The purpose of this study was to assess the relative antioxidant activities of a range of carotenes and xanthophylls through the extent of their abilities to scavenge the ABTS "+ radical cation. The results show that the relative abilities of the carotenoids to scavenge the ABTS "÷ radical cation are influenced by the presence of functional groups with increasing polarities, such as carbonyl and hydroxyl groups, in the terminal rings, as well as by the number of conjugated double bonds.

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In 1876, Fenton described a colored product obtained on mixing tartaric acid with hydrogen peroxide and a low concentration of a ferrous salt. Full papers in 1894 and 1896 showed the product was dihydroxymaleic acid. Haber, Weiss and Willstätter proposed in 1932-1934 the involvement of free hydroxyl radicals in the iron(II)/hydrogen peroxide system, and Baxendale and colleagues around 1950 suggested that superoxide reduces the iron(III) formed on reaction, explaining the catalytic nature of the metal. Since Fridovich and colleagues discovered the importance of superoxide dismutase in 1968, numerous studies have sought to explain the deleterious effects of cellular oxidative stress in terms of superoxide-driven Fenton chemistry. There remain questions concerning the involvement of free hydroxyl radicals or reactions of metal/oxo intermediates. However, these outstanding questions may obscure a wider appreciation of the importance of Fenton chemistry involving hypohalous acids rather than hydrogen peroxide as the oxidant.

dynamics are predominantly governed by the homolytic dissociation energy of the +'C-H bond, although other factors such as solvent reorganization should be taken into account to a minor extent.Acknowledgment. This work has been jointly supported by the CNRS, the Office of Basic Energy Sciences of the US Department of Energy. P.H. thanks the NIST for allowing him to carry out the laser flash photolysis experiments and Dr. R. E. Huie for his help in using the laser system.

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