Relative stability of carotenoid radical cations and homologue tocopheroxyl radicals. A real time kinetic study of antioxidant hierarchy

Real time detection following laser flash photolysis of transient carotenoid radical cations and tocopheroxyl radicals formed in chloroform and bleaching of the carotenoids has allowed interaction between carotenoids and tocopherols to be studied. It is found that alpha-, beta-, and gamma-tocopherol reduce all the carotenoid radical cations investigated whereas the delta-tocopheroxyl radical can be reduced by lycopene and beta-carotene. Astaxanthin, canthaxanthin, and beta-apo-8'-carotenal radical cations are scavenged rapidly by all four tocopherol homologues whereas the other carotenoid radical cations react much more slowly with the tocopherols. The results allow the antioxidant hierarchy to be established: alpha-tocopherol > lycopene approximately beta-tocopherol approximately gamma-tocopherol > beta-carotene > zeaxanthin approximately delta-tocopherol > lutein > echinenone >> canthaxanthin approximately beta-apo-8'-carotenal > astaxanthin.