Structure−Activity Relationship of the Free-Radical-Scavenging Reaction by Vitamin E (α-, β-, γ-, δ-Tocopherols) and Ubiquinol-10:  pH Dependence of the Reaction Rates

• Published in: The journal of physical chemistry. B 2007-01, Vol.111 (3), p.652-662
• Main Authors: Mukai, Kazuo, Tokunaga, Aiko, Itoh, Shingo, Kanesaki, Yu, Ohara, Keishi, Nagaoka, Shin-ichi, Abe, Kouichi
• Format: Article
• Language: English
• Subjects: Animals; Ethanol - chemistry; Free Radical Scavengers - chemistry; Humans; Hydrogen-Ion Concentration; Kinetics; Mice; Micelles; Structure-Activity Relationship; Ubiquinone - analogs & derivatives; Ubiquinone - chemistry; Vitamin E - chemistry
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp0650580

The reaction rates (k s) of vitamin E (α-, β-, γ-, δ-tocopherols, TocH), ubiquinol-10, and related antioxidants (tocol, ubiquinol-0, and hydroquinone) with aroxyl (ArO·) radical have been measured in micellar solution by stopped-flow spectrophotometer. The k s values increased in the order of hydroquinone < tocol < δ-TocH < ubiquinol-0 < γ-TocH ≈ β-TocH < ubiquinol-10 < α-TocH at pH 4∼8. The antioxidants which have lower oxidation potentials showed higher reactivities. The k s values of α-, β-, γ-, δ-tocopherol, and tocol remained constant between pH 4 and 10, and decreased rapidly at pH 11∼12 by increasing pH value. From the pH dependence of k s values, the pK a values (= 13.1∼12.6) have been determined for these tocopherols. The k s values of ubiquinol-10 also remained constant between pH 4 and 9, and increased rapidly at pH 9.5. Ubiquinol-10 is dibasic acid and can exist in three different molecular forms, depending on the pH value. By comparing the k s values with the mole fraction of each molecular form of ubiquinol-10, the reaction rate k s1 (= 1.21 × 105 M-1s-1) for the undissociated form, k s2 (= 1.04 × 106 M-1s-1) for monoanion and k s3 (= 0 M -1s -1) for dianion, and the pK a1 and pK a2 values (= 11.4 and 12.7) were determined. The k s2 value is 8.6 times as large as the k s1 value. Similar analyses were performed for ubiquinol-0 and hydroquinone. It was found that the relative ratio of k s values (100:21:20:2.9) of α-, β-, γ-, δ-tocopherols in micellar dispersion has good correlation with the relative biopotency ratios for rat fetal resorption, rat hemeolysis, and chicken muscle dystrophy. The relative antioxidant activities of α-tocopherol and ubiquinol-10 have been discussed based on the k s values obtained and their concentrations in serum and several tissues (heart, muscle, liver, kidney, and brain).