IMPORTANCE OF CAROTENOID STRUCTURE IN RADICAL-SCAVENGING REACTIONS

The rate of (parallel) reaction between carotenoids and phenoxyl radic al to yield (i) carotenoid/ phenoxyl adducts and (ii) carotenoid radic al cations (in effect regenerating the phenol by reduction) has been f ound to increase for an increasing number of(coplanar) conjugated doub le bonds in the carotenoid and to decrease in the presence of hydroxy and especially keto groups. The reactions to yield the carotenoid radi cal have half-lives of tenths of milliseconds, and when studied by tra nsient visible and near-infrared absorption spectroscopy using nanosec ond laser flash photolysis for radical generation, the relative rates in di-tert-butyl peroxide/benzene (7/3, v/v) at 20 degrees C were lyco pene (1.66), beta-carotene (1), zeaxanthin (0.79), lutein (0.70), and echinenone (0.68), while canthaxanthin and beta-apo-8'-carotenal hardl y reacted and astaxanthin not at all. When formed, the carotenoid radi cals are rather stable (second-order decay on a millisecond time scale ). The lower the energy of the near-infrared transition in the radical cation, the more effective is the carotenoid as phenoxyl scavenger, a nd lycopene is concluded to be the more efficient antioxidant.