REAPPRAISAL OF THE TOCOPHEROXYL RADICAL REACTION WITH BETA-CAROTENE -EVIDENCE FOR OXIDATION OF VITAMIN-E BY THE BETA-CAROTENE RADICAL-CATION

Photobleached beta-carotene (Car) is regenerated in hexane on a micros econd timescale in the presence of alpha-tocopherol (TOH) but not when alpha-tocopherol is absent, as studied by laser flash photolysis. bet a-Carotene radical cations (Car(.+)) likewise react with (excess) alph a-tocopherol: Car(.+) + TOH --> Car + TO. + H+ (second-order rate cons tant of k = 1.7 +/- 0.1 . 10(7) M-1 s(-1) in homogeneous di-tert-butyl peroxide/benzene at 20 degrees C) rather than alpha-tocopheroxyl radic als (TO.) reacting with beta-carotene. In hexane, hexane radicals form ed 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-toco pherol mixtures resulting from alpha-tocopheroxyl radical oxidation of beta-carotene. Steady-state radiolysis experiments confirmed that alp ha-tocopherol protects beta-carotene from oxidation by hexane radicals . In both solvent systems, beta-carotene is regenerated from the radic al cation by alpha-tocopherol rather than alpha-tocopherol being regen erated by beta-carotene from the alpha-tocopheroxyl radical.