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Reaction kinetics in the Trolox equivalent antioxidant capacity (TEAC) assay between ABTS(+•) 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) cation radical and compounds with different structure, molecular weight, number of OH groups, and redox potential were investigated by recording loss of ABTS(+•) absorbance (734 nm) continuously over time. Curves showed six distinguishable kinetic patterns, including both immediate and extended reaction components. Radical quenching rates in the immediate component most relevant to reactions in foods and tissues depended on phenol structure and steric accessibility to the hindered radical, while reaction stoichiometry correlated with the number of phenol groups (>0.81) but not redox potential. Current assay procedures measure antioxidant capacity under conditions not relevant to actual applications and do not determine radical quenching rates. Results raise serious questions regarding the ability of reactions with the hindered ABTS(+•) to rank actual radical quenching by compounds with different structures and invalidate reporting antioxidant activity as Trolox equivalents.
Tian et al. (Thu,) studied this question.