflected involves the loss of a proton by a coordinated water molecule. Data on E_m versus pH for Mn^IV/Mn^III hematoporphyrin IX indicate the presence of two redox-linked proton functions in Mn^IV hematoporphyrin IX having pK′ values below 10. It is suggested that these reflect the loss of a proton by each of two water molecules, one in coordination position 5 and the other in coordination position 6. Two redox-linked proton functions were determined in Mn^II hematoporphyrin IX having pK′ values of 6.9 and 12.8, and the latter constant was assigned to the loss of a proton by a coordinated water molecule. The results are related to existing data on metalloporphyrin systems, and it is pointed out that Mn^II hematoporphyrin IX is a strong reductant whereas Mn^IV hematoporphyrin IX is a very strong oxidant, a fact of possible interest in photosynthesizing systems.