Abstract

The aromatic nucleophilic substitution reaction of 3,6-dichloro-1,2,4,5- tetrazine (DCT) with a series of biothiols RSH: (cysteine, homocysteine, cysteinyl-glycine, N-acetylcysteine, and glutathione) is subjected to a kinetic investigation. The reactions are studied by following spectrophotometrically the disappearance of DCT at 370nm. In the case of an excess of N-acetylcysteine and glutathione, clean pseudo first-order rate constants (kobs1) are found. However, for cysteine, homocysteine and cysteinyl-glycine, two consecutive reactions are observed. The first one is the nucleophilic aromatic substitution of the chlorine by the sulfhydryl group of these biothiols (RSH) and the second one is the intramolecular and intermolecular nucleophilic aromatic substitutions of their alkylthio with the amine group of RSH to give the di-substituted compound. Therefore, in these cases, two pseudo first-order rate constants (kobs1 and kobs2, respectively) are found under biothiol excess. Plots of kobs1 versus free thiol concentration at constant pH are linear, with the slope (kN) independent of pH (from 6.8 to 7.4). The kinetic data analysis (Brønsted-type plot and activation parameters) is consistent with an addition-elimination mechanism with the nucleophilic attack as the rate-determining step. Copyright © 2014 John Wiley & Sons, Ltd.