Kinetics of oxidation of histidine with Mn(III) in an aqueous sulfuric acid medium

The kinetics of Mn(III) oxidation of L-histidine in an aqueous sulfuric acid medium have been studied spectrophotometrically. The reaction shows a first-order dependence both on [Mn(III)] and [histidine] in a moderately sulfuric acid medium. The reduced product of the oxidant, Mn(II), retarded the rate of the reaction and shows an inverse first order both in [Mn(II)] and [H+]. The effect of the ionic strength on the reaction rate was also studied. There is evidence for the existence of a free radical as a transient species. The dependence of the reaction rate on the temperature was examined, and the activation parameters were evaluated using an Eyring plot. Finally, a mechanism consistent with the observed kinetic data was proposed and discussed.