Stopped - flow Investigation of the Reaction of Vitamin C with Tocopheroxyl Radical in Aqueous Triton X - 100 Micellar Solutions

A kinetic study of the reaction between vitamin C (Lascorbic acid, A S H ~ ) and a tocopheroxyl radical (7tert-butyl-6-isopropyltocopheroxyl) in Triton X-100 micellar solution has been performed using stoppedflow spectrophotometry. The second-order rate constants (k,) obtained showed notable pH dependence with a broad maximum around pH 8. For instance, the kz values obtained were 26 M” s-l at pH 3,322 M” s-l at pH 7, and 273 M” s” at pH 10. A good correlation between the rate constants and the mole fraction of ascorbate monoanion (ASH-) was observed, showing that ascorbate (ASH-) can regenerate the tocopherol from tocopheroxyl in biological systems. Furthermore, the results indicate that reduced ascorbic acid (ASH,) does not have the ability to regenerate the tocopherol in aqueous solution. On the other hand, it was found that AsHz can reduce the tocopheroxyl to tocopherol in benzene/ethanol (2:l) mixtures, although the rate of reaction is only -15% of that observed in micellar solution at pH 7.