Measurements of Singlet Oxygen-Quenching Activity of Vitamin E Homologs and Palm Oil and Soybean Extracts in a Micellar Solution

Recently, a new assay method that can quantify the singlet oxygen-absorption capacity (SOAC) of antioxidants (AO) and food extracts in homogeneous organic solvents has been proposed. In the present study, second-order rate constants (k(Q)) for the reaction of singlet oxygen (O-1(2)) with vitamin E homologs (-, -, -, and -tocopherols [Toc] and -, -, -, and -tocotrienols [Toc-3]) were measured in an aqueous Triton X-100 (5.0wt%) micellar solution (pH7.4). Toc-3 showed k(Q) values larger than those of Toc in a micellar solution, although Toc and Toc-3 showed the same k(Q) values in a homogeneous solution. Similar measurements were performed for 5 palm oil extracts 1-5 and one soybean extract 6, which contained different concentrations of Toc, Toc-3, and carotenoids. It has been clarified that the O-1(2)-quenching rates (k(Q)) (that is, the relative SOAC value) obtained for extracts 3-6 may be explained as the sum of the product kQAO--of the rate constant and the concentration ([AO-i]/100) of AO-i contained. The UV-vis absorption spectra of Toc and Toc-3 were measured in a micellar solution and chloroform. The results obtained demonstrated that the k(Q) values of AO in homogeneous and heterogeneous solutions vary notably depending on (1) polarity (dielectric constant [epsilon]) of the reaction field between O-1(2) and AO, (2) the local concentration of AO, and (3) the mobility of AO in solution. The results suggest that the SOAC method is applicable to the measurement of O-1(2)-quenching activity of general food extracts in a heterogeneous micellar solution.