Kinetics of parallel electron transfer from beta-carotene to phenoxyl radical and adduct formation between phenoxyl radical and beta-carotene

Phenoxyl radicals generated by laser flash photolysis were found to react with beta-carotene with concomitant beta-carotene bleaching in two parallel reactions with similar rates: (i) formation of a beta-carotene adduct with a (pseudo) first order rate constant of 1-1.5 . 10(4) s(-1) with absorption maximum around 800 nm, and (ii) formation of a beta-carotene radical cation with a (pseudo) first order rate constant of 2-3 . 10(4) s(-1) with absorption maximum around 920 nm. Both beta-carotene radicals decay on a similar time scale and have virtually disappeared after 100 ms, the beta-carotene adduct by a second order process. Oxygen had 170 effect on beta-carotene bleaching or radical formation and decay. The reduction of phenoxyl radicals by beta-carotene may prove important for an understanding of how beta-carotene acts as an antioxidant.