Permanganate oxidation of a-amino acids: kinetic correlations for the nonautocatalytic and autocatalytic reaction pathways

Abstract

The reactions of permanganate ion with seven -amino acids in aqueous KH2PO4-K2HPO4 buffers have been followed spectrophotometrically at two different wavelengths: 526 nm (decay of MnO4 - ) and 418 nm (formation of colloidal MnO2 ). All the reactions studied were autocatalyzed by colloidal MnO2, the contribution of the autocatalytic reaction pathway decreasing in the order glycine > L-threonine > L-alanine > L-glutamic acid > L-leucine > Lisoleucine > L-valine. The rate constants corresponding to the non-autocatalytic and autocatalytic pathways have been obtained by means of either a differential rate law or an integrated one, the latter requiring the use of an iterative method for its implementation. The activation parameters for the two pathways have been determined and analyzed in order to obtain statistically significant correlations for the series of reactions studied. The activation enthalpy of the non-autocatalytic pathway showed a strong, positive dependence on the standard Gibbs energy for the dissociation of the protonated amino group of the -amino acid. Linear enthalpy-entropy correlations were found for both pathways, leading to isokinetic temperatures of 370 ± 21 K (non-autocatalytic) and 364 ± 28 K (autocatalytic). Mechanisms in agreement with the experimental data are proposed for the two reaction pathways.