A kinetic study of the quenching reaction of singlet oxygen (1O2) with eight kinds of carotenoids and α-tocopherol was performed in ethanol/chloroform/D2O (50:50:1, v/v/v) solution at 35 °C. The overall rate constants, kQ (=kq+kr, physical quenching+chemical reaction), for the reaction of carotenoids with 1O2 were measured, using the competition reaction method, where endoperoxide was used as a singlet oxygen generator, 2,5-diphenyl-3,4-benzofuran (DPBF) as an UV-vis absorption prove, and α-tocopherol as a standard compound. The rate constants, kQ (S) and kQ (t1/2), were determined by analyzing the first-order rate constant (S) and the half-life (t1/2) of the decay curve of DPBF with carotenoids, respectively, showing good accordance with each other. Similar measurements were performed for tomato and carrot extracts. From the results, a new assay method that can quantify the singlet oxygen absorption capacity (SOAC) of antioxidants, including carotenoids, α-tocopherol, and vegetable extracts, has been proposed.

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The reaction rates (ks) of vitamin E (alpha-, beta-, gamma-, delta-tocopherols, TocH), ubiquinol-10, and related antioxidants (tocol, ubiquinol-0, and hydroquinone) with aroxyl (ArO(.-)) radical have been measured in micellar solution by stopped-flow spectrophotometer. The ks values increased in the order of hydroquinone < tocol < delta-TocH < ubiquinol-0 < gamma-TocH approximately beta-TocH < ubiquinol-10 < alpha-TocH at pH 4 approximately 8. The antioxidants which have lower oxidation potentials showed higher reactivities. The ks values of alpha-, beta-, gamma-, delta-tocopherol, and tocol remained constant between pH 4 and 10, and decreased rapidly at pH 11 approximately 12 by increasing pH value. From the pH dependence of ks values, the pKa values (= 13.1 approximately 12.6) have been determined for these tocopherols. The ks values of ubiquinol-10 also remained constant between pH 4 and 9, and increased rapidly at pH 9.5. Ubiquinol-10 is dibasic acid and can exist in three different molecular forms, depending on the pH value. By comparing the ks values with the mole fraction of each molecular form of ubiquinol-10, the reaction rate ks1 (= 1.21 x 10(5) M(-1)s(-1)) for the undissociated form, ks2 (= 1.04 x 10(6) M(-1)s(-1)) for monoanion and ks3 (= 0 M(-1)s(-1)) for dianion, and the pKa1 and pKa2 values (= 11.4 and 12.7) were determined. The ks2 value is 8.6 times as large as the ks1 value. Similar analyses were performed for ubiquinol-0 and hydroquinone. It was found that the relative ratio of ks values (100:21:20:2.9) of alpha-, beta-, gamma-, delta-tocopherols in micellar dispersion has good correlation with the relative biopotency ratios for rat fetal resorption, rat hemeolysis, and chicken muscle dystrophy. The relative antioxidant activities of alpha-tocopherol and ubiquinol-10 have been discussed based on the ks values obtained and their concentrations in serum and several tissues (heart, muscle, liver, kidney, and brain).

Recently a new assay method that can quantify the singlet oxygen absorption capacity (SOAC) of antioxidants was proposed. In the present work, kinetic study of the reaction of singlet oxygen ((1)O(2)) with carotenoids and vegetable extracts has been performed in ethanol/chloroform/D(2)O (50:50:1, v/v/v) solution at 35 °C. Measurements of the second-order rate constants (k(Q)(S)) and the SOAC values were performed for eight kinds of carotenoids and three kinds of vegetable extracts (red paprika, carrot, and tomato). Furthermore, measurements of the concentrations of the carotenoids included in vegetable extracts were performed, using a HPLC technique. From the results, it has been clarified that the total (1)O(2)-quenching activity (that is, the SOAC value) for vegetable extracts may be explained as the sum of the product {Σ k(Q)(Car-i)(S) [Car-i](i)} of the rate constant (k(Q)(Car-i)(S)) and the concentration ([Car (i)]) of carotenoids included in vegetable extracts.