Development of a Singlet Oxygen Absorption Capacity (SOAC) Assay Method. Measurements of the SOAC Values for Carotenoids and α‑Tocopherol in an Aqueous Triton X‑100 Micellar Solution

• Published in: Journal of agricultural and food chemistry Vol. 65; no. 4; pp. 784 - 792
• Main Authors: Mukai, Kazuo, Ouchi, Aya, Azuma, Nagao, Takahashi, Shingo, Aizawa, Koichi, Nagaoka, Shin-ichi
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 01.02.2017
• Subjects: alpha-Tocopherol - chemistry; Antioxidants - chemistry; Carotenoids - chemistry; Kinetics; Micelles; Octoxynol - chemistry; Singlet Oxygen - chemistry; Spectroscopy, Near-Infrared - methods; singlet oxygen; carotenoids; endoperoxide; kinetic study; quenching rate; micellar solution; SOAC value; α-tocopherol
• ISSN: 0021-8561; 1520-5118
• DOI: 10.1021/acs.jafc.6b04329

Recently, a new assay method for the quantification of the singlet oxygen absorption capacity (SOAC) of antioxidants (AOs) and food extracts in homogeneous organic solvents was proposed. In this study, second-order rate constants (k Q) for the reaction of singlet oxygen (1O2) with eight different carotenoids (Cars) and α-tocopherol (α-Toc) were measured in an aqueous Triton X-100 (5.0 wt %) micellar solution (pH 7.4, 35 °C), which was used as a simple model of biomembranes. The k Q and relative SOAC values were measured using ultraviolet–visible (UV–vis) spectroscopy. The UV–vis absorption spectra of Cars and α-Toc were measured in both a micellar solution and chloroform, to investigate the effect of solvent on the k Q and SOAC values. Furthermore, decay rates (k d) of 1O2 were measured in 0.0, 1.0, 3.0, and 5.0 wt % micellar solutions (pH 7.4), using time-resolved near-infrared fluorescence spectroscopy, to determine the absolute k Q values of the AOs. The results obtained demonstrate that the k Q values of AOs in homogeneous and heterogeneous solutions vary notably depending on (i) the polarity [dielectric constant (ε)] of the reaction field between AOs and 1O2, (ii) the local concentration of AOs, and (iii) the mobility of AOs in solution. In addition, the k Q and relative SOAC values obtained for the Cars in a heterogeneous micellar solution differ remarkably from those in homogeneous organic solvents. Measurements of k Q and SOAC values in a micellar solution may be useful for evaluating the 1O2 quenching activity of AOs in biological systems.