Probing Antioxidant-Related Properties for Phenolic Compounds.

In this work, properties related to antioxidant-potential mechanisms (such as the bond dissociation enthalpy, BDE, for the homolytic cleavage of the O-H bond and ionization energies, IEs) were determined for phenol, pyrocatechol, and gallic acid (GA). Both the protonated and deprotonated forms of GA were investigated. The Feller-Peterson-Dixon (FPD) composite method was employed with a variety of computational approaches, i.e., density functional theory, Möller-Plesset perturbation theory, and coupled-cluster-based methods, in combination with large correlation consistent basis sets with extrapolation to the complete basis set limit and consideration of core electron correlation effects. FPD results were compared to experimental and computational data available in the literature, presenting good agreement. For example, the FPD BDE (298 K) obtained for phenol, which was based on valence-correlated MP2/CBS calculations with contributions from correlating all electrons, was determined to be 87.56 kcal/mol, a value that is 0.42 kcal/mol lower than the result obtained in the most recent experiments, 87.98 ± 0.62. Calibration against coupled-cluster calculations was also carried out for phenol. We expect that the outcomes gathered here may help in establishing a general protocol for computational chemists that are interested in determining antioxidant-related properties for phenolic compounds with considerable accuracy as well as to motivate future IE measurements (particularly for GA) to be accomplished in the near future.

A computational study on the reaction between fisetin and 2,2-diphenyl-1-picrylhydrazyl (DPPH).

The strategy of investigating the antioxidant potential of flavonols through the explicit modeling of chemical reactions (initiated to be employed in a previous work from our group) was taken further in this work. Therefore, a theoretical investigation on the reaction between fisetin and 2,2-diphenyl-1-picrylhydrazyl (DPPH) is presented. All the computations were performed using the density functional theory with the B3LYP functional along with the 6-31G(d,p) basis set. Structural, energetic quantities (ΔG and ΔG++), and reaction rates were probed in order to provide information on the antioxidant activity and to explore the contributions of each hydroxyl group to the referred property. According to the results obtained for the thermodynamic properties, fisetin presents antioxidant potential similar to quercetin (behavior that is also observed experimentally). In addition, the order of contribution of each OH group to the antioxidant potential was found to be 4'-ArOH (the most contributor, presenting ΔG = -5.17 kcal/mol) → 3'-ArOH (ΔG = -3.35 kcal/mol) → 3-ArOH (ΔG = -1.64 kcal/mol) → 7-ArOH (ΔG = 7.72 kcal/mol). These observations are in consistent agreement with the outcomes of other computational investigations performed using bond dissociation enthalpies (BDEs) as descriptors for the antioxidant activity. Therefore, the methodology employed in this work can be used as an alternative for probing antioxidant potential of compounds derived from fisetin. Graphical Abstract Illustrative scheme of the PES mapping in terms of hydrogen atom transfer from fisetin 3-ArOH to the nitrogen centered DPPH.

Evaluation of the antioxidant potential of myricetin 3-O-α-L-rhamnopyranoside and myricetin 4'-O-α-L-rhamnopyranoside through a computational study.

In this work, we present a computational study on the antioxidant potential of myricetin 3-O-α-L-rhamnopyranoside (Compound M3) and myricetin 4'-O-α-L-rhamnopyranoside (Compound M4'). Structural parameters, bond dissociation enthalpies (BDEs), ionization potentials (IPs), proton dissociation enthalpies (PDEs), proton affinities (PAs), and electron transfer enthalpies (ETEs), which are properties connected with different mechanisms related to antioxidant activity, were determined using density functional theory (DFT) with B3LYP, LC-ωPBE, M06-2X, and BMK functionals along with the 6-311G(d,p) and 6-311+G(d,p) basis sets in the gas phase, water, and pentylethanoate. The values obtained were compared with results previously available in the literature for myricetin (the parent molecule and a well-known antioxidant) and myricetin 3,4'-di-O-α-L-rhamnopyranoside (Compound M3,4'). As the BDEs are considerably lower than the IPs, the HAT mechanism is preferred over SET for the compounds M3 and M4'. The present study indicates Compound M3 as having its lowest bond dissociation enthalpy from the several different OH groups with similar value to the lowest for myricetin (74.72 kcal/mol versus 74.8 kcal/mol, respectively, at the B3LYP/6-311G(d,p) level of theory in the gas phase) and, thus, presenting antioxidant potential as good as its parent molecule. On the other hand, Compound M4' presented 78.97 kcal/mol as the lowest BDE at the B3LYP/6-311G(d,p) level of theory in the gas phase, that is very close to the 78.34 kcal/mol computed using the same approach for Compound M3,4'. Therefore, the present investigation indicated Compound M4' as being a slightly inferior antioxidant (with antioxidant potential comparable to Compound M3,4') than Compound M3. In addition, the inclusion of the sugar moiety studied here in the position 4'-ArOH of myricetin seems to have a more marked impact (downward) on the antioxidant activity than the glycosylation in the position 3-ArOH.

A computational investigation on the antioxidant potential of myricetin 3,4'-di-O-α-L-rhamnopyranoside.

In this work, we present a computational study on the antioxidant potential of myricetin 3,4[Formula: see text]-di-O-α-L-rhamnopyranoside (Compound M). A density functional theory (DFT) approach with the B3LYP and LC-ωPBE functionals and with both the 6-311G(d,p) and 6-311+G(d,p) basis sets was used. The focus of the investigation was on the structural and energetic parameters including both bond dissociation enthalpies (BDEs) and ionization potentials (IPs), which provide information on the potential antioxidant activity. The properties computed were compared with BDEs and IPs available in the literature for myricetin, a compound well known for presenting antioxidant activity (and the parent molecule of the compound of interest in the present work). Myricetin 3,4[Formula: see text]-di-O-α-L-rhamnopyranoside presented the lowest BDE to be 79.13 kcal/mol (as determined using B3LYP/6-311G(d,p) in water) while myricetin has a quite similar value (within 3.4 kcal/mol). IPs computed in the gas phase [B3LYP/6-311G(d,p)] are 157.18 and 161.4 kcal/mol for myricetin 3,4[Formula: see text]-di-O-α-L-rhamnopyranoside and myricetin, respectively. As the values of BDEs are considerably lower than the ones probed for IPs (in the gas phase or in any given solvent environment), the hydrogen atom transfer mechanism is preferred over the single electron transfer mechanism. The BDEs obtained suggest that myricetin 3,4[Formula: see text]-di-O-α-L-rhamnopyranoside can present antioxidant potential as good as the parent molecule myricetin (a well-known antioxidant). Therefore, experimental tests on the antioxidant activity of Compound M are encouraged.