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 MnIII/MnII hematoporphyrin IX between pH 6.7 and 13.6 and the system MnIV/MnIII hematoporphyrin IX between pH 9.9 and 13.6. Both systems show evidence for a redox-linked proton function in MnIII 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 Em versus pH for MnIV/MnIII hematoporphyrin IX indicate the presence of two redox-linked proton functions in MnIV 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 MnII 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 MnII hematoporphyrin IX is a strong reductant whereas MnIV hematoporphyrin IX is a very strong oxidant, a fact of possible interest in photosynthesizing systems.
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