Free Radical Reactions with Alpha-Tocopherol and N-Stearoyl Tryptophan Methyl Ester in Micellar Solutions

Abstract

The rate constants have been measured for one-electron oxidation by N3. and Br2-. of N-stearoyl tryptophan methyl ester (STME) and alpha-tocopherol (alpha-T) in micelles of sodium dodecyl sulphate (SDS) or tetradecyl trimethyl ammonium bromide (TTAB). Compared with analogous reactions of tryptophan and Trolox C in aqueous solution, the rate constants for oxidation in micellar solution by N3. are reduced by 30-70%. The micellar charge increased the rate of oxidation of STME by Br2-. in TTAB micelles by almost an order of magnitude, compared with the reaction of Br2-. with tryptophan in aqueous solution. In SDS micelles the rate of oxidation of STME by Br2-. was reduced more than 30-fold. Quenching of fluorescence from STME in micelles by acrylamide confirmed the accessibility of the indole ring to the aqueous solvent. The rate of repair of the neutral STME radical by alpha-T in TTAB micelles was found to be accelerated by a factor of at least 27, compared with the similar reaction between Trolox C and tryptophan radicals in aqueous solution.