Kinetic Study of the Scavenging Reaction of the Aroxyl Radical by Eight Kinds of Vegetable Oils in Solution

A detailed kinetic study was performed for the reaction of the aroxyl radical (ArO•) with eight vegetable oils 1–8, which contain different concentrations of α-, β-, γ-, and δ-tocopherols and -tocotrienols (-Tocs and -Toc-3s). The second-order rate constants (k_s) and aroxyl radical absorption capacity (ARAC) values for the reaction of ArO• with vegetable oils 1–8 (rice bran 1, perilla 2, rapeseed 3, safflower 4, grape seed 5, sesame 6, extra virgin olive 7, and olive oils 8) were measured in ethanol/chloroform/D₂O (50:50:1, v/v/v) solution at 25 °C using stopped-flow spectrophotometry. The k_s value (16.1 × 10⁻³ L g⁻¹ s⁻¹) of rice bran oil 1 with the highest activity was 8.0 times larger than that (2.02 × 10⁻³) of olive oil 8 with the lowest activity. The concentrations (in mg 100 g⁻¹) of α-, β-, γ-, and δ-Tocs and -Toc-3s contained in the vegetable oils 1–8 were determined using high performance liquid chromatography-mass spectrometry/mass spectrometry (HPLC-MS/MS). From these results, it was clarified that the ArO•-scavenging rates (k_s) (i.e., the relative ARAC value) obtained for the vegetable oils 1–8 may be well explained as the sum of the product (Formula presented.) of the rate constant ((Formula presented.)) and the concentration ([AOH-i]/10⁵) of AOH-i (Tocs and Toc-3s) included in vegetable oils. The results suggest that the ARAC assay method might be used in the evaluation of antioxidant activity of general food extracts.