Antioxidant activities of vitamin E analogues in water and a Kamlet - Taft β-value for water

Rate constants for hydrogen atom abstraction from two water-soluble α-tocopherol (α-TOH, vitamin E) analogues by tert-butoxyl radicals have been measured at room temperature in four solvents, including water. The results imply that H-atom abstraction from α-TOH is 3.9 times slower in water than in benzene and yield a 'reliable' Kamlet-Taft β solvent parameter for water of 0.31. Literature rate constants for H-atom abstraction by peroxyl radicals from α-TOH in SDS micelles and in phospholipid micelles are, respectively, about 100 and 1000 times lower than in styrene. Since only a small fraction of the observed rate diminution can be attributed to hydrogen bonding of α-TOH to water it is concluded that in these heterogeneous systems much of the α-TOH is physically inaccessible to the attacking radicals. Whether this is also true for α-TOH in the biological membranes in living animals remains to be determined.