KINETICS AND MECHANISM OF THE OXIDATION OF 0-MERCAPTOBENZOIC ACID (A MODEL FOR PEPSIN) BY METHYLENE BLUE IN METHANOL-WATER MEDIUM

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Abstract
The oxidation of o-mercaptobenzoic acid (TSA: a model for pepsin) by methylene blue (MB) has been investigated kinetically in aqueous methanol and in presence of sulphuric acid. The principal reactants interact in a molar ratio of 2:1 forming diphenyldisulfide-2,2′-dicarboxylic acid as the oxidation product. The order in the oxidant is unity while the order in the substrate varies from zero to half at its larger concentrations. The rate depends on pH showing an optimum. The order in hydrogen ion increases from 0.04 to 0.1 when the runs are made at low concentrations of TSA (ca. 1.25 × M). The variation in ionic strength does not influence the rate but an increase in dielectric constant increases the rate. Here again, the rate attains a limiting value after a particular concentration of methanol (50% vh). The addition of reaction products retards the rate showing a variation in order of reaction in methylene blue from unity to zero on adding the leuco base. The possibility of the participation of half reduced MB was checked by making the run in presence of acrylonitrile and it was overruled. Activation parameters were evaluated and a reaction scheme presuming the participation of triplet methylene blue has been proposed.

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