Isolation and the characterization of the degradation products of the mediator ABTS-derived radicals formed upon reaction with polyphenols

Two degradation products were obtained from the incubation of the widely used 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid), ABTS, radical cations with the polyphenols, (+)-catechin, (−)-epicatechin, and phloroglucinol in acetate buffer (pH 5). The products were purified by reversed-phase chromatography and characterized by UV–visible detection, mass spectrometry, and 1H NMR spectroscopy. The data allowed us to identify the degradation products as 3-ethyl-6-sulfonate benzothiazolinone imine and the corresponding sulfoxide, 3-ethyl-6-sulfonate benzothiazolone. Elemental composition strongly supported the proposed structures. Our results unequivocally demonstrated that ABTS radicals are not as stable as usually claimed because they could be degraded upon interaction with polyphenols, in addition to being reduced by these antioxidants back to the parent compound. Therefore, it is concluded that caution must be exercised in using ABTS radicals as a basis for the evaluation of antioxidant capacities of pure compounds and/or complex mixtures.