Kinetics and mechanism of regeneration of β-carotene from tert-butoxyl radical induced β-carotene radical cation by α-tocopherol: A synergistic interaction

G Vijayalakshmi, M Adinarayana

Abstract

The rates of oxidation of α -tocopherol and β -carotene by tert-butoxyl radicals (t-BuO • ) were studied spectrophotometrically. Radicals (t-BuO • ) were generated by the photolysis of tert-butyl hy droperoxide (t-BuOOH) in presence of tert-butyl alcohol to scavenge • OH radicals. The rates and the quantum yields ( φ ) of oxidation of α - tocopherol by t-BuO • radicals were determined in the absence and presen ce of varying concentrations of β - carotene. An increase in the concentration of β -carotene was found to decrease the rate of oxidati on of α - tocopherol, suggesting that β -carotene and α -tocopherol competed for t-BuO • radicals. From competition kinetics, the rate constant of β -carotene reaction with t-BuO • was calculated to be 5.54 × 10 8 dm 3 mol -1 s -1 . The quantum yields φ expt and φ cal values suggested that α -tocopherol not only protected β -carotene from t-BuO • radicals, but also repaired β -carotene radicals, formed by the reaction of β -carotene with t-BuO • radicals.