Kinetics of Parallel Electron Transfer from β-Carotene to Phenoxyl Radical and Adduct Formation Between Phenoxyl Radical and β-Carotene

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Abstract
Phenoxyl radicals generated by laser flash photolysis were found to react with β-carotene with concomitant β-carotene bleaching in two parallel reactions with similar rates: (i) formation of a β-carotene adduct with a (pseudo) first order rate constant of 1-1.5 ± 104 s-1 with absorption maximum around 800 nm, and (ii) formation of a β-carotene radical cation with a (pseudo) first order rate constant of 2-3 ± 104 s-1 with absorption maximum around 920 nm. Both β-carotene radicals decay on a similar time scale and have virtually disappeared after 100 ms, the β-carotene adduct by a second order process. Oxygen had no effect on β-carotene bleaching or radical formation and decay. The reduction of phenoxyl radicals by β-carotene may prove important for an understanding of how β-carotene acts as an antioxidant.