Astaxanthin radicals cations

CCI3O2 reacts with ascorbic and uric acid [71], as well as bilirubin [72] and glutathione [73] via electron transfer. However, with tryptophan and carotenoids another reaction also occurs, suggested to be radical addition [74, 75]. For the carotenoids the proposed adduct decays to yield more radical cation and for the carotenoid, astaxanthin, the radical cation is not formed initially but is formed solely through the proposed addition radical [75]. The one electron reduction potential of astaxanthin radical cation has been shown to be higher than several other carotenoids [76], so it may be that it is very close to that of CCI3O2 so that electron transfer is very slow. 

One possible mechanism responsible for cooperative action of antioxidants is reduction of a semi-oxidized carotenoid by another antioxidant. Carotenoid cation radicals can be reduced, and therefore recycled to the parent molecule, by a-tocopherol, ascorbate, and melanins (Edge et al., 2000b El-Agamey et al., 2004b) (Figure 15.5). Interestingly, lycopene can reduce radical cations of other carotenoids, such as astaxanthin, (3-carotene, lutein, and zeaxanthin (Edge et al., 1998). 

Fig. 1. Electron transfer from the radical cation of astaxanthin to lycopene measured by absrobance (A).

Radical cations of astaxanthin (17) were generated by oxidation with FeCl3at 150 K [110]. 

Although carotene radical cations are too short-lived to determine their reversible reduction potentials directly, the (relative ease( of electron transfer for seven carotenoids has been determined by pulse radiolysis. In the series astaxanthin > 3-apo-8-8 -carotenal > canthaxantin > lutein > zeaxanthin > 3-carotene > lycopene, lycopene is the strongest reductant. It can, for example, reduce lutein cation radicals to lutein, whereas P-carotene cannot (Edge et al., 1998). 

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