Reactivity of β-carotene towards peroxyl radicals studied by laser flash and steady-state photolysis

Abstract

Peroxyl radicals, as model for peroxyl radicals formed during autoxidation of lipids, have been generated in three solvent systems (cyclohexane, tetrahydrofuran and tert-butanol/water) by steady-state and laser flash photolysis, and their reaction with β-carotene studied. Steady-state photolysis experiments showed that alkyl, alkoxyl and alkylperoxyl radicals all react with β-carotene. However, laser flash photolysis experiments indicated that the reaction with peroxyl radicals (second-order rate constant estimated to be less than 10 6 M −1 s −1) is slower than with alkyl and alkoxyl radicals, and that β-carotene is hence a poor direct scavenger of peroxyl radicals. Scavenging of peroxyl radicals by β-carotene is suggested not to proceed via electron transfer but rather by adduct formation and/or hydrogen abstraction. For different phenoxyl radicals, differences in reactivity towards β-carotene seem to be correlated with standard reduction potential.