HEXADECYLPYRIDINIUM CHLORIDE MICELLES FOR THE SIMULTANEOUS KINETIC DETERMINATION OF CYSTEINE AND CYSTINE BY THEIR INDUCTION OF THE IODINE-AZIDE REACTION

The associate formed by triiodide ion and hexadecylpyridinium chloride (cetylpyridinium chloride; CPC) micelles was used to enhance the kine tic spectrophotometric determination of cysteine and cystine by their inductor effect on the iodine-azide reaction. The reaction rate was mo nitored at 500 nm, the maximum absorption wavelength of the triiodide- CPC associate. The micellar medium allowed the determination of cystei ne and cystine at concentrations between 1 and 15 ng ml(-1) and 0.2 an d 1.3 mu g ml(-1), respectively, the detection limits (3 sigma) for th e two inductors being 0.6 and 0.12 mu g ml(-1), respectively, Cysteine and cystine were simultaneously determined by measuring the absorbanc e at two fixed times, 3 min and Ih. Cysteine:cystine ratios between 1: 50 and 1:300 were determined with relative standard deviations of 1.2% for cysteine and 2.3% for cystine. The proposed method was applied fo r the determination of cysteine in pharmaceuticals.