Importance of Carotenoid Structure in Radical-Scavenging Reactions

Abstract

The rate of (parallel) reaction between carotenoids and phenoxyl radical to yield (i) carotenoid/phenoxyl adducts and (ii) carotenoid radical cations (in effect regenerating the phenol by reduction) has been found to increase for an increasing number of (coplanar) conjugated double bonds in the carotenoid and to decrease in the presence of hydroxy and especially keto groups. The reactions to yield the carotenoid radical have half-lives of tenths of milliseconds, and when studied by transient visible and near-infrared absorption spectroscopy using nanosecond laser flash photolysis for radical generation, the relative rates in di- tert -butyl peroxide/benzene (7/3, v/v) at 20 °C were lycopene (1.66), β-carotene (1), zeaxanthin (0.79), lutein (0.70), and echinenone (0.68), while canthaxanthin and β-apo-8‘-carotenal hardly reacted and astaxanthin not at all. When formed, the carotenoid radicals 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, and lycopene is concluded to be the more efficient antioxidant. Keywords: β-Carotene; lycopene; lutein; zeaxanthin; echinenone; β-apo-8‘-carotenal; canthaxanthin; astaxanthin; phenoxyl radical; antioxidant synergism; carotenoid radicals