The reaction of metmyoglobin with strong oxidizing agents

Abstract

Sodium chlorite, potassium periodate, potassium persulfate with Ag ions present, potassium molybdicyanide and potassium chloriridate all react with metmyoglobin to give compounds with the same spectroscopic characteristics as those of the intermediate compound produced by the peroxides, a one-equivalent oxidation product which can be denoted by the symbol FepIV. The oxidizing agents containing O2, e.g., potassium periodate, react slowly with met-myoglobin, but the reactions with the electron transfer reagents are extremely rapid with velocity constants greater than 105 1. mole -1 sec.-1, even in alkaline solutions. Potassium molybdi-cyanide only oxidizes metmyoglobin above pH 11.0. At pH 8.6 and below, an equimolar concentration of potassium molybdo-cyanide reduces the intermediate compound. In the reaction between potassium chloriridate and metmyoglobin at pH 8.6 the plot of percentage oxidation against the ratio [IrCl6-2]/ [MetMb] is linear, a mole ratio of about 1.5 being required for complete reaction. In solutions of pH about 7.0 the intermediate compound is formed in an equilibrium reaction with potassium chloriridate, the constant (K) having a value of 0.20 [plus or minus] 0.01 at pH 6.82, 20.4[degree] and I = 0.042. The cationic electron transfer oxidizing agents, ruthenic and osmic tris-dipyridyl complexes, do not form the intermediate compound with metmyoglobin. They appear to react preferentially with other groups rather than with the Fe atom. However, the corresponding osmous complex reduces the intermediate compound at all pH values below about 8.6. The variation of E[image]o with pH for the FepIV / Fep3+ couple, inferred from the threshold pH values observed in the action of several of the oxidizing and reducing agents, is discussed together with its bearing on possible structures for the higher oxidation state. The papers of Polonovski, Jayle, Glotz and Graudet, who suggested between 1938 and 1941 that quadrivalent Fe compounds participate in hemo-protein reactions, are critically examined.