A critical appraisal of the use of the antioxidant capacity (TEAC) assay in defining optimal structures

In the Trolox Equivalent Antioxidant Capacity (TEAC) assay, i.e. the capacity of a compound to scavenge the ABTS radical (ABTS(circle)), is assessed. The aim of the present study is to evaluate the applicability of the TEAC assay to predict the antioxidant effectivity of a compound. For this purpose the TEAC assay is compared with other screening assays, such as superoxide scavenging, peroxynitrite scavenging and lipid peroxidation. Of the structurally related compounds, catechol, resorcinol and hydroquinone, resorcinol has the highest TEAC. In contrast, resorcinol appears to have a much lower antioxidant activity than catechol and hydroquinone in other in vitro assays. Similar discrepancies were observed with the flavonoids, chrysin and galangin. The TEAC values of chrysin and galangin are comparable, whereas galangin appears to be a much better antioxidant in other assays. The relatively high TEAC values of chrysin and resorcinol are due to the ability of the reaction products, formed by the reaction of the parent compound with ABTS(circle), to further react with ABTS(circle). With catechol, hydroquinone and galangin, these reaction products do not react with ABTS(circle) and therefore make no contribution to the TEAC. The possible contribution of reaction products to the TEAC of a compound hampers the use of the TEAC assay for constructing structure-activity relationships (SAR).