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All-trans beta-carotene radical anion efficiently transfers an electron to oxygen but the reverse reaction is not observed and, instead we suggest the formation of a beta-carotene-superoxide radical addition complex. On the other hand, all-trans lycopene undergoes a reversible electron transfer with the superoxide radical. This distinctive behaviour may be related to the anti-cancer properties of these molecules.

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The reactivity of carotenoids with ROS including singlet oxygen (102), superoxide radical anions (02.-and cc1302.) is reported. The effects of environment are marked, thus while lycopene reacts with 1 0 2 somewhat faster ( 1 . 6~) than pcarotene in simple solvents the trend is reversed in micellar systems (0.3~). In fact, for the twelve carotenoids studied in detergents, the reactivity depends mainly on the size of the carotenoid. In cellular systems the highest protection of the cell from photo-oxidative damage was given by lycopene followed by astaxanthin then p-carotene with only a small protection by canthaxanthin. All twelve carotenoids studied react efficiently with 1 0 2 (k=1-20 x 109 M-1s-1) in simple solvents and micelles while the reactivity with oxy-radicals is quite different. Thus, for example, &carotene reacts with 02.-to form an addition complex whereas with lycopene reversible electron transfer is observed.Consistent with such marked differences pcarotene--readily transfers an electron to lycopene. Carotenoids also react with the oxy-radical from carbon tetrachloride (cc1302.) to yield radical cations plus another product which may be an addition complex analogous to that produced with 02.-.Abbreviations used : Reactive oxy-species = ROS