The singlet oxygen and carotenoid interaction.

Second-order rate constants kQ for the quenching of O2(1 delta g) by carotenoids were determined at room temperature in benzene and toluene using the technique of time-resolved luminescence. Of the C40 pigments studied, lycopene was found to be the most efficient quencher, but the increased efficiency compared with all-trans beta-carotene was less than previously reported. The efficiency of quenching of O2(1 delta g) was extended to a number of solvents with varying viscosities. kQ was found to be inversely proportional to solvent viscosity, although the relationship is not simply linear. The results suggest the involvement of thermodynamic factors. The efficiency of deactivation of O2(1 delta g) was found to increase with the number of conjugated carbon-carbon double bonds, i.e. kQ(C60) greater than kQ(C50) greater than kQ(C40). A number of xanthophylls were included in this study; it would appear that an epoxide group rather than carbonyl or hydroxyl substituents increase the reactivity of the carotenoid with respect to O2(1 delta g).