Re-appraisal of the Tocopheroxyl Radical Reaction with β-Carotene: Evidence for Oxidation of Vitamin E by the β-Carotene Radical Cation

Abstract

Photobleached β-carotene (Car) is regenerated in hexane on a microsecond timescale in the presence of α-tocopherol (TOH) but not when α-tocopherol is absent, as studied by laser flash photolysis. β-Carotene radical cations (Car⁺) likewise react with (excess) α-tocopherol: Car⁺ TOH→ Car + TO· + H⁺ (second-order rate constant of k = 1.7 ± 0.1 · 10⁷ M^-1 s^-1 in homogeneous di-tert-butylperoxide/ benzene at 20°C) rather than α-tocopheroxyl radicals (TO·) reacting with β-carotene. In hexane, hexane radicals formed by pulse radiolysis react considerably faster with β-carotene (k = 2.1 ± 0.1 · 10⁹ M^-1 s^-1) than with α-tocopherol (k = 4.9 ± 0.1 · 10⁶ M^-1 s^-1). No evidence was obtained for a slower rate of β-carotene radical cation formation in β-carotene/α-tocopherol mixtures resulting from α-tocopheroxyl radical oxidation of β-carotene. Steady-state radiolysis experiments confirmed that α-tocopherol protects β-carotene from oxidation by hexane radicals. In both solvent systems, β-carotene is regenerated from the radical cation by α-tocopherol rather than α-tocopherol being regenerated by β-carotene from the α-tocopheroxyl radical.

Key Words:

• β-carotene
• α-tocopherol
• β-carotene radical cation
• α-tocopheroxyl radical
• laser flash photolysis
• pulse radiolysis