Manganese(III) Oxidation of L-Lysine and L-Histidine in Pyrophosphate Solution: A Kinetic and Mechanistic Study

Abstract

Kinetic studies on the oxidation of two basic amino acids (AA), L-lysine and L-histidine, by manganese(III) ions have been performed in pyrophosphate solution at 328 K and pH 1.3. Manganese(III) pyrophosphate solution was prepared by a known chemical method and the nature of the oxidizing species present in the solution was determined by spectrophotometric and redox potential measurements. The reaction follows the rate law: rate = k [Mn(III)] [AA] [H⁺][x/[Mn(II)]^y [P₂O₇ ⁴⁻]^z] where x, y, and z are all fractional. The effects of added anions, fluoride, chloride, and perchlorate, and of varying solvent composition were studied. A solvent isotope effect in D₂O medium was investigated. The dependence of the rate on temperature was studied and activation parameters were evaluated from the Arrhenius and Eyring plots. A mechanism, which is consistent with the kinetic data, has been discussed.