Kinetic and mechanistic studies of oxidation of arginine, histidine, and threonine in alkaline medium by N‐chloro‐N‐sodio‐p‐toluenesulfonamide

Abstract

AbstractThe kinetics of oxidation of arginine, histidine, and threonine by chloramine‐T (CAT) have been investigated in alkaline medium at 35°C. The rates are first order in both [CAT] and [amino acid] and inverse fractional order in [OH−] for arginine and histidine. The rate is independent of [OH−] for threonine. Variation of ionic strength and addition of the reaction product, p‐toluenesulfonamide, or Cl− ions had no effect on the rate. A decrease of the dielectric constant of the medium by adding methanol decreased the rate with arginine, while the rates increased with histidine and threonine. The solvent isotope effect was studied using D2O. (kobs)/(kobs) was found to be 0.55 and 0.79 for arginine and histidine, respectively. The reactions were studied at different temperatures, and activation parameters have been computed. The oxidation process in alkaline medium, under conditions employed in the present investigations, has been shown to proceed via two paths, one involving the interaction of RNHCl (formed rapidly from RNCl−), with the amino acid in a slow step to form monochloroamino acid, which subsequently interacts with another molecule of RNHCl in a fast step to give the products, p‐toluenesulfonamide (RNH2), and the corresponding nitrile of the amino acid (R'CN). The other path involves the interaction of RNCl− with the amino acid in a similar way to give RNH2 and R'CN. Mechanisms proposed and the derived rate laws are consistent with the observed kinetics. The rate constants predicted using the derived rate laws, as [OH−] varies, are in excellent agreement with the observed rate constants, thus justifying these rate laws and hence the proposed mechanistic schemes.