Manganese (III) oxidation of some neutral amino acids in pyrophosphate medium: A kinetic and mechanistic study

Abstract

Kinetics of oxidation of L-serine, L-phenylalanine, DL-alanine, DL -valine and DL -threonine (AA) by manganese (III) ions have been studied as a function of pH (0.91-1.45) at 328K. The reaction shows a first order dependence each in [Mn¹¹¹]o and [AA]o, inverse fractional order each in the added [Na₄P₂O₇] and [Mn¹¹] and a fractional order in [H⁺]. Effect of the added anions and varying solvent composition have also been studied. Solvent isotope studies have been made in D₂O medium. Activation parameters are computed from Arrhenius-Eyring plots. An isokinetic relationship is noted and the isokinetic temperature P=340K, indicating enthalpy control on the reactions. Taft linear free energy relationship is observed with r = -0.43 indicating an electron deficiency transition state. A mechanism consistent with the observed results is proposed and discussed.