Free Radical Scavenging by Natural Polyphenols: Atom versus Electron Transfer

Abstract

Polyphenols (synthetically modified or directly provided by human diet) scavenge free radicals by H-atom transfer and may thus decrease noxious effects due to oxidative stress. Free radical scavenging by polyphenols has been widely theoretically studied from the thermodynamic point of view whereas the kinetic point of view has been much less addressed. The present study describes kinetic-based structure-activity relationship for quercetin. This compound is very characteristic of the wide flavonoid subclass of polyphenols. H-atom transfer is a mechanism based on either atom or electron transfer. This is analyzed here by quantum chemical calculations, which support the knowledge acquired from experimental studies. The competition between the different processes is discussed in terms of the nature of the prereaction complexes, the pH, the formation of activated-deprotonated forms, and the atom- and electron-transfer efficiency. The role of the catechol moiety and the 3-OH group of quercetin as scavengers of different types of free radicals (CH3OO(•), CH3O(•), (•)OH, and (•)CH2OH) is rationalized. Identifying the exact mechanism and accurately evaluating kinetics is of fundamental importance to understand antioxidant behavior in physiological environments.

Nephroprotective Effect of Nasturtium Officinale (Watercress) Ethanol Extract and Vitamin E on Vancomycin-Induced Nephrotoxicity in Rats

“…In our earlier study we found that NOE exhibited a noticeable anti-inflammatory activity in several animal models of inflammation (17). Moreover, free radical scavenging properties of phenolic and flavonoid compounds were confirmed in some studies (37)(38)(39). In this study, phenolic and flavonoid contents of NOE detected to be 71 ± 6.23 gallic acid equivalents/g dried extract and 95.26 ± 7.6 mg rutin equivalents/g dried extract, respectively.…”

Therapeutic detoxification of quercetin against carbon tetrachloride-induced acute liver injury in mice and its mechanism

“…After treatment with quercetin, the increased liver GSH level was reduced, which suggests that oxidative stress injuries were ameliorated. There are already existing reports that state that quercetin has a free radical scavenging activity (Pascoal et al, 2011;di Meo et al, 2013), so we can speculate that quercetin itself may have the capacity to scavenge free radicals, and thus substitute for the function of GSH, which leads to a decrease of GSH levels in quercetin-treated mice.…”

Metal chelation, radical scavenging and inhibition of Aβ42 fibrillation by food constituents in relation to Alzheimer’s disease

“…S9) (Di Meo et al, 2013;Litwinienko & Ingold, 2007). In addition to being influenced by the properties of the reactants, the relative contribution of each mechanism to the overall process is highly dependent on the ability of the solvent to promote ionisation of the phenol and to accept hydrogen bonds (HBs) (Litwinienko & Ingold, 2007).…”

“…In addition to being influenced by the properties of the reactants, the relative contribution of each mechanism to the overall process is highly dependent on the ability of the solvent to promote ionisation of the phenol and to accept hydrogen bonds (HBs) (Litwinienko & Ingold, 2007). The rate constant for electron transfer (k ET ) (the rate limiting step of SPLET (Di Meo et al, 2013)) is normally much greater than the rate constant for HAT (k HAT ), hence SPLET can be the prevailing mechanism even at very low phenoxide concentrations (Litwinienko & Ingold, 2007). In solvents with high dielectric constants and good anion solvation abilities (e.g.…”