Abstract
The kinetics of the oxidation of L-tryptophan by water-soluble colloidal MnO2 (prepared from potassium permanganate and sodium thiosulfate solutions) has been carried out in aqueous perchloric acid medium at different temperatures. Monitoring the disappearance of the MnO2 spectrophotometrically at 390 nm was used to follow the kinetics. The first-order kinetics with respect to [L-tryptophan] at low concentrations shifted to zero-order at higher concentrations. The reaction followed first-order with respect to [MnO2] but fractional-order with respect to [HClO4]. Adding trapping agents enhanced the rate of the reaction. The Arrhenius and Eyring equations were found valid for the reaction between 35°C and 55°C and different activation parameters (Ea, ΔH#, ΔS#) have been evaluated. On the basis of various observations and product characterization a plausible mechanism has been envisaged for the reaction taking place at the colloid surface. The results suggest formation of an adsorption complex between L-tryptophan and MnO2. The complex decomposes in a rate-determining step, leading to the formation of free radical, which again reacts with the colloidal MnO2 in a subsequent fast step to yield products. Freundlich isotherm is used to explain the adsorption of L-tryptophan on the colloidal MnO2.
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