Kinetic Analysis of Oxidation of Dipeptides by Sodium N-Bromobenzenesulfonamide in Acid Medium: a Mechanistic Approach

The kinetics of the oxidation of five dipeptides (DPP) viz., glycylglycine (Gly-Gly), L-alanyl-L-alanine (Ala-Ala), L-valyl-L-valine (Val-Val), L-leucyl-L-leucine (LeuLeu), and phenylglycyl-phenylglycine (Phg-Phg) by sodium N-bromobenzenesulfonamide or bromamine-B (BAB) in presence of HClO4 was studied at 40 °C. The five reactions followed identical kinetics with a first-order dependence on [BAB] and fractional order in [DPP]. At [H+] > 0.04 mol dm^-3, the rate was inverse fractional in [H^+], but zero order at lower [H^+] (=< 0.04 mol dm^-3). A variation of the ionic strength or dielectric constant of the medium and the addition of halide ions and benzenesulfonamide had no effect on the rate of the reaction. Proton inventory studies were made in H2O-D2O mixtures for all five dipeptides. A MichaelisMenten type mechanism has been suggested to explain the results. The decomposition and equilibrium constants were evaluated. The oxidation products were identified. The isokinetic temperature was 360 K, indicating the enthalpy to be a controlling factor. The rate of oxidation increased in the order Phg-Phg > Ala-Ala > Val-Val > Leu-Leu > Gly-Gly. The kinetics of oxidation of dipeptides was compared with those of their corresponding monomer amino acids, namely Phenylglycine, alanine, valine, leucine, and glycine. A general mechanism was proposed and the derived rate law are consistent with the observed kinetics.