Autoxidation of oxymyoglobin. A nucleophilic displacement mechanism

In the presence of a salt in an excess amount, the oxidation of native MbO2 was enhanced considerably above the normal autoxidation in buffr alone with the formation of the corresponding MetMb . anion complex. This anion-induced oxidation of MbO2 was measured for each salt at some 30 different values of pH in 0.1 M buffer at 25 degrees C. The anions examined were SCN-, F-, OCN-, N3- and CN-. The resulting pH dependence shows that the reaction involves two types of the displacement processes of O2- from MbO2 by the anion, i.e. those with and without proton assistance. The Brønsted plots for the rate constants versus the pKa values of the conjugate acids of anions indicate that the displacing oxidation of MbO2 proceeds by way of a nucleophilic attack of anions on the iron center and that both H2O and OH- can react with native MbO2 as the most common nucleophiles in vivo. These findings lead to a view that the proton-catalyzed nucleophilic displacement of O2- from MbO2 by an entering water molecule, or SN2 mechanism with proton assistance, is the basis for most of the autoxidation reaction under normal conditions.