Kinetic Study of the Tocopherol Regeneration Reaction by Biological Hydroquinones in Micellar Solution

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 112; no. 3; pp. 448 - 456
• Main Authors: Itoh, Shingo, Nagaoka, Shin-ichi, Mukai, Kazuo
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 24.01.2008
• Subjects: Animals; Antioxidants - chemistry; Antioxidants - metabolism; Ascorbic Acid - chemistry; Ascorbic Acid - metabolism; Humans; Hydroquinones - chemistry; Hydroquinones - metabolism; Kinetics; Lipid Peroxidation - drug effects; Micelles; Molecular Structure; Solutions; Tocopherols - chemistry; Tocopherols - metabolism
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp075894p

The rate constants (k r) of the regeneration reaction of 7-t-butyl-5-isopropyltocopheroxyl with ubiquinol-10 (UQ10H2), ubiquinol-0 (UQ0H2), α-, β-, and γ-tocopherolhydroquinones (α-, β-, γ-TQH2), 2,3,5-trimethyl-1,4-hydroquinone (TMQH2), vitamin K3 hydroquinone (VK3H2), and vitamin C (Vit C) have been measured in 2-propanol/water and micellar solutions by a stopped-flow spectrophotometer. The k r values of these hydroquinones (HQs) in micellar solution remained constant at pHs of 6−9 and increased rapidly by increasing the pH value. The k r values decreased in the order of VK3H2 > γ-TQH2 ≥ α-TQH2 > β-TQH2 ≥ UQ10H2 ≥ TMQH2 > UQ0H2 >> Vit C at pHs of 6−9. These HQs are dibasic acids and can exist in three different molecular forms, depending on pH. By comparing the k r values with the mole fraction of each molecular form of the HQs, the reaction rate k r1 for the undissociated form, k r2 for the monoanion, and k r3 for the dianion and the pK a1 and pK a2 values were determined. It has been found that the k r values of UQ10H2, α-TQH2, β-TQH2, and γ-TQH2 (plastoquinol model) are 460, 1430, 494, and 1530 times larger than that of Vit C at pH 7.0, respectively, although the values are similar to that of Vit C in 2-propanol/water. The biological HQs and Vit C coexist in many tissues of animals and plants, and thus, the relative antioxidant activities of HQs and Vit C have been tentatively discussed based on the products of k r values by concentrations in several tissues. The results suggest that these HQs show high activity for the tocopherol regeneration in biological systems.