Kinetic Study of the Quenching Reaction of Singlet Oxygen by Eight Vegetable Oils in Solution

A kinetic study of the reaction of singlet oxygen (¹O₂) with eight vegetable oils 1-8 containing different concentrations of tocopherols (Tocs) and tocotrienols (Toc-3s) was performed. The second-order rate constants (k_Q) for the reaction of ¹O₂ with vegetable oils 1-8 (rice bran, perilla, rape seed, safflower, grape seed, sesame, extra virgin olive, and olive oils) were measured in ethanol/chloroform/D₂O (50:50:1, v/v/v) solution at 35°C using UV-vis spectrophotometry. Furthermore, comparisons of k_Q values determined for the above oils 1-8 with the sum of the product {∑k_Q^AO-i [AO-i]/10⁵} of the k_Q^AO-i values obtained for each antioxidant (AO-i) and concentration (in mg/100 g) ([AO-i]/10⁵) of AO-i (Tocs and Toc-3s) contained in the oils 1-8 were performed. The observed k_Q values were not reproduced by the k_Q values calculated using only the concentrations of the four Tocs and Toc-3s. These results suggest that the contribution of fatty acids contained in the oils 1-8 is also necessary to fully explain the k_Q values. Recently, the second-order rate constants (k_S) for the reaction of aroxyl radical (ArO・) with the same vegetable oils 1-8 were measured in the same solvent at 25℃ using stopped-flow spectrophotometry (Ref. 23). The k_S values obtained could be well explained as the sum of the product {Σ k_S^AO-i [AO-i]/10⁵} of the k_S^AO-i and the [AO-i]/10⁵ of AO-i (Tocs and Toc-3s) contained in the vegetable oils.