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 (kQ ) 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 kQ values larger than those of Toc in a micellar solution, although Toc and Toc-3 showed the same kQ 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 (kQ ) (that is, the relative SOAC value) obtained for extracts 3-6 may be explained as the sum of the product <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mfenced><mml:mrow><mml:mi>Σ</mml:mi><mml:msubsup><mml:mi>k</mml:mi><mml:mi>Q</mml:mi><mml:mrow><mml:mi>AO</mml:mi><mml:mo>-</mml:mo><mml:mi>i</mml:mi></mml:mrow></mml:msubsup><mml:mfenced><mml:mrow><mml:mi>AO</mml:mi><mml:mo>-</mml:mo><mml:mi>i</mml:mi></mml:mrow></mml:mfenced><mml:mo>/</mml:mo><mml:mn>100</mml:mn></mml:mrow></mml:mfenced></mml:math> of the rate constant ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mi>k</mml:mi><mml:mi>Q</mml:mi><mml:mrow><mml:mi>AO</mml:mi><mml:mo>-</mml:mo><mml:mi>i</mml:mi></mml:mrow></mml:msubsup></mml:math> ) 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 kQ 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.