A RAPID SCREENING-TEST TO DETERMINE THE ANTIOXIDANT POTENCIES OF NATURAL AND SYNTHETIC ANTIOXIDANTS

This paper presents a simple and convenient method for measuring antioxidant efficiencies in a model system consisting of micelles of sodium dodecyl sulfate (SDS) with added linoleic acid. The analytical method involves following the development of absorption at 234 nm due to the conjugated diene hydroperoxide of linoleic acid; 2,2'-azobis(2-amidinopropane) dihydrochloride (ABAP) is used as the initiator. The antioxidant efficiency of an antioxidant is defined as AE = k(inh)/k(p), where k(inh) is the rate constant for reaction of the peroxyl radical from linoleic acid with the antioxidant (eq 7 in the text) and k(p) is the propagation rate constant for autoxidation of linoleic acid (eq 5). The relative antioxidant efficiency (RAE) is defined as the AE of the test compound divided by that for alpha-tocopherol; thus, RAE values are equal to k(inh)/k(inh,alpha-tocopherol). Since the absolute value of the rate constant for reaction of alpha-tocopherol with peroxyl radicals is known in micellar systems, the absolute value of k(inh) for a given antioxidant can be calculated if required. To establish the reliability of the method we report the AE of 17 synthetic and naturally occurring antioxidants and compare values obtained by this method to others in the literature. For neutral antioxidants, the same AE values are obtained in both negatively-charged micelles of SDS and positively-charged micelles of hexadecyl trimethylammonium bromide (HDTBr). The relative antioxidant efficiencies are reported for more than 40 synthetic compounds that are potential antioxidants or anti-inflammatory compounds; most of these compounds are tocopherol, ascorbate, or 2-hydroxytetronic acid analogues. Although the charge type of the micelle has no effect on the RAE of neutral antioxidants, micelles of different charge types do have an effect on the RAE value of charged antioxidants such as ascorbic acid, other 2-hydroxytetronic acids, and Trolox C. In 0.10 M SDS micelles, ascorbic acid and alpha-tocopherol are shown to have an additive effect, not a synergistic effect, as antioxidants.