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 α-tocopherol (α-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 α-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.

Key Words:

• α-Tocopherol
• free radicals
• tryptophan
• pulse radiolysis
• electron transfer
• fluorescence quenching