Re-appraisal 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 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 x 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 x 10(9) M(-1) s(-1)) than with alpha-tocopherol (k = 4.9 +/- 0.1 x 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.
Mortensen
A
A
Department of Dairy and Food Science, Royal Veterinary and Agricultural University, Frederiksberg C, Denmark.
Skibsted
L H
LH
Willnow
A
A
Everett
S A
SA
D002412:Cations; D005609:Free Radicals / Q000737:chemistry*; D006586:Hexanes / Q000737:chemistry; D010084:Oxidation-Reduction; D010782:Photolysis; D011675:Pulse Radiolysis; D014810:Vitamin E / Q000737:chemistry*; D019207:beta Carotene / Q000737:chemistry*
D002412:Cations; D005609:Free Radicals; D006586:Hexanes; D019207:beta Carotene; C066323:tocopheroxy radical; D014810:Vitamin E