Kinetic study of the reaction of vitamin C with vitamin E radicals (tocopheroxyls) in solution.
New stable vitamin E radicals (7-tert-butyl-5-isopropyltocopheroxyl (4), 5,7-diisopropyltocopheroxyl (5), 7-tert-butyl-5-methyltocopheroxyl (6), and 5,7-diethyltocopheroxyl (7] with two bulky alkyl substituents at ortho positions (C-5 and C-7) have been prepared, and the reaction rates of vitamin C (ascorbic acid (1) and 6-O-stearyl ascorbic acid (2] with these tocopheroxyl radicals in benzene/ethanol/water (2:1:0.1, v/v) solution have been determined spectrophotometrically, using a stopped-flow technique. The second-order rate constants, k2, obtained vary in the order of 10(3), and decrease dramatically in the order 7 greater than 6 greater than 5 greater than 4, as the size of two ortho-alkyl groups in tocopheroxyl increases. The result suggests that the effect of steric hindrance on the reaction rate is considerable. These reaction rates were compared with those of vitamin C with alpha-tocopheroxyl reported by Packer et al. (Nature 278 (1979) 737-738) and Scarpa et al. (Biochim. Biophys. Acta 801 (1984) 215-219).
Mukai
K
K
Department of Chemistry, Faculty of Science, Ehime University, Matsuyama, Japan.
Nishimura
M
M
Ishizu
K
K
Kitamura
Y
Y
D001205:Ascorbic Acid; D055598:Chemical Phenomena; D002621:Chemistry; D005609:Free Radicals; D007700:Kinetics; D013053:Spectrophotometry; D014810:Vitamin E
D005609:Free Radicals; D014810:Vitamin E; D001205:Ascorbic Acid