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In this paper, we report the results of a laser flash photolysis study of the reactions of a range of carotenoids with acylperoxyl radicals in polar and nonpolar solvents. The results show, for the first time, that carotenoid addition radicals do not react with oxygen to form carotenoid peroxyl radicals; an observation which is of significance in relation to antioxidant/pro-oxidant properties of carotenoids. Acylperoxyl radicals, generated by photolysis of ketone precursors in oxygenated solvents, display high reactivity toward carotenoids in both polar and nonpolar solvents, but the nature of the carotenoid radicals formed is dependent on solvent polarity. In hexane, acylperoxyl radicals react with carotenoids with rate constants in the region of 10(9) M(-1) s(-1) and give rise to transient absorption changes in the visible region that are attributed to the formation of addition radicals. All of the carotenoids show bleaching in the region of ground-state absorption and, with the exception of 7,7'-dihydro-beta-carotene (77DH), no distinct absorption features due to addition radicals are observed beyond the ground state absorption region. For 77DH, the addition radical displays an absorption band that is spectrally resolved from the parent carotenoid absorption. The rate of decay of the 77DH addition radical is unaffected by oxygen in the concentration range 10(-4)-10(-2) M, suggesting that these resonance-stabilized carbon-centered radicals are not scavenged by oxygen. At low incident laser intensities, the 77DH addition radical decay kinetics are 1st order with k(1) approximately 4 x 10(3) s(-1) at room temperature. The 1st order decay is attributed to an intramolecular cyclization process, which is supported by the substantial negative entropies of activation obtained from measurements of the decay rate constants for different 77DH addition radicals as a function of temperature. No transient absorption features are observed in the red or near-infrared regions in hexane for any of the carotenoids studied. In polar solvents such as methanol, acylperoxyl radicals also react with carotenoids with rate constants in the region of 10(9) M(-1) s(-1), but give rise to transient absorption changes in both the visible and the red/near-infrared regions, where it is evident that there are two distinct species. For 77DH, the addition radical absorption around 450 nm is still evident, although its kinetic behavior differs from its behavior in hexane. For 77DH and zeta-carotene (zeta-CAR) the spectral and kinetic resolution of the various absorption bands simplifies kinetic analysis. The kinetic evidence suggests that addition radical formation precedes formation of the two near-infrared absorbing species, and that the kinetics of the addition radical decay match the kinetics of formation of the first of these species (NIR1, absorbing at shorter wavelengths). The decay of NIR1 leads to NIR2, which is attributed to the carotenoid radical cation. The solvent dielectric constant dependence of the relative amounts of NIR1 and NIR...

Direct observation of reversible oxygen addition to a carotenoid-derived carbon-centered neutral radical is reported for the first time. The influence of temperature on the observed reaction kinetics has been used to obtain kinetic and thermodynamic parameters relating to the reversible addition of oxygen to the carotenoid radical obtained from reaction of 7,7'-dihydro-beta-carotene (77DH) with phenylthiyl radical (PhS.) in benzene. In addition, the rate constant for oxygen addition to the equivalent beta-carotene (beta-CAR) derived radical is also reported. [reaction: see text]

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Carotenoids play diverse roles in biology and medicine. Both the quenching of singlet oxygen (energy transfer) and interaction with oxy-radicals (electron transfer, H-atom transfer and addition reactions) are key processes in understanding many of these roles. Much previous work in 'simple' solvents is reviewed and new results in cell membrane models are presented. The possible consequences of using carotenoids as dietary supplements are discussed.