Single Electron Transfer Followed By Proton Transfer Research Articles

Antioxidant properties of α-amino acids: a density functional theory viewpoint

The antioxidant properties of 21 proteinogenic amino acids (AAs) and 3,4-dioxophenylanine (DOPA) have been studied in implicit water using density functional theory (DFT). All the calculations have been performed according to three oxidation mechanisms: (1) hydrogen-atom transfer (HAT); (2) single electron transfer followed by proton transfer (SET-PT); and (3) sequential proton-loss electron transfer (SPLET). As a result, five AAs with the highest antioxidant capacity have been established: DOPA, selenocysteine (Sec), tyrosine (Tyr), cysteine (Cys), and tryptophan (Trp). Also, global reactivity in terms of hardness/softness has been evaluated, as well as Fukui indices of local reactivity. Trp has been determined as the most reactive molecule, whereas selenium atom of Sec has been established as the most reactive atom. All the findings are in agreement with the recent literature on both experimental and theoretical studies of amino acids antioxidant activity. However, to the best of my knowledge, the calculations for one electron redox reactions of zwitterionic amino acids in implicit water have been performed for the first time.

Unlocking the potential of Allium dictyoprasum C.A. Meyer ex Kunth: quantum chemical insights into radical scavenging, chemical composition, phenolic content, and antimicrobial activity

ABSTRACT Allium dictyoprasum C.A. Meyer ex Kunth (A. dictyoprasum) underwent comprehensive analysis, encompassing quantum chemical computations to assess its radical scavenging potential, chemical and elemental composition, total phenolic content, and antimicrobial activity. Experimental and theoretical investigations focused on elucidating the radical scavenging properties of polyhydroxy phenolic compounds present in the plant. Quantum chemical calculations were employed to evaluate the antioxidants employed to evaluate selected polyhydroxy phenolic molecules including flavonoids, hydrocinnamic acid derivatives, and hydroxybenzoic acid derivatives from natural sources. Thermochemical parameters of these compounds were calculated by the B3LYP/6–311 G++(d,p) level in both gas and solvent phases to elucidate the radical scavenging mechanism including hydrogen atom transfer (HAT), single electron transfer followed by proton transfer (SET-PT) and sequential proton loss electron transfer (SPLET). Analysis of A. dictyoprasum extracts obtained via various extraction methods revealed the presence of several major compounds, including dimethyl trisulfide, 3,5-Dihydroxy-6-methyl-2,3-dihydro-4 H-pyran-4-one, 2-Methoxy-4-vinylphenol, Dimethyl phthalate, Methyl palmitate, Methyl oleate, Methyl stearate, (9Z)-9-Octadecenamide. Notably, Malic acid and Quinic acid were identified as major compounds, with concentrations of 43.31 and 17.47 mg kg−1 extract, respectively, based on LC-MS/MS analysis. The total phenolic content of the extract was measured as 17.83 mg gallic acid/mL, while its free radical scavenging activity was 80.89% per mg/mL. Elemental analysis revealed significant levels of Mg, K, Na, Fe, and P, with minor concentrations of elements such as Ti, Tl, B, and Be. Furthermore, A. dictyoprasum exhibited notable antibacterial activity against various bacteria strains, surpassing the efficacy of some commercial antibiotics.

Computer-aided drug design supporting sunscreen research: a showcase study using previously synthesized hybrid UV filter-antioxidant compounds.

Although quantum mechanical calculations have proven effective in accurately predicting UV absorption and assessing the antioxidant potential of compounds, the utilization of computer-aided drug design (CADD) to support sustainable synthesis research of new sunscreen active ingredients remains an area with limited exploration. Furthermore, there are ongoing concerns about the safety and effectiveness of existing sunscreens. Therefore, it remains crucial to investigate photoprotection mechanisms and develop enhanced strategies for mitigating the harmful effects of UVR exposure, improving both the safety and efficacy of sunscreen products. A previous study conducted synthesis research on eight novel hybrid compounds (I-VIII) for use in sunscreen products by molecular hybridization of trans-resveratrol (RESV), avobenzone (AVO), and octinoxate (OMC). Herein, time-dependent density functional theory (TD-DFT) calculations performed in the gas phase on the isolated hybrid compounds (I-VIII) proved to reproduce the experimental UV absorption. Resveratrol-avobenzone structure-based hybrids (I-IV) present absorption maxima in the UVB range with slight differences between them, while resveratrol-OMC structure-based hybrids (V-VIII) showed main absorption in the UVA range. Among RESV-OMC hybrids, compounds V and VI exhibited higher UV absorption intensity, and compound VIII stood out for its broad-spectrum coverage in our simulations. Furthermore, both in silico and in vitro analyses revealed that compounds VII and VIII exhibited the highest antioxidant activity, with compound I emerging as the most reactive antioxidant within RESV-AVO hybrids. The study suggests a preference for the hydrogen atom transfer (HAT) mechanism over single-electron transfer followed by proton transfer (SET-PT) in the gas phase. With a strong focus on sustainability, this approach reduces costs and minimizes effluent production in synthesis research, promoting the eco-friendly development of new sunscreen active ingredients. The SPARTAN'20 program was utilized for the geometry optimization and energy calculations of all compounds. Conformer distribution analysis was performed using the Merck molecular force field 94 (MMFF94), and geometry optimization was carried out using the parametric method 6 (PM6) followed by density functional theory (DFT/B3LYP/6-31G(d)). The antioxidant behavior of the hybrid compounds (I-VIII) was determined using the highest occupied molecular orbital (εHOMO) and the lowest unoccupied molecular orbital (εLUMO) energies, as well as the bond dissociation enthalpy (BDE), ionization potential (IP), and proton dissociation enthalpy (PDE) values, all calculated at the same level of structural optimization. TD-DFT study is carried out to calculate the excitation energy using the B3LYP functional with the 6-31G(d) basis set. The calculated transitions were convoluted with a Gaussian profile using the Gabedit program.

Theoretical Exploration of Enhanced Antioxidant Activity in Copper Complexes of Tetrahydroxystilbenes: Insights into Mechanisms and Molecular Interactions.

A theoretical investigation was conducted using DFT/PW91/TZP/DMSO calculations on a complete set of exhaustive lists of 18 compounds resulting from the complexation of trans-2,4,3',5'-tetrahydroxystilbene (T-OXY) and cis-2,4,1',3'-tetrahydroxystilbene (C-OXY) with copper metal cations (Cu+ and Cu2+). The ligand-binding sites are the critical points of Quantum Theory of Atoms in Molecules (QTAIM) analysis on neutral and deprotonated ligands. Various mechanisms, including hydrogen atom transfer (HAT), sequential proton loss electron transfer (SPLET), single electron transfer followed by proton transfer (SET-PT), and bond dissociation energy (BDE(E0)) calculations, were employed to quantify the antioxidant activity. The BDE(E0) mechanism emerged as the most suitable approach for such analyses to evaluate the departure of hydrogen atoms since the results show the HAT mechanism is the most likely occurring. Particularly intriguing were the anionic Cu+ complexes with ligands adopting trans configurations and deprotonated conformations, displaying superior antioxidant activity compared to their counterparts. Remarkably, a single ligand within the Cu+ complex exhibited exceptional antioxidant prowess, yielding a BDE(E0) value of 91.47 kcal/mol. Furthermore, a complex involving two deprotonated ligands demonstrated antioxidant activity of 31.12 kcal/mol, signifying its potential as a potent antiradical agent, surpassing T-OXY by a factor of 3.91 and even surpassing the antioxidant efficiency of Vitamin C.

Mechanism of Antiradical Activity of Coumarin-Trihydroxybenzohydrazide Derivatives: A Comprehensive Kinetic DFT Study.

As part of this study, the mechanisms of the antioxidant activity of previously synthesized coumarin-trihydrobenzohydrazine derivatives were investigated: (E)-2,4-dioxo-3-(1-(2-(2″,3″,4″-trihydroxybenzoyl)hydrazineyl)ethylidene)chroman-7-yl acetate (1) and (E)-2,4-dioxo-3-(1-(2-(3″,4″,5″-trihydroxybenzoyl)hydrazineyl)ethylidene)chroman-7-yl acetate (2). The capacity of the compounds to neutralize HO• was assessed by EPR spectroscopy. The standard mechanisms of antioxidant action, Hydrogen Atom Transfer (HAT), Sequential Proton Loss followed by Electron Transfer (SPLET), Single-Electron Transfer followed by Proton Transfer (SET-PT), and Radical Adduct/Coupling Formation (RAF/RCF) were examined using the QM-ORSA methodology. It was estimated that the newly synthesized compounds, under physiological conditions, exhibited antiradical activity via SPLET and RCF mechanisms. Based on the estimated overall rate constants (koverall), it can be concluded that 2 exhibited a greater antiradical capacity. The obtained values indicated a good correlation with the EPR spectroscopy results. Both compounds exhibit approximately 1.5 times more activity in comparison to the precursor compound used in the synthesis (gallic acid).

Enrichment, analysis, identification and mechanism of antioxidant components in Toona sinensis

• Toona sinensis has strong antioxidant activity. • Liquid-liquid-refining extraction was developed to enrich antioxidants from extract. • Pentagalloyl glucose and ethyl gallate were the main antioxidants. • Single electron transfer followed by proton transfer is main antioxidant mechanism. The tender leaves of Toona sinensis not only taste delicious, but also have a wide range of biological activities, including strong antioxidant activity. In this study, a method based on liquid-liquid-refining extraction and bioassay-guided separation were established and developed for enrichment, analysis and identification of antioxidant components in T. sinensis . Two main antioxidants, 1,2,3,4,6-penta-O-galloyl-β-D-glucose (PGG) and ethyl gallate (EG) with strong antioxidant capacity were responsible for the antioxidant activity of T. sinensis . The EC 50 of EG scavenging ABTS· + and DPPH· were 4.46 ± 0.05 μg/mL and 7.61 ± 0.13 μg/mL, and the EC 50 of PGG were 12.90 ± 0.16 μg/mL and 16.29 ± 0.20 μg/mL. The thermodynamic parameters of various pathways indicated that single electron transfer followed by proton transfer (SET-PT) was the main antioxidant mechanism of PGG and EG. Furthermore, the results of frontier molecular orbital theory and reaction kinetics showed that PGG had low energy gap and reaction energy barrier, which was the fundamental reason why its antioxidant activity was better than EG. Graphical Abstract. .

Radical scavenging activity of three Scirpusins: A kinetic and mechanistic study

The strong antioxidant activity of Scirpusin, a polyphenolic compound also known as resveratrol or piceatannol dimer, toward •OH/•OOH in gas phase and solvents was investigated through systematic thermodynamics and kinetics analyses using the density functional theory method. All mechanisms, including hydrogen atom transfer (HAT), single electron transfer followed by proton transfer (SET-PT), sequential proton loss electron transfer (SPLET), and radical adduct formation (RAF), were investigated. The transition state theory was used to calculate potential energy surfaces and rate constants for HAT and RAF. The results revealed that Scirpusins exhibited considerable antioxidant activity, which followed the sequence: Scirpusin-B > Scirpusin-C ≈ Scirpusin-A, the amount of o-bisphenol is essential to enhance their antioxidant activity. Furthermore, Scirpusin-A, Scirpusin-B, and Scirpusin-C with structures of 13′-OH, 12′-OH, and 2-OH, respectively, shared the same parent nucleus but exhibited distinct strongest reactive sites. This result can provide a guidance for Scirpusin application in the future.

A broad assessment of Eremurus spectabilis M. Bieb: chemical and elemental composition, total phenolic and antimicrobial activity analysis, and quantum chemical calculations of radical scavenging potential

ABSTRACT Eremurus spectabilis M. Bieb was extensively investigated experimentally and theoretically, including the antioxidant properties of compounds such as flavonoids, hydroxycinnamic acid derivatives, hydroxybenzoic acid derivatives, and organic acids. Antioxidant activity was investigated using the Density Functional Theory (DFT) method based on three known mechanisms: hydrogen atom transfer (HAT), single electron transfer followed by proton transfer (SET-PT), and sequential proton loss electron transfer (SPLET). Subcritical water extraction (SWE), soxhlet extraction (SE), and solvent extraction (SOE) techniques were applied in the extraction process. Malic acid was the major compound with an extract concentration of 38,532.84 ± 1849.58 µg analyte/kg, total phenolics, and free radical scavenging activity were 10.67 mg gallic acid/mL extract and 73.89% per mg/mL extract, respectively. P, Fe, Na, Mg, K, and Ca were the main elements. The antibacterial activity of E. spectabilis against seven bacteria was evaluated, and it was found to be higher than the commercial antibiotics P10 and AMC30.

Computational insights into the antioxidant and antidiabetic mechanisms of cannabidiol: An in vitro and in silico study

The present study was aimed at investigating the molecular mechanism involved the antioxidant and antidiabetic activities of cannabidiol using in vitro and in silico models. The antioxidant activities of cannabidiol were determined by assaying its ability to scavenge 2,2-diphenyl-1-picryl-hydrazyl-hydrate (DPPH) radical and reduce ferric ion (Fe3+). The antidiabetic activity of cannabidiol was determined by its ability to inhibit α-glucosidase activity. The DPPH scavenging activity of cannabidiol was further investigated using the Density Functional Theory (DFT) analysis. The molecular interactions of cannabidiol and α-glucosidase activity was determined via molecular docking and molecular dynamics simulation. Cannabidiol significantly scavenged DPPH radical and reduced Fe3+, with IC50 values of 211.93 and 101.72 μg/mL, respectively. It also inhibited the activity of α-glucosidase, with an IC50 value of 311.26 μg/mL. DFT analysis revealed sequential proton loss followed by electron transfer (SPLET) as the preferred mechanism by which cannabidiol scavenge DPPH, followed by hydrogen atom transfer (HAT) and single electron transfer followed by proton transfer (SET-PT). The highest occupied molecular orbital (HOMO) orbitals were delocalised on the benzene ring and OH groups, while the lowest energy orbital (LUMO) orbitals were distributed on both the parent and substituent molecules. Cannabidiol bounded favourably to α-glucosidase enzyme, with ΔG, root mean square deviation (RMSD), root mean square fluctuation (RMSF) values of −28.49 kcal/mol, 1.36 Å and 0.835 Å as well as higher correlated motions. Therefore, the antioxidant activity of cannabidiol involves the abstraction of its H1 by the radical and delocalization of the radical’s unpaired electron. Its antidiabetic activity can be attributed to the potent inhibitory effect on α-glucosidase, as well as its high affinity with the enzyme.

Fermentation of Agaricus bisporus for antioxidant activity: response surface optimization, chemical components, and mechanism

Agaricus bisporus is one of the most widely cultivated edible mushrooms in the world. The chemical components of A. bisporus have a wide range of biological activities. In order to deeply understand the antioxidant properties of A. bisporus, this study conducted an investigation on the components of A. bisporus fermentation. Through the single factor experiment and response surface optimization, it was found that when the C/N ratio was 45:1, the inoculum concentration was 10%, and the fermentation time was 7 d, the n-butanol extract of the fermentation product had the strongest scavenging capacity for free radical generated through 2,2'-azino-bis(3-ethylbenzothiazoline)-6-sulphonic acid (ABTS·+). The concentration for 50% of the maximal effect (EC50) was 0.33 ± 0.01 mg/mL. Moreover, in order to identify the two main components, the elution–extrusion counter-current chromatography (EECCC) was employed for separation, where 5,5'-oxy-dimethyl-bis(2-furfuraldehyde) and 5-(butoxymethyl) furfural were obtained. The antioxidant activity of 5,5'-oxy-dimethyl-bis(2-furfuraldehyde) (EC50 = 0.26 ± 0.01 mg/mL) was superior to that of 5-butylmethyl furfural (EC50 = 1.52 ± 0.02 mg/mL), indicating that 5,5'-oxy-dimethyl-bis(2-furfuraldehyde) was the main antioxidant in the fermentation products. The thermodynamic parameters and frontier molecular orbitals of 5,5'-oxy-dimethyl-bis (2-furanaldehyde) was evaluated by density functional theory (DFT). The result indicated 5,5'-oxy-dimethyl-bis(2-furanaldehyde) scavenged free radicals in polar media through single electron transfer followed by proton transfer (SET-PT).

Fisetin and Robinetin antiradical activity under solvent effect: density functional theory study

The structural and antioxidant activity of two flavonols, namely, Fisetin and Robinetin, have been investigated employing the density functional theory (DFT) using B3LYP functional and 6-311++G (d, p) basis set. The calculations were performed in the gas phase and under the solvent effect of water, dimethylsulfoxide (DMSO), methanol, and benzene. The Hydrogen-Atom Transfer (HAT), single Electron Transfer Followed by Proton Transfer (SET-PT), and sequential Proton Loss Electron Transfer (SPLET) mechanisms were investigated to rationalize the radical scavenging capacities and to identify the favored antioxidant mechanism. Hence, the bond dissociation enthalpies (BDE) ionization potential (IP), IE, proton dissociation enthalpy (PDE), proton affinity (PA), and electron Transfer enthalpy (ETE) related to each mechanism were reported and discussed in function of the solvent effect. For both flavonols, the results showed that 4'-OH hydroxyl is the preferred active site following the trend 4'-OH > 3'-OH > 3-OH > (5'-OH) > 7-OH. Besides, the HAT mechanism is energetically the most favored pathway. The energetically favored solvents follow the trends water > DMSO > benzene > methanol and benzene > DMSO > methanol > water, for Fisetin and Robinetin, respectively.

Tactfully improve the antioxidant activity of 2′-hydroxychalcone with the strategy of substituent, solvent and intramolecular hydrogen bond effects

The substituent and solvent effects on the antioxidant activity of 2′-hydroxychalcone (2′Cha) were systematically explored in gas and three solvent phases by designing six novel compounds (2′Cha-3′NH2, 2′Cha-4′NH2, 2′Cha-5′NH2, 2′Cha-3′CN, 2′Cha-4′CN, and 2′Cha-5′CN) based on the density functional theory (DFT) approach. Significant parameters involved in three primary antioxidant mechanisms, namely the hydrogen atom transfer (HAT), single electron transfer followed by proton transfer (SET-PT), and sequential proton loss electron transfer (SPLET) were computed to confirm the thermodynamically preferred reaction pathway in different media. Besides, 4′-hydroxychalcone (4′Cha) was introduced to unveil the intramolecular hydrogen bond (IHB) effect on the radical scavenging capacity of 2′Cha utilizing the DFT and 2,2-Diphenyl-1-picrylhydrazyl (DPPH) ultraviolet–visible (UV–Vis) spectroscopic methods. Energy barriers and kinetic parameters for HAT reaction of 2′Cha and 4′Cha with hydroperoxyl (HOO) radical were calculated to further compare the radical-trapping process. The photophysical properties of 2′Cha in benzene, chlorobenzene and ethanol are in good agreement with the measured experimental data and the excited-state antiradical capacity improving mechanism was also confirmed experimentally. The current work not only provides valuable guidance for synthesizing high-efficiency antioxidants but also exploits the potential application fields for 2′Cha.

Theoretical study on the radical scavenging activity and mechanism of four kinds of Gnetin molecule

As an important subgroup of resveratrol oligomers, Gnetins received much attention due to their antioxidants. The four Gnetin molecules are divided into two major categories according to different structures, type-A (Gnetin-C, Gnetin-D) and type-B (Gnetin-L, Gnetin-F). Density functional theory (DFT) has been performed thermodynamically and kinetically in detail to analyze the structure and antioxidant activity of four Gnetins toward OH/OOH radical in the gas and solvents phase with four possible antioxidant mechanisms, namely, Hydrogen-atom transfer (HAT), Single electron transfer followed by proton transfer (SET-PT), Sequential proton-loss electron transfer (SPLET), and Radical adduct formation (RAF). From these calculations; Gnetins' order of antioxidant activity was estimated as: Gnetin-C ≈ Gnetin-L > Resveratrol > Gnetin-D > Gnetin-F. All investigations suggested that type A has a higher radical scavenging activity compared to type B. On the basis of the structure–activity relationship, type A structure may have more vital antioxidant potential in the future.

Mechanism of Antiradical Activity of Newly Synthesized 4,7-Dihydroxycoumarin Derivatives-Experimental and Kinetic DFT Study.

Coumarin derivatives have proven beneficial biological activities, but the mechanism of their radical scavenging potency is not fully understood. In this study, the antiradical capacity of two newly synthesized 4,7-dihydroxycoumarin derivatives: (E)-3-(1-((3-hydroxy-4-methoxyphenyl)amino)-ethylidene)-2,4-dioxochroman-7-yl acetate (A-3OH) and (E)-3-(1-((4-hydroxy-3-methoxyphenyl)amino)ethylidene)-2,4-dioxochroman-7-yl acetate (A-4OH) towards HO• were examined by Electron Paramagnetic Resonance (EPR) Spectroscopy and Density Functional Theory (DFT). The compounds were fully characterized by the elemental microanalysis, IR, and NMR spectroscopies. The effect of pH on the acid–base equilibria is separately discussed and the predominant species at the physiological pH were determined. Several common mechanisms (Hydrogen Atom Transfer (HAT), Single-Electron Transfer followed by Proton Transfer (SET-PT), Sequential Proton Loss followed by Electron Transfer (SPLET), Radical Adduct Formation (RAF), and Intramolecular Hydrogen Atom ion (iHAA)) of radical scavenging were investigated based on thermodynamic and kinetic parameters. EPR results indicated that both compounds significantly reduce the amount of present HO•. The results of the kinetic DFT study demonstrated that both compounds predominantly exhibit antiradical capacity through HAT and SPLET mechanisms. The estimated overall rate constants (koverall) proved that A-4OH shows better antioxidant capacity than A-3OH which is well-correlated with the results obtained by EPR measurement.

A Computational Study of the Reactions between Dehydrozingerone Derivatives and the Hydroperoxyl Radical in Aqueous and Lipid Media

Dehydrozingerone (the half-analogue of curcumin) is a natural product that is isolated from ginger rhizomes. Recent studies have shown that this compound displays several physiological activities including antioxidant properties, and it can scavenge oxygen-free radicals like hydroxyl, peroxyl and superoxide radicals. In this work, the thermodynamic and kinetic aspects of the antiradical activity of the natural dehydrozingerone DHZ0 (R=H) and the designed derivatives DHZ1 (R=CHO), DHZ2 (R=OMe) and DHZ3 (R=NMe2) towards the hydroperoxyl radical (HOO•) were investigated at the M06-2X/6-311[Formula: see text]G([Formula: see text],[Formula: see text]) level of theory. The calculations have been performed in gas phase, pentyl ethanoate and water solvents using the implicit SMD solvation model. Four potential mechanisms have been investigated, namely, hydrogen atom transfer (HAT), single-electron transfer followed by proton transfer (SET-PT), sequential proton loss electron transfer (SPLET) and radical adduct formation (RAF). The obtained results show that the SET-PT and RAF mechanisms are endergonic processes and are thermodynamically disfavored, whereas the exergonic HAT process is predicted as the most thermodynamically favored mechanism in all media. The SPLET mechanism was excluded due the minor proportions of the anion species at physiological pH. Kinetic calculations show that DHZ3 reacts with HOO• 1860, 1250 and 215 times faster than DHZ0 in gas phase, pentyl ethanoate, and water, respectively. Based on thermodynamic and kinetic calculations, the designed compound DHZ3 is predicted as a good candidate for hydroperoxyl scavenging in both polar and nonpolar media. Drug likeness evaluation of the studied DHZ derivatives shows that the pharmacokinetic parameters satisfy all the Lipinski and Veber rules.

Improving the antioxidant activity of natural antioxidant honokiol by introducing the amino group.

Exploring and synthesizing the compounds with stronger antioxidant activity have always been the goal of researchers. Herein, the substitution effects of the amino (NH2-) group with the excellent electron-donating ability in different positions on the antioxidant activity of Honokiol (Hon) were systematically explored by using the quantum chemistry calculation based on the density functional theory method. The three possible antioxidant mechanisms of Hon and its four NH2-substituted derivatives (Hon1-Hon4), containing the hydrogen atom transfer (HAT), single electron transfer followed by proton transfer (SET-PT), and sequential proton loss electron transfer (SPLET), were explored in depth considering the gas and solvent phases. In addition, the frontier molecular orbital energies, natural bond orbital (NBO) charge population, and global descriptive parameters were used to study their antioxidant activity. The results indicate that compared with the original molecule Hon, the NH2 substituents would have the stronger antioxidant activity. Moreover, the radical scavenging process of Hon and its derivatives has a disposition to the HAT and SPLET mechanisms in the gas and solvent phases, respectively. Meaningfully, owing to the lowest bond dissociation enthalpy and proton affinity values, Hon4 would show the most prominent antioxidant activity by comparison with the other compounds. In conclusion, this work will provide the purposeful reference for designing and synthesizing the antioxidants with more outstanding performance.

The antioxidant potential of retrochalcones isolated from liquorice root: A comparative DFT study

Polyphenolic compounds are known to exhibit potent antioxidant properties owing to the presence of various phenolic groups. The present study reports the antioxidant potentials of six retrochalcones, namely echinatin, and licochalcone A, B, C, D and E, isolated from the root of the Glycyrrhiza species, toward various reactive oxygen and nitrogen species. Different mechanistic pathways, viz. hydrogen atom transfer (HAT), single electron transfer (SET), single electron transfer followed by proton transfer (SET-PT), and sequential proton loss electron transfer (SPLET), have been considered. In addition, two other pathways, i.e. sequential double proton loss electron transfer (SdPLET) and sequential proton loss hydrogen atom transfer (SPLHAT), which are significant for the scavenging of reactive species by the mono-deprotonated forms of retrochalcones, have also been considered. All the calculations were performed using density functional theory at the B3LYP/6–311++G** level in the gas phase and in aqueous solution. The results suggest the predominance of the HAT mechanism in the gas phase, while in aqueous solution, the SPLET mechanism is thermodynamically favored. The possibility of SdPLET increases at higher pH.

The important role of benzylic C[sbnd]H bond in the antioxidant behaviours of the xanthones

Xanthones are the major active substances in the mangosteen pericarp with well-documented antioxidant activities. The antioxidant activity of 13 different xanthones identified from the mangosteen pericarp was examined in the gas, benzene and water phases using density functional theory. We found that the benzylic C groups play great contribution to the antioxidant activity of the xanthones. The thermodynamic preferred free radical scavenging mechanisms vary in different phases. In the gas and benzene phases, hydrogen atom transfer (HAT) was the most preferred mechanism in the free radical scavenging reaction of xanthones and the benzylic CH groups are the most favourable antioxidant sites. In these phases, α-mangostin is the best antioxidant. In water phase, sequential proton loss electron transfer (SPLET) was more favourable than HAT and single electron transfer followed by proton transfer (SET-PT) mechanisms and the phenolic OH groups play the most important role in the free radical trapping progress. Besides, γ-mangostin is better antioxidant than other xanthones in water phase.

Comparative theoretical researches on the anti-oxidant activity of δ-viniferin and ε-viniferin

Viniferin, the dimer of resveratrol, is an important natural product class with anti-oxidant activities. δ-viniferin and ε-viniferin are two different isomers of viniferin. In this context, a systematic theoretical study by density functional theory (DFT) calculations is carried out on these two isomers’ radical scavenging activity, which focuses on the thermodynamically preferred mechanism, anti-oxidant sites, and anti-oxidant activity sequence of δ-viniferin and ε-viniferin. Three radical scavenging mechanisms named hydrogen atom transfer (HAT), single electron transfer followed by proton transfer (SET-PT) and sequential proton loss electron transfer (SPLET) were discussed. The anti-oxidative capacities of δ-viniferin and ε-viniferin in the gas phase and solvents were elucidated by comparing the thermodynamic parameters such as bond dissociation enthalpy (BDE), ionization potential (IP), proton dissociation enthalpy (PDE), proton affinity (PA), and electron transfer enthalpy (ETE). The computational results showed that δ-viniferin is a more potent anti-oxidant than ε-viniferin. The obtained results may have some important implications for better understanding the relationship between structure and anti-oxidant activity, designing more active anti-oxidants, and providing theoretical guidance for developing and utilizing resveratrol derivatives.

DFT study of the radical scavenging activity of isoxanthohumol, humulones (α-acids), and iso-α-acids from beer

Humulones and iso-humulones are potent natural antioxidants found in beer. In this study, density functional theory (DFT) method was applied for elucidating the structure-antioxidant activity relationship and molecular mechanism of antioxidant activity of eight bioactive humulones previously identified in different beer samples: isoxanthohumol, (R)- and (S)-adhumulone, cis- and trans-iso-adhumulone, cis- and trans-iso-n-humulone, and desdimethyl-octahydro-iso-cohumulone. The calculated bond dissociation enthalpies (BDEs) suggest that desdimethyl-octahydro-iso-cohumulone was the most potent compound with BDEs 5.1 and 23.9 kJ/mol lower compared to the values for resveratrol in gas phase and water, respectively. The enolic –OH is the most reactive site for hydrogen atom transfer (HAT). The presence of β-keto group with respect to enolic –OH diminishes the HAT potency via the formation of a strong intramolecular hydrogen bond. Another common antioxidant mechanism, single electron transfer followed by proton transfer (SET-PT), is only feasible for isoxanthohumol. The results of this study indicate a strong correlation between the increased antioxidant activity of beer products and the higher content of reduced iso-α-acids.