Thiamine Derivatives of Disulfide Type. VII. Kinetics between Thiamine Tetrahydrofurfuryl Disulfide and L-Cysteine or Its Derivatives

Abstract

The kinetics in aqueous solution on the exchange reaction between thiamine tetrahydrofurfuryl disulfide and thiols, i.e., L-cysteine, N-acetyl-L-cysteine, and glutathione, was conducted at the experimental condition of 15° to 37° and pH 3.5 to 8.5 where it was proved that the main reaction could be followed from the formation of thiamine neglecting the possible side reactions examined by paper partition chromatographic procedure. The following conclusions were drawn from the result presented. 1) The formation of thiamine followed the second order reaction concluded as SN2 and was converted to a first order kinetics by the addition of excess amount of reactant. 2) From the analysis of pH-rate profile of the reaction, the following two elementary reactions were postulated, i.e., between the molecular or ionized form of thiamine derivative and thiol anion, which well explained the result obtained. 3) No detectable effect on rate was proved for the ionic strength and concentration of buffer solution used. 4) The kinetical parameters for each elementary reaction, i.e., specific rate constant, activation energy, frequency factor, and activation entropy, were tabulated. 5) The activation energies were around 10 kcal/mole. Larger negative activation entropy was found in reactions between molecular form of thiamine derivative and thiol anions than ones between ions. The reactivity of thiols was -NACyS->-GS->CyS-.