| Abstract
| - Diphenylpicrylhydrazyl (DPPH) is widely used for quickly assessing the ability of polyphenols to transferlabile H atoms to radicals, a likely mechanism of antioxidant protection. This popular test generallypays no attention to the kinetics of H atom transfer, which however could be even more importantthan the total H-atom-donating capacities (stoichiometry, EC50) typically evaluated. In the presentwork, a series of dietary polyphenols belonging to the most representative families (flavonols fromonion, flavanol monomers and oligomers from barley, and caffeic acid and caffeoyl esters fromartichoke and endive) are characterized not only by their total stoichiometries (ntot) but also by theirrate constants of first H atom abstraction by DPPH (k1), deduced from the kinetic analysis of thedecay of the DPPH visible band following addition of the antioxidant. The mildly reactive DPPH radicalallows a good discrimation between polyphenols, as demonstrated by the relatively large ranges ofk1 (ca. 400−5000 M-1 s-1) and ntot (ca. 1−5) values typically measured with antioxidants having asingle polyphenolic nucleus. With antioxidants displaying more than one polyphenolic nucleus(procyanidin oligomers, dicaffeoyl esters), the kinetic analysis makes it possible to demonstratesignificant differences in reactivity between the subunits (two distinct k1 values whose ratio lies in therange 3−10) and nonadditive stoichiometries. Keywords: Polyphenol; flavonol; flavanol; procyanidin; caffeic acid ester; antioxidant; DPPH; kinetics
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