Abstract
The kinetics of the oxidation of L-tryptophan by water-soluble colloidal MnO
2
(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 MnO
2
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 [MnO
2
] but fractional-order with respect to [HClO
4
]. 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 (E
a
,
Δ
H
#
,
Δ
S
#
) 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 MnO
2
. The complex decomposes in a rate-determining step, leading to the formation of free radical, which again reacts with the colloidal MnO
2
in a subsequent fast step to yield products. Freundlich isotherm is used to explain the adsorption of L-tryptophan on the colloidal MnO
2
.