Antioxidant capacity of reaction products limits the applicability of the Trolox Equivalent Antioxidant Capacity (TEAC) assay

The Trolox Equivalent Antioxidant Capacity (TEAC) assay is based on the scavenging of the 2,2'-azinobis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) radical (ABTS(*)) converting it into a colorless product. The degree of decolorization induced by a compound is related to that induced by trolox, giving the TEAC value. The assay is frequently used for constructing structure activity relationships (SARs). HPLC analysis of the reaction mixture, obtained after scavenging of ABTS(*) by the flavonoid chrysin, shows that a product is formed that also reacts with ABTS(*). The product has a higher antioxidant capacity and reacts faster with ABTS(*) than the parent compound, chrysin. In contrast to the reaction product of chrysin, the reaction product of trolox, which is formed during scavenging of ABTS(*), i.e. trolox quinone, does not react with ABTS(*). The experiments show that the TEAC is the antioxidant capacity of the parent compound plus the potential antioxidant capacity of the reaction product(s). This means that the TEAC assay does not necessarily reflect the antioxidant effect of only one structure. This hampers the applicability of the assay for the construction of SARs and for ranking antioxidants.