Cyclodextrins as Secondary Antioxidants:  Synergism with Ascorbic Acid

The preservation of the antioxidant capacity of ascorbic acid by 2-hydroxypropyl-β-cyclodextrins has been studied. For this, a model system consisting of ascorbic acid, H2O2, a phenolic compound, 2,2,5,7,8-pentamethylchroman-6-ol (PMC), and commercial soybean lipoxygenase was used. In this system, ascorbic acid was oxidized by PMC radicals generated by the hydroperoxidative activity of lipoxygenase, giving rise to a biphasic curve, in which the inflection point represents the time at which all the ascorbic acid has been consumed (tip). This tip depends on the enzyme, ascorbic acid, and PMC concentration. When 2-hydroxypropyl-β-cyclodextrins were included in the system, the tip increased, indicating that these cyclodextrins were acting as secondary antioxidants, so preserving the antioxidant capacity of the system. A mathematical model to explain the effect of cyclodextrins on ascorbic acid oxidation in this experimental system is described, giving a constant, Kc, which represents the preservation capacity of a particular antioxidant system produced by cyclodextrins. Keywords: Ascorbic acid; cyclodextrins; lipoxygenase; hydroperoxidase; PMC