Thiamine Derivatives of Disulfide Type. IV. The Exchange Reaction between Thiamine Propyl Disulfide and L-Cysteine

Experimental conditions for the kinetic study of exchange reaction between thiamine propyl disulfide (TPD) and L-cysteine were examined where the possible side reactions shown in Chart 1 proved by thin-layer chromatography could be safely ignored. The following conclusions were drawn from the results obtained. 1) It was proved that the formation of thiamine was the main product and the formation of propylmercaptan was negligible when the reactants were mixed in a molar ratio of 1 : 1 and an initial concentration of 1×10-3∼5×10-5 molar. 2) The exchange reaction followed second order kinetics examined from half-life, the plot of integrated equation, and the result of pseudo first order reaction converted by experimental conditions. The rate constants observed were 1.723 and 444.4 L. mole-1 min. -1at 15°and pH 4.6 and 8.0, respectively. 3) The rate constant of the thiamine formed from TPD and propylmercaptan which was the largest side reaction among the various possibilities, was 0.607 L. mole-1 min. -1 at pH 4.6 and 15°as a second order reaction.