Acidity of Hydroxyl Groups: An Overlooked Influence on Antiradical Properties of Flavonoids

Abstract

The reactions of 10 flavonoids with 2,2-diphenyl-1-picrylhydrazyl radical (dpph(*)) carried out in alcohols always occur significantly faster than in acidified alcohols or in dioxane. These fast kinetics benefit from the contribution of the electron transfer from a flavonoid anion to a radical, a mechanism known as Sequential Proton-Loss Electron-Transfer (SPLET), which adds to the kinetics of single-step Hydrogen Atom Transfer (HAT)/Proton Coupled Electron Transfer (PCET) processes (see Acc. Chem. Res. 2007, 40 , 222.). The domination of SPLET over HAT/PCET in case of a flavonoid reacting with electron-deficient radicals such as peroxyls or dpph(*) in polar solvents explains the enhancement of antioxidant activity of 3-hydroxyflavone. It also elucidates the great acceleration in the reactions of dpph(*) with quercetin, morin, galangin, and 7,8-dihydroxyflavone. The analysis of structure-acidity and structure-activity relationships for 10 flavonoids clearly indicates that hydroxyl group at position 7 is the most acidic site. Thus, in polar solvents this group can participate in radical reaction via SPLET. In nonpolar solvents the most active site in quercetin (a flavonoid antioxidant commonly found in plants) is 3',4'-dihydroxyl moiety and HAT/PCET occurs. However, in ionization-supporting solvents an anion formed at position 7 is responsible for very fast kinetics of quercetin/dpph(*) reaction because both mechanisms participate: HAT (from catechol moiety in ring B) and SPLET (from ionized 7-hydroxyl in ring A). Because of conjugation of rings A, B, and C the final structure of the formed quercetin radical (or quercetin anion radical) is the same for the SPLET and HAT/PCET mechanisms.

The Methodology Applied in DPPH, ABTS and Folin-Ciocalteau Assays Has a Large Influence on the Determined Antioxidant Potential

“…12,[28][29][30][31] The rate constants however depend greatly on the type of antioxidant and the solvent composition. 3,12 In the present section we have not focused on the kinetics of the fast phases but rather on the amplitude that is directly proportional to the determined AOP, in order to show how incubation time influences its value.…”

Monitoring of Fluorescence Characteristics of Satsuma Mandarin (Citrus unshiu Marc.) during the Maturation Period

“…On the other hand, PMFs in the flesh fluctuated unsteadily, declining in August, stabilizing in September-October, and suddenly increasing again in November, which might be caused by changes in acid content. In addition, the solubility of PMFs in the water phase of biological systems increases with acidity [31].…”

Theoretical Study of Primary Antioxidant Action Thermodynamics

“…Numerous experimental reports on antioxidant action of flavonoids and other polyphenols have been published in last two decades (Craft et al 2012, Gülçin 2012, Musialik et al 2009, RiceEvans et al 1996, Procházková et al 2011). However, experimental values of reaction enthalpies related to the three mechanisms of their primary antioxidant action are still not available.…”

“…HAT, SET-PT and SPLET, were studied for mono-substituted phenols, thiophenols, tocopherols, chromans and flavonoids. Because flavonoids and other polyphenols contain several phenolic OH groups with different acidities, their antioxidant action in aqueous solutions depends on pH, since their deprotonated forms show considerably different effect (Lemanska et al 2001, Musialik et al 2009, Álvarez-Diduk et al 2013. Therefore, our recent work is devoted to the study of hydrogen atom transfer and deprotonation of flavonoid phenoxide anions.…”