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

Abstract

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 (¹ O₂ ) with vitamin E homologs (α-, β-, γ-, and δ-tocopherols [Toc] and α-, β-, γ-, and δ-tocotrienols [Toc-3]) were measured in an aqueous Triton X-100 (5.0 wt%) micellar solution (pH 7.4). 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 ¹ O₂ -quenching rates (k_Q ) (that is, the relative SOAC value) obtained for extracts 3-6 may be explained as the sum of the product $\left(\Sigma k_Q^{\mathrm{AO}-i}\left(\mathrm{AO}-i\right)/100\right)$ of the rate constant ( $k_Q^{\mathrm{AO}-i}$ ) 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 [ε]) of the reaction field between ¹ O₂ 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 ¹ O₂ -quenching activity of general food extracts in a heterogeneous micellar solution.

Keywords: Kinetic study; Palm oil extracts; Quenching reaction; SOAC value; Singlet oxygen; Vitamin E homologs.