Reaction of Ascorbate with the α-Tocopheroxyl Radical in Micellar and Bilayer Membrane Systems

The reaction by which the antioxidant capacity of α-tocopherol is sustained by recycling of the α-tocopheroxyl radical in membranes or lipoproteins by aqueous ascorbate has been studied by laser flash photolysis in model micellar and membrane systems. In bilayers of dimyristoylphosphatidylcholine at 35°C the measured second-order rate constant was 3 × 105 M−1 s−1, or about five times slower than previously reported in a solvent system. The rate of reaction was decreased on addition of negatively charged lipid (dipalmitoylphosphatidic acid) and increased by addition of positively charged lipid (didodecyldimethylammonium bromide). These effects of bilayer charge were suppressed by increasing the ionic strength of the aqueous medium. Micellar charge also had an effect on the pH dependence of the reaction rate. Arrhenius data showed that the enthalpy of activation was effectively zero for the reaction in solution between ascorbate and radicals of water-soluble tocopherol analogues, but was positive in membrane and micellar systems. In all cases the entropy of activation was strongly negative. The kinetic deuterium isotope ratio varied between 3 and 8. The data strongly support a concerted electron and proton transfer mechanism for the reaction between α-tocopheroxyl radical and ascorbate.