Abstract
A kinetic study of the quenching reaction of singlet oxygen (1O2) with 25 natural antioxidants (including 8 carotenoids, 6 tocopherol derivatives, ubiquinol-10, 3 caffeic acids, 6 catechins, and vitamin C) was performed in ethanol/chloroform/D2O (50:50:1, v/v/v) and ethanol solutions at 35°C. The second-order rate constants, kQ, for the reaction of antioxidants 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 was used 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 antioxidants, respectively, showing good accordance with each other. The method is useful to evaluate the 1O2-quenching activity of lipophilic- and hydrophilic-antioxidants having five orders of magnitude different rate constants from 1.38×1010 M－1s－1 for lycopene to 2.71×105 for ferulic acid. Similar measurements were performed for 30 kinds of food extracts including vegetable, fruit, and edible oil extracts. From the results, a new assay method which can quantify the singlet oxygen absorption capacity (SOAC) of natural antioxidants and food extracts has been proposed.
