REACTIVITY OF BETA-CAROTENE TOWARDS PEROXYL RADICALS STUDIED BY LASERFLASH AND STEADY-STATE PHOTOLYSIS

Peroxyl radicals, as model for peroxyl radicals formed during autoxida tion of lipids, have been generated in three solvent systems (cyclohex ane, tetrahydrofuran and tert-butanol/water) by steady-state and laser flash photolysis, and their reaction with beta-carotene studied. Stea dy-state photolysis experiments showed that alkyl, alkoxyl and alkylpe roxyl radicals all react with beta-carotene. However, laser flash phot olysis 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 beta-carotene is hence a poor direct scavenger of peroxyl radicals. Scavenging of p eroxyl radicals by beta-carotene is suggested not to proceed via elect ron transfer but rather by adduct formation and/or hydrogen abstractio n. For different phenoxyl radicals, differences in reactivity towards beta-carotene seem to be correlated with standard reduction potential. (C) 1998 Federation of European Biochemical Societies.