The reaction between metmyoglobin and hydrogen peroxide

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Abstract
Metmyoglobin forms a complex with H2O2 throughout the pH range 5-12. Only within the pH range 8-9 is this reaction unattended by oxidative attack on the porphyrin ring. On the alkaline side the attack on the ring seems to give cholemyoglobin, for a regular decrease in the intensity of the absorption bands is observed both in the visible and Soret regions of the spectrum. In solns. more acid than pH 8 a different type of oxidative attack occurs, for no regular decrease in intensity is observed, but there is a shift of the band maxima. These side reactions are not due to the reaction of the complex itself with metmyoglobin, but arise by some other mechanism during the formation of the complex. The system MetMB+H2O2 is not an equilibrium system. The effect of concn. on the formation of the complex is only very small over a 10-fold change of absolute concn. The oxidizing equivalent of the complex is unity. By adding a reducing agent to the metmyoglobin before H2O2, it is possible to account for the 2 oxidizing equivalents of the peroxide. This indicates the reaction to be of the type MetMB+H2O2[long dash][forward arrow]complex+X. A tentative mechanism is proposed identifying the complex with ferrylmyoglobin, a compound of quadrivalent Fe, and X with the OH radical. The high oxidizing activity of the OH radical could account for the oxidative side reactions.