A theoretical study of phenolic compounds with antioxidant properties

Rational Design of a Low-Data Regime of Pyrrole Antioxidants for Radical Scavenging Activities Using Quantum Chemical Descriptors and QSAR with the GA-MLR and ANN Concepts

A series of pyrrole derivatives and their antioxidant scavenging activities toward the superoxide anion (O₂^•-), hydroxyl radical (^•OH), and 1,1-diphenyl-2-picryl-hydrazyl (DPPH^•) served as the training data sets of a quantitative structure-activity relationship (QSAR) study. The steric and electronic descriptors obtained from quantum chemical calculations were related to the three O₂^•-, ^•OH, and DPPH^• scavenging activities using the genetic algorithm combined with multiple linear regression (GA-MLR) and artificial neural networks (ANNs). The GA-MLR models resulted in good statistical values; the coefficient of determination (R²) of the training set was greater than 0.8, and the root mean square error (RMSE) of the test set was in the range of 0.3 to 0.6. The main molecular descriptors that play an important role in the three types of antioxidant activities are the bond length, HOMO energy, polarizability, and AlogP. In the QSAR-ANN models, a good R² value above 0.9 was obtained, and the RMSE of the test set falls in a similar range to that of the GA-MLR models. Therefore, both the QSAR GA-MLR and QSAR-ANN models were used to predict the newly designed pyrrole derivatives, which were developed based on their starting reagents in the synthetic process.

Electrophilicity index revisited

This review aims to be a comprehensive, authoritative, critical, and accessible review of general interest to the chemistry community; because the electrophilicity index is a very useful global reactivity descriptor defined within a conceptual density functional theory framework. Our group has also introduced electrophilicity based new global and local reactivity descriptors and also new associated electronic structure principles, which are important indicators of structure, stability, bonding, reactivity, interactions, and dynamics in a wide variety of physico-chemical systems and processes. This index along with its local counterpart augmented by the associated electronic structure principles could properly explain molecular vibrations, internal rotations and various types of chemical reactions. The concept of the electrophilicity index has been extended to dynamical processes, excited states, confined environment, spin-dependent and temperature-dependent situations, biological activity, site selectivity, aromaticity, charge removal and acceptance, presence of external perturbation through solvents, external electric and magnetic fields, and so forth. Although electrophilicity and its local variant can adequately interpret the behavior of a wide variety of systems and different physico-chemical processes involving them, their predictive potential remains to be explored. An exhaustive review on all these aspects will set the tone of the future research in that direction.

Deep eutectic solvent assisted synthesis of dihydropyrimidinones/thiones via Biginelli reaction: theoretical investigations on their electronic and global reactivity descriptors

DES 2 (ZrOCl2·8H2O-ethylene glycol at 1 : 2 ratio) was used as a catalyst for the synthesis of dihydropyrimidinones/thiones via a Biginelli reaction.

Experimental and theoretical study on structure-tautomerism among edaravone, isoxazolone, and their heterocycles derivatives as antioxidants

Edaravone is a heterocyclic pyrazolone compound. It has pronounced effect against free radicals, however renal and hepatic disorders have been reported. Isoxazolones are considered bioisosteric analogues of pyrazolones and may have comparable properties. Thus, we investigated the structural and electronic influences for edaravone, isoxazolone, and their tautomers on antioxidant process. Structure and tautomerism study among edaravone, isoxazolone and their heterocycles derivatives were related to antioxidant mechanisms by using the hybrid DFT method B3LYP with the basis sets 6-31++G(2d,2p). The C—H tautomer was the most stable and energetically favored among them. Intramolecular N—H—N hydrogen bonds and polar medium were responsible for the low energy differences among all possible tautomers. N—H tautomers in both systems proved to be better antioxidant by SET (single electron transfer), while O—H tautomers were better antioxidant on HAT (homolytic hydrogen atom transfer) mechanism. Theoretical calculation showed that edaravone is more potent than phenylisoxazolone, however, both has similar antioxidant scavenging on experimental DPPH. The carbonyliminic system played a very important role in the antioxidant activity for both studied classes.

QSARs for phenols and phenolates: oxidation potential as a predictor of reaction rate constants with photochemically produced oxidants

Quantitative structure-activity relationships (QSARs) for prediction of the reaction rate constants of phenols and phenolates with three photochemically produced oxidants, singlet oxygen, carbonate radical, and triplet excited state sensitizers/organic matter, are developed. The predictive variable is the one-electron oxidation potential (E₁), which is calculated for each species using density functional theory. The reaction rate constants are obtained from the literature, and for singlet oxygen, are augmented with new experimental data. Calculated E₁ values have a mean unsigned error compared to literature values of 0.04-0.06 V. For singlet oxygen, a single linear QSAR that includes both phenols and phenolates is developed that predicts experimental rate constants, on average, to within a factor of three. Predictions for only 6 out of 87 compounds are off by more than a factor of 10. A more limited data set for carbonate radical reactions with phenols and phenolates also gives a single linear QSAR with prediction of rate constant being accurate to within a factor of three. The data for the reactions of phenols with triplet state sensitizers demonstrate that two sensitizers, 2-acetonaphthone and methylene blue, most closely predict the reactivity trend of triplet excited state organic matter with phenols. Using sensitizers with stronger reduction potentials could lead to overestimation of rate constants and thus underestimation of phenolic pollutant persistence.

Theoretical Reactivity Study of Indol-4-Ones and Their Correlation with Antifungal Activity

Chemical reactivity descriptors of indol-4-ones obtained via density functional theory (DFT) and hard–soft acid–base (HSAB) principle were calculated to prove their contribution in antifungal activity. Simple linear regression was made for global and local reactivity indexes. Results with global descriptors showed a strong relationship between antifungal activity vs. softness (S) (r = 0.98) for series I (6, 7a–g), and for series II (8a–g) vs. chemical potential (µ), electronegativity (χ) and electrophilicity (ω) (r = 0.86), p < 0.05. Condensed reactivity descriptors s_k⁺, ω_k⁻ for different fragments had strong relationships for series I and II (r = 0.98 and r = 0.92). Multiple linear regression was statistically significant for S (r = 0.98), η (r = 0.91), and µ/ω (r = 0.91) in series I. Molecular electrostatic potential maps (MEP) showed negative charge accumulation around oxygen of carbonyl group and positive accumulation around nitrogen. Fukui function isosurfaces showed that carbons around nitrogen are susceptible to electrophilic attack, whereas the carbon atoms of the carbonyl and phenyl groups are susceptible to nucleophilic attack for both series. The above suggest that global softness in conjunction with softness and electrophilicity of molecular fragments in enaminone systems and pyrrole rings contribute to antifungal activity of indol-4-ones.

Antioxidant Properties of Kynurenines: Density Functional Theory Calculations

Kynurenines, the main products of tryptophan catabolism, possess both prooxidant and anioxidant effects. Having multiple neuroactive properties, kynurenines are implicated in the development of neurological and cognitive disorders, such as Alzheimer's, Parkinson's, and Huntington's diseases. Autoxidation of 3-hydroxykynurenine (3HOK) and its derivatives, 3-hydroxyanthranilic acid (3HAA) and xanthommatin (XAN), leads to the hyperproduction of reactive oxygen species (ROS) which damage cell structures. At the same time, 3HOK and 3HAA have been shown to be powerful ROS scavengers. Their ability to quench free radicals is believed to result from the presence of the aromatic hydroxyl group which is able to easily abstract an electron and H-atom. In this study, the redox properties for kynurenines and several natural and synthetic antioxidants have been calculated at different levels of density functional theory in the gas phase and water solution. Hydroxyl bond dissociation enthalpy (BDE) and ionization potential (IP) for 3HOK and 3HAA appear to be lower than for xanthurenic acid (XAA), several phenolic antioxidants, and ascorbic acid. BDE and IP for the compounds with aromatic hydroxyl group are lower than for their precursors without hydroxyl group. The reaction rate for H donation to *O-atom of phenoxyl radical (Ph-O*) and methyl peroxy radical (Met-OO*) decreases in the following rankings: 3HOK ~ 3HAA > XAA_OXO > XAA_ENOL. The enthalpy absolute value for Met-OO* addition to the aromatic ring of the antioxidant radical increases in the following rankings: 3HAA* < 3HOK* < XAA_OXO* < XAA_ENOL*. Thus, the high free radical scavenging activity of 3HAA and 3HOK can be explained by the easiness of H-atom abstraction and transfer to O-atom of the free radical, rather than by Met-OO* addition to the kynurenine radical.

Kynurenines, the tryptophan metabolites with multiple biological activities, regulate the production of reactive oxygen species (ROS) during several neurodegenerative diseases. Many experiments show that kynurenines can be both prooxidants and antioxidants depending on their concentration, mode of action, and cell redox potential. However, there is lack of computational studies of kynurenines properties which could help us better understand the biophysical mechanism of their antioxidant activity. We performed the computations of kynurenines' hydrogen and electron donating power, both in the gas phase and in water solution. We found that aromatic hydroxyl group facilitates hydrogen and electron abstraction by kynurenines, in agreement with experimental data and computations earlier performed for phenolic antioxidants. We revealed the correlations of kynurenines' antioxidant power with their electronic structure, charge, and surroundings. We also found that 3-hydroxykynurenine and 3-hydroxyanthranilic acid can fastly quench free radicals by hydrogen atom donation. Hence both of them are potent antioxidants. The therapeutic strategy may be to inhibit their oxidative dimerization leading to ROS production.

Characterization of phenolic constituents inhibiting the formation of sulfur-containing volatiles produced during garlic processing

Garlic (Allium sativum L.), which is a widely distributed plant, is globally used as both spice and food. This study identified five novel phenolic compounds, namely, 8-(3-methyl-(E)-1-butenyl)diosmetin, 8-(3-methyl-(E)-1-butenyl)chrysin, 6-(3-methyl-(E)-1-butenyl)chrysin, and Alliumones A and B, along with nine known compounds 6-14 from the ethanol extract of garlic. The structures of these five novel phenolic compounds were established via extensive 1D- and 2D-nuclear magnetic resonance spectroscopy experiments. The effects of the phenolic compounds isolated from garlic on the enzymatical or nonenzymatical formation of sulfur-containing compounds produced during garlic processing were examined. Compound 12 significantly reduced the thermal decomposition of alliin, whereas compound 4 exhibited the highest percentage of alliinase inhibition activity (36.6%).

Gallic acid: a versatile antioxidant with promising therapeutic and industrial applications

Oxidative stress, a result of an overproduction and accumulation of free radicals, is the leading cause of several degenerative diseases such as cancer, atherosclerosis, cardiovascular diseases, ageing and inflammatory diseases.

Design and evaluation of 4-aminophenol and salicylate derivatives as free-radical scavenger

This theoretical and experimental study describes the design and evaluation of the free-radical scavenging effect for the molecular association of 4-aminophenol and salicylate derivatives. For this purpose, we employed theoretical methods for the selection of antioxidant drugs and the rapid methods of evaluation: the 1,1-diphenyl-2-picrylhydrazyl radical and the thiobarbituric acid reactive substances in the lipid peroxidation initiated by Fe(2+) and ascorbic acid in human erythrocytes. The associate derivatives exhibited a more potent inhibition than the salicylic acid, while the benzoyl compound exhibited a more potent inhibition than paracetamol. The molecular parameters related to the electron distribution and structure (ionization potential and energy of the highest occupied molecular orbital) correlated very well with the antioxidant action of the compounds studied here in different tests.

Theoretical study on the antioxidant properties of 2'-hydroxychalcones: H-atom vs. electron transfer mechanism

The free radical scavenging activity of six 2'-hydroxychalcones has been studied in gas phase and solvents using the density functional theory (DFT) method. The three main working mechanisms, hydrogen atom transfer (HAT), stepwise electron-transfer-proton-transfer (ET-PT) and sequential-proton-loss-electron-transfer (SPLET) have been considered. The O-H bond dissociation enthalpy (BDE), ionization potential (IP), proton affinity (PA) and electron transfer energy (ETE) parameters have been computed in gas phase and solvents. The theoretical results confirmed the important role of the B ring in the antioxidant properties of hydroxychalcones. In addition, the calculated results matched well with experimental values. The results suggested that HAT would be the most favorable mechanism for explaining the radical-scavenging activity of hydroxychalcone in gas phase, whereas SPLET mechanism is thermodynamically preferred pathway in aqueous solution.

Density functional theory (DFT) study of edaravone derivatives as antioxidants

Quantum chemical calculations at the B3LYP/6–31G* level of theory were employed for the structure-activity relationship and prediction of the antioxidant activity of edaravone and structurally related derivatives using energy (E), ionization potential (IP), bond dissociation energy (BDE), and stabilization energies (ΔE_iso). Spin density calculations were also performed for the proposed antioxidant activity mechanism. The electron abstraction is related to electron-donating groups (EDG) at position 3, decreasing the IP when compared to substitution at position 4. The hydrogen abstraction is related to electron-withdrawing groups (EDG) at position 4, decreasing the BDE_CH when compared to other substitutions, resulting in a better antioxidant activity. The unpaired electron formed by the hydrogen abstraction from the C–H group of the pyrazole ring is localized at 2, 4, and 6 positions. The highest scavenging activity prediction is related to the lowest contribution at the carbon atom. The likely mechanism is related to hydrogen transfer. It was found that antioxidant activity depends on the presence of EDG at the C₂ and C₄ positions and there is a correlation between IP and BDE. Our results identified three different classes of new derivatives more potent than edaravone.

The basic antioxidant structure for flavonoid derivatives

An antioxidant structure-activity study is carried out in this work with ten flavonoid compounds using quantum chemistry calculations with the functional of density theory method. According to the geometry obtained by using the B3LYP/6-31G(d) method, the HOMO, ionization potential, stabilization energies, and spin density distribution showed that the flavonol is the more antioxidant nucleus. The spin density contribution is determinant for the stability of the free radical. The number of resonance structures is related to the π-type electron system. 3-hydroxyflavone is the basic antioxidant structure for the simplified flavonoids studied here. The electron abstraction is more favored in the molecules where ether group and 3-hydroxyl are present, nonetheless 2,3-double bond and carbonyl moiety are facultative.

A THEORETICAL STUDY ON THE ANTIOXIDANT PROPERTY OF GALLIC ACID AND ITS DERIVATIVES

We analyze the antioxidant property of four phenolic compounds i.e., gallic acid, methyl gallate, ethyl gallate, and 4, 5-dihydroxy 3-methoxy benzoic acid through the determination of bond dissociation enthalpy (BDE), vertical ionization potential ( IP v) in gas phase as well as in six different solvent medium. The theoretical trends of these phenolic compounds, based on BDE and IP, is compared with the scavenging activity towards HOCl . In addition, we compute and analyze the values of the density-based reactivity descriptors such as chemical potential, hardness, electrophilicity, and local softness.

Comparative study on antioxidant capacity of flavonoids and their inhibitory effects on oleic acid-induced hepatic steatosis in vitro

Nonalcoholic fatty liver disease (NAFLD) is the most common liver disease and its incidence is rising worldwide. We compared the antioxidant capacity of seventeen flavonoids with their inhibitory effects on oleic acid-induced triglyceride (TG) over-accumulation in HepG2 cells. The results showed significant correlations (P < 0.01) between the inhibition of intracellular TG levels and the suppression effects on reactive oxygen species. Nevertheless, the radical-reducing activities of flavonoids assessed by chemical assays (cyclic voltammetry and Folin-Ciocalteu reagent assay) were poorly correlated with their intracellular TG inhibitory effects. The relationships between structural properties of flavonoids and their inhibitory effects on TG over-accumulation were discussed.

Structure of dihydrochalcones and related derivatives and their scavenging and antioxidant activity against oxygen and nitrogen radical species

Quantum mechanical calculations at B3LYP/6-31G** level of theory were employed to obtain energy (E), ionization potential (IP), bond dissociation enthalpy (O-H BDE) and stabilization energies (ΔE_iso) in order to infer the scavenging activity of dihydrochalcones (DHC) and structurally related compounds. Spin density calculations were also performed for the proposed antioxidant activity mechanism of 2,4,6-trihydroxyacetophenone (2,4,6-THA). The unpaired electron formed by the hydrogen abstraction from the phenolic hydroxyl group of 2,4,6-THA is localized on the phenolic oxygen at 2, 6, and 4 positions, the C₃ and C₆ carbon atoms at ortho positions, and the C₅ carbon atom at para position. The lowest phenolic oxygen contribution corresponded to the highest scavenging activity value. It was found that antioxidant activity depends on the presence of a hydroxyl at the C2 and C4 positions and that there is a correlation between IP and O-H BDE and peroxynitrite scavenging activity and lipid peroxidation. These results identified the pharmacophore group for DHC.

3D-QSAR investigation of synthetic antioxidant chromone derivatives by molecular field analysis

A series of 7-hydroxy, 8-hydroxy and 7,8-dihydroxy synthetic chromone derivatives was evaluated for their DPPH free radical scavenging activities. A training set of 30 synthetic chromone derivatives was subject to three-dimensional quantitative structure-activity relationship (3D-QSAR) studies using molecular field analysis (MFA). The substitutional requirements for favorable antioxidant activity were investigated and a predictive model that could be used for the design of novel antioxidants was derived. Regression analysis was carried out using genetic partial least squares (G/PLS) method. A highly predictive and statistically significant model was generated. The predictive ability of the developed model was assessed using a test set of 5 compounds (r²_pred = 0.924). The analyzed MFA model demonstrated a good fit, having r² value of 0.868 and cross-validated coefficient r²_cv value of 0.771.