Reactive oxygen species are potentially damaging molecules. An important function of antioxidants is to intercept harmful triplet states, in order to prevent the formation of singlet oxygen, or to quench singlet oxygen directly. However, antioxidants are also reactive towards other active oxygen species such as the hydroxyl radical, the superoxide anion and the non-excited oxygen ground state in the presence of radical initiators. It is well known that¯avonoids and carotenoids show strong antioxidant properties. Polyenes and carotenoids are the best known among the compounds that quench singlet oxygen by ef®cient energy transfer. A large number of modi®ed, synthetic analogues and derivatives have been synthesised to prepare even better quenchers than the natural carotenoids. Phenols are also excellent chain-breaking antioxidants. Recently, many indigoid dyes (including bacterial indigoids) were studied, with the remarkable result that most, but not all, members of this class of chromophores quench singlet oxygen at the diffusion limit and some of them are excellent radical traps. It has been shown in this study that a quantitative assessment of antioxidant properties of avonoids, carotenoids, phenols and natural indigoids can be achieved using the following three assays: (1) oxygen pressure dependence; (2) peroxide formation; (3) singlet oxygen quenching. Reactivities towards both excited states and ground state radicals can be properly described by these assays. The remarkable role of b-carotene as an`unusual antioxidant' (Burton GW and Ingold KU, Science 224: 569±573 (1984)) in reactions using various oxygen pressures becomes clearer. The socalled`pro-oxidant effects' concern primarily the antioxidant itself and its degradation, since no or very little damage to the substrate occurs in this type of experiment. Three main categories of antioxidants may be classi®ed: (1) excellent antioxidants that perfectly quench excited states as well as ground state radicals (eg actinioerythrol, astaxanthin); (2) good antioxidants that strongly inhibit peroxide formation but are less ef®cient in quenching excited states (eg¯avonols, tocopherols) or lead to considerable degradation of the antioxidant itself (eg b-carotene, lycopene); (3) moderate antioxidants that fail to excel in both reactivities (eg z-carotene,¯avone).

Abstract

Effective chain length / Second-order quenching rate constantsThe bimolecular rate constants k, for quenching of singlet oxygen (lAS state) by 26 different natural and novel synthetic carotenoids were determined at 37 "C in a mixture of chloroform and ethanol. The steady-state technique used involves the generation of by thermal decomposition of disodium 3,3'-naphtalene-l,4-diyl-dipropionate endoperoxide (NDP02) and the detection of its luminescence intensity at 1270 nm.Excitation energies (xJ', llA, -+ llB,) and absorption maxima (430-590 nm) vary in the broadest range. Deeply coloured blue carotenoids are also included in the studies for the first time. An empirical correlation between the X,X* (llA, + llB,) excitation energy and carotenoid structure (effective chain length N,,,) was found: E ( S ) = 12642 cm-l + 92027 cm-l X l/Neff. The quenching ability of the investigated carotenoids depends on the excitation energy of their transition at long wavelengths in a characteristic way showing as limiting factors either the thermal Arrhenius activation or the diffusion-controlled rate. This dependence and the suspected relationship between singlet E ( S ) and triplet E(T) energies, respectively, are discussed.

Abstract. A family of¯uorescent styryl dyes was synthesized to apply them as probes that monitor the ion-translocating activity of the Na,K-ATPase and the SR Ca-ATPase, similar to the widely used dye RH421. All dyes had the same chromophore but they di ered in the length of the spacer between chromophore and polar head, an isothiocyanate group, and in the lengths of the two identical acyl chains, which form the tail of the dye molecules. A number of substrate-dependent partial reactions of both P-type ATPases a ected the¯uorescence intensity, and the magnitude of the¯uorescence changes was used to characterize the usefulness of the dyes for further application. The experimental results indicate that electrochromy is the major mechanism of these dyes. While in the case of the Na,K-ATPase a single dye, 5QITC, showed larger¯uorescence changes than all others, in the case of the SR Ca-ATPase all dyes tested were almost equal in their¯uorescence responses. This prominent di erence is interpreted as a hint that the position of the ion binding sites in both ion pumps may di er signi®cantly despite their otherwise closely related structural features. Quench experiments with spin-labeled lipids in various positions of their fatty acids were used to gain information on the depth of the chromophore of the di erent dyes within the membrane dielectric, however, the spatial resolution was so poor that only qualitative information on the position of the chromophore in the lipid phase could be obtained.

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