Kinetic and Mechanistic Investigation of Os(VIII)-Catalyzed L-Tryptophan Oxidation by Hexacyanoferrate(III) in CTAB Micellar Medium

Abstract

The kinetics of Os(VIII)-accelerated L-tryptophan (Trp) oxidation by hexacyanoferrate(III) in CTAB micellar medium were investigated by measuring the decline in absorbance at 420 nm, which corresponds to [Fe(CN)₆]^3–. By adjusting one variable at a time, the progression of the reaction has been inspected as a function of [OH^–], ionic strength, [CTAB], [Os(VIII)], [Trp], [Fe(CN)$_{6}^{{3 - }}$], and temperature using the pseudo-first-order condition. The results show that [CTAB], [Trp], and [OH^–] are the critical parameters with a discernible influence on reaction rate. The reaction rate is independent of the [Fe(CN)$_{6}^{{3 - }}$]; hexacyanoferrate(III) is merely used up to regenerate the Os(VIII) during the reaction. In the investigated concentration range of Os(VIII), as well as at lower [OH^–] and [Trp], the reaction displays first-order kinetics with respect to [Os(VIII)], [OH^–], and [Trp] but follows less than unit order at larger Trp and alkali concentrations. The linear increase in reaction rate with added electrolyte is indicative of a positive salt effect. CTAB significantly catalyzes the process, and once at a maximum, the rate remains almost constant as [CTAB] increases. The observed decrease in CTAB CMC could be attributed to reduced repulsion between the positive charge heads of surfactant molecules caused by the negatively charged [Fe(CN)₆]^3–, OH^–, and [OsO₅(OH)]^3–.