The antioxidant activities of 18 typical phenolic acids were investigated using 2, 2′-diphenyl-1-picrylhydrazyl (DPPH) and ferric ion reducing antioxidant power (FRAP) assays. Five thermodynamic parameters involving hydrogen atom transfer (HAT), single-electron transfer followed by proton transfer (SET-PT), and sequential proton-loss electron transfer (SPLET) mechanisms were calculated using density functional theory with the B3LYP/UB3LYP functional and 6–311++G (d, p) basis set and compared in the phenolic acids. Based on the same substituents on the benzene ring, -CH2COOH and -CH = CHCOOH can enhance the antioxidant activities of phenolic acids, compared with -COOH. Methoxyl (-OCH3) and phenolic hydroxyl (-OH) groups can also promote the antioxidant activities of phenolic acids. These results relate to the O-H bond dissociation enthalpy of the phenolic hydroxyl group in phenolic acids and the values of proton affinity and electron transfer enthalpy (ETE) involved in the electron donation ability of functional groups. In addition, we speculated that HAT, SET-PT, and SPLET mechanisms may occur in the DPPH reaction system. Whereas SPLET was the main reaction mechanism in the FRAP system, because, except for 4-hydroxyphenyl acid, the ETE values of the phenolic acids in water were consistent with the experimental results.

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Structure-antioxidant activity relationship of methoxy, phenolic hydroxyl, and carboxylic acid groups of phenolic acids.

Among the multiple components of propolis, flavonoids contribute greatly to the antioxidant activities of propolis. Flavonoids mainly exist in the form of sugar-conjugated derivatives. Quercetin glycosides represent the predominant flavonoid fraction in propolis. In this work, density functional theory (DFT) calculations were applied to analyze the antioxidative properties of quercetin and its glucosides in the gas and in the liquid phase (ethanol, water). Three main antioxidant mechanisms, hydrogen atom transfer (HAT), single electron transfer followed by proton transfer (SET-PT) and sequential proton loss electron transfer (SPLET) were used to analyze the antioxidative capacity of the investigated compounds. Solvent effects dominantly affect SET-PT and SPLET. Thus, the thermodynamically preferred mechanism can be altered. HAT and SPLET are the thermodynamically dominant mechanisms in gas and solvent phases, respectively. Therefore, in the gas phase, the sequence of the antioxidative capacity is similar with the bond dissociation enthalpy values: quercetin > quercetin-5-O-glucoside > quercetin-7-O-glucoside > quercetin-3-O-glucoside > quercetin-3′-O-glucoside > quercetin-4′-O-glucoside. While, in the solvent phases, the sequence is similar with the proton affinity values: quercetin-4′-O-glucoside > quercetin-5-O-glucoside > quercetin > quercetin-3-O-glucoside > quercetin-7-O-glucoside > quercetin-3′-O-glucoside. OH groups in B-ring and C-ring contribute mainly to the antioxidative activities of quercetin and glucosides compared with A-ring.

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Antioxidant Activity of Quercetin and Its Glucosides from Propolis: A Theoretical Study.

https://bib.irb.hr/datoteka/841875.Ami_et_al_Food_Chemistry_218_2017_144-151.pdf

Free radical scavenging potency of quercetin catecholic colonic metabolites: Thermodynamics of 2H+/2e- processes.

We present a computational study on two flavonols that were recently isolated from Loranthaceae family plant extracts: kaempferol 3-O-α-L-arabinofuranosyl-(1 → 3)-α-L-rhamnoside and quercetin 3-O-α-L-arabinofuranosyl-(1 → 3)-α-L-rhamnoside. Their structures and energetics have been investigated at the density functional level of theory, up to B3LYP/6-31+G(d,p), incorporating solvent effects with polarizable continuum models. In addition, their potential antioxidant activities were probed through the computation of the (i) bond dissociation enthalpies (BDEs), which are related to the hydrogen-atom transfer mechanism (HAT), and (ii) ionization potentials (IPs), which are related to the single-electron transfer mechanism (SET). The BDEs were determined in water to be 83.23 kcal/mol for kaempferol 3-O-α-L-arabinofuranosyl-(1 → 3)-α-L-rhamnoside and 77.49 kcal/mol for quercetin 3-O-α-L-arabinofuranosyl-(1 → 3)-α-L-rhamnoside. The corresponding IPs were obtained for both compounds as 133.38 and 130.99 kcal/mol, respectively. The BDEs and IPs are comparable to those probed for their parental molecules kaempferol and quercetin; this is in marked contrast to previous studies where glycosylation at the 3-position increases the corresponding BDEs, and, hence, decreases subsequent antioxidant activity. The BDEs and IPs obtained suggest both compounds are promising for antioxidant activity and thus further experimental tests are encouraged.

A DFT investigation on the structural and antioxidant properties of new isolated interglycosidic O-(1 → 3) linkage flavonols.

https://bib.irb.hr/datoteka/841878.Dimitri_Markovi_et_al_Food_Chem_218_2017_440-446.pdf

Antiradical activity of delphinidin, pelargonidin and malvin towards hydroxyl and nitric oxide radicals: The energy requirements calculations as a prediction of the possible antiradical mechanisms.

Kamila Dropková

Papers

Reaction enthalpies of OH bonds splitting-off in flavonoids: The role of non-polar and polar solvent