Abstract
The reaction of a 2,2-diphenyl-1-picrylhydrazyl radical (dpph˙) with phenols carried out in alcohols is a frequently used assay for estimation of the antiradical activity of phenolic compounds. The rates of reactions of dpph˙ with five phenols (ArOH: unsubstituted phenol, 4-hydroxyacetophenone, two calix[4]resorcinarenes and baicalein) measured in methanol indicate the different kinetics of the process for very diluted phenols compared to their non-diluted solutions. This effect was explained as dependent on the ratio [ArO−]/[ArOH] and for diluted ArOH corresponds to an increased contribution of much faster electron transfer (ET, ArO−/dpph˙) over the Hydrogen Atom Transfer (HAT, ArOH/dpph˙). Simplified analysis of the reaction kinetics resulted in estimation of kET/kHAT ratios for each studied ArOH, and in calculation of the rate constants kET. Described results are cautionary examples of how the concentration of a phenol might change the reaction mechanism and the overall kinetics of the observed process.
Highlights
The stable 2,2-diphenyl-1-picrylhydrazyl radical, abbreviated as DPPH or dpphc, is broadly employed for quick assessment of the radical scavenging abilities of natural and synthetic compounds
From the ionized fraction of ArOH. This two-step mechanism is described as Sequential Proton-Loss Electron Transfer (SPLET),3,4 with kET >> kHAT, see Chart 1A, and even traces of ArOÀ causing an enormous increase of overall rate of reaction 1.2 Mixed mechanism in alcohols excludes dpphc as mimetic to peroxyl radicals but, regardless of this controversy,5 reaction 1 is one of the most frequently used colorimetric assays employed for quick assessment of the antiradical abilities of natural and synthetic compounds, with more than 40 thousand papers
Another problem with the dpphc assay is that IC50 parameter re ects the position of redox equilibrium between dpphc and the tested compound, established a er the incubation time, and gives no information about the kinetics and stoichiometry of the reaction.5b,d
Summary
The stable 2,2-diphenyl-1-picrylhydrazyl radical, abbreviated as DPPH or dpphc, is broadly employed for quick assessment of the radical scavenging abilities of natural and synthetic compounds. This two-step mechanism is described as Sequential Proton-Loss Electron Transfer (SPLET), with kET >> kHAT, see Chart 1A, and even traces of ArOÀ causing an enormous increase of overall rate of reaction 1.2 Mixed mechanism in alcohols excludes dpphc as mimetic to peroxyl radicals but, regardless of this controversy, reaction 1 is one of the most frequently used colorimetric assays employed for quick assessment of the antiradical abilities of natural and synthetic compounds, with more than 40 thousand papers. Reaction 1 can be (and usually is) reversible, because the bond dissociation enthalpy, BDE, in dpph-H (78.9 kcal molÀ1) is lower than BDEO–H for majority of phenols, and IC50 measurements can be misleading Another problem was noticed by Foti et al who reported that for some phenols reaction 1 exhibits non-integer order in [ArOH].4–7. We present the results obtained for: phenol (1), 4-hydroxyacetophenone (2), C-methylcalix[4]resorcinarene (3), C-undecylcalix[4] resorcinarene (4), and baicalein (5, the only natural compound within this series, being an interesting example of ArOH with a strongly acidic catechol moiety), see Chart 1B
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