Re-appraisal of the tocopheroxyl radical reaction with β-carotene:: Evidence for oxidation of vitamin E by the β-carotene radical cation

Photobleached beta-carotene (Car) is regenerated in hexane on a microsecond timescale in the presence of alpha-tocopherol (TOH) but not when alpha-tocopherol is absent, as studied by laser flash photolysis. beta-Carotene radical cations (Car(.+)) likewise react with (excess) alpha-tocopherol: Car(.+) + TOH --> Car + TO. + H+ (second-order rate constant of k = 1.7 +/- 0.1 . 10(7) M-1 s(-1) in homogeneous di-tert-butylperoxide/benzene at 20 degrees C) rather than alpha-tocopheroxyl radicals (TO.) reacting with beta-carotene. In hexane, hexane radicals formed by pulse radiolysis react considerably faster with beta-carotene (k = 2.1 +/- 0.1 . 10(9) M-1 s(-1)) than with alpha-tocopherol (k = 4.9 +/- 0.1 . 10(6) M-1 s(-1)). No evidence was obtained for a slower rate of beta-carotene radical cation formation in beta-carotene/alpha-tocopherol mixtures resulting from alpha-tocopheroxyl radical oxidation of beta-carotene. Steady-state radiolysis experiments confirmed that alpha-tocopherol protects beta-carotene from oxidation by hexane radicals. In both solvent systems, beta-carotene is regenerated from the radical cation by alpha-tocopherol rather than alpha-tocopherol being regenerated by beta-carotene from the alpha-tocopheroxyl radical.