Thiamine Derivatives of Disulfide Type. V. Kinetic Studies of the Reaction between Thiamine Propyl Disulfide or Homologes and L-Cysteine

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The Kinetics between thiamine propyl disulfide or its homologes and L-cysteine were studied under the experimental conditions determined in the preceding paper. The effects of alkyl substitution of TPD on kinetic parameter were also studied, using thiamine benzyl disulfide, thiamine allyl disulfide, and thiamine hydroxyethyl disulfide. The following conclusions were drawn from the results presented. 1) From an analysis of the pH-rate profile of the reaction which followed a second order kinetics throughout the pH range of 3.5 to 13.2, the reaction was revealed as the combination of the following elementary reactions. [numerical formula] where HG₁-SS-Pr is the protonated form of TPD, NH₃⁺-Cy-S^- is protonated cysteine anion, and k₁, k₂ and k₃ are specific rate constants of individual elementary reactions. The values determined were 1.82, 0.261 and 1.10 in 10⁴L. mole^-1 min. ^-1for k₁, k₂ and k₃, respectively. 2) No detectable effect of ionic strength and buffer concentration on the rate constant was observed at pH 4.0 on Reaction (I). 3) The activation energy obtained from an Arrhenius type plot was 12.3 and 10.9 kcal./mole for Reaction (I) and (II) after correction for ionic dissociation of thiol and amino groups in cysteine. 4) From the kinetic data tabulated on TPD derivatives, the order of the reactivity was TBD>TAD-TOED>TPD. The activation energies of Reaction (I) were about 10 to 12 kcal./mole.