A DFT Study on the Structural and Antioxidant Properties of Three Flavonols

The structural and antioxidant activity properties of three flavonols kaempferol, galangin and morin have been investigated at density functional level of theory with the aim of verifying experimental findings. The potentialities of antioxidant activity are highly related to their capabilities to scavenge free radicals. Two potential working mechanisms of the hydrogen-atom transfer and single-electron transfer are reported by which antioxidants can play their role. Two parameters of the O-H bond dissociation enthalpy (BDE) and ionization potential (IP) in the presence of water medium are computed to estimate the antioxidant capacities. Results indicate that the order of antioxidant efficacies predicted theoretically in this work is in agreement with that reported by experimental results of oxygen radical-scavenging capacity (ORAC) assay. This demonstrates the importance of the hydrogen-atom and single-electron transfer mechanisms to explain their capacities to scavenge peroxyl radical.     

三种黑米花色苷自由基清除能力的理论评价

用一种组合的密度泛函理论方法计算了三种黑米花色苷的O-H键解离焓和电离势,以此作为理论指标评价其自由基清除能力。计算结果发现花色苷的电离势和总抗氧化能力存在良好的线性关系,提示电离势表征的质子转移伴随电子转移的反应机制在清除自由基的过程中发挥着重要作用。     

Quantitative elucidation of the molecular mechanisms of hydroxyl radical quenching reactivity of phenolic compounds

Reported discrepancies have confused the understanding of the molecular mechanisms of antioxidant reactivity somewhat. The consequent problems necessitate systematic investigations on the molecular orbital features of antioxidants and their correlation with antioxidant potentials. In the present work, phenolic compounds as typical antioxidants were selected to investigate their hydroxyl radical-scavenging properties, and the related mechanisms of action were studied theoretically by computational chemistry. A good correlation was observed between antioxidant activity and theoretical parameters, such as O-H bond dissociation energy (BDE), ionization potential (IP), enthalpy of electron transfer (E(a)), chemical hardness (HOMO-LUMO gap), and spin delocalization of the phenoxyl radicals (D(s)(r)). The results demonstrate that the molecular mechanisms regulating the antioxidant action were more complex than hydrogen or electron-transfer processes and explain previous contradictions. Meanwhile, a satisfactory quantitative structure-activity relationship (QSAR) model was established which should be of predictive value in evaluating or screening hydroxyl radical-scavenging antioxidants.     

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.     

A theoretical study on the structure-activity relationships of metabolites of folates as antioxidants and its implications for rational design of antioxidants

To elucidate the structure-activity relationships of metabolites of folates as antioxidants, the O-H bond dissociation enthalpies (BDEs) and ionization potentials (IPs) for these compounds were calculated by density functional theory (DFT) on B3LYP/6-31+G(,3pd) level. Accordingly, the antioxidant activity difference for metabolites of folates can be elucidated by O-H BDE and IP values and can be further explained in terms of electronic effect and intramolecular hydrogen bond effect of substituents. Furthermore, the potential of the active center of metabolites of folates, 4-hydroxypyrimidine (4-HP), as lead antioxidant, was evaluated by comparing the BDEs and IPs of 4-HP with those of 5-hydroxypyrimidine (5-HP). It was revealed that 4-HP and 5-HP held identical IPs, but the O-H BDE of the former was 22.84 kcal/mol higher than that of the latter, which meant 4-HP was inert in H-atom donation. Nevertheless, the O-H BDE of 4-HP was very sensitive to the substituents, which made NH2-derivatives of 4-HP very active as antioxidants. Therefore, 4-HP is also a potential lead antioxidant and deserves attention in rational design of antioxidants.     

The ortho hydroxy-amino group: another choice for synthesizing novel antioxidants

Four ortho hydroxy-amino derivatives have been designed based on the structures of flavonoids to explore the effect of the ortho hydroxy-amino group on the antioxidant properties of molecules, and their bond dissociation enthalpies (BDE), ionization potentials (IP), the highest occupied molecular orbitals (HOMO), and spin densities have been calculated. The results reveal that the ortho hydroxy-amino group plays an important role in promoting the antioxidant properties of molecules because of its lowering effect on BDE, IP, and spin density. The derivatives with ortho hydroxy-amino group show stronger antioxidant activity than the derivatives with mono hydroxy or ortho dihydroxy group. Thus, the ortho hydroxy-amino group can be used as another potential functional group to synthesize novel antioxidants as guessed.     

The influence of pH on antioxidant properties and the mechanism of antioxidant action of hydroxyflavones

The effect of the pH on antioxidant properties of a series of hydroxyflavones was investigated. The pKa of the individual hydroxyl moieties in the hydroxyflavones was compared to computer-calculated deprotonation energies. This resulted in a quantitative structure activity relationship (QSAR), which enables the estimation of pKa values of individual hydroxyl moieties, also in hydroxyflavones for which these pKa values are not available. Comparison of the pKa values to the pH-dependent antioxidant profiles, determined by the TEAC assay, reveals that for various hydroxyflavones the pH-dependent behavior is related to hydroxyl moiety deprotonation, resulting in an increase of the antioxidant potential upon formation of the deprotonated forms. Comparison of these experimental results to computer calculated O-H bond dissociation energies (BDE) and ionization potentials (IP) of the nondeprotonated and the deprotonated forms of the various hydroxyflavones indicates that especially the parameter reflecting the ease of electron donation, i.e., the IP, and not the BDE, is greatly influenced by the deprotonation. Based on these results it is concluded that upon deprotonation the TEAC value increases (radical scavenging capacity increases) because electron-, not H*-, donation becomes easier. Taking into account that the mechanism of radical scavenging antioxidant activity of the neutral form of the hydroxyflavones is generally considered to be hydrogen atom donation, this implies than not only the ease of radical scavenging, but also the mechanism of antioxidant action changes upon hydroxyflavone deprotonation.     

Flavonoids from Rhamnus alaternus L. (Rhamnaceae): Kaempferol 3-O-β-isorhamninoside and rhamnocitrin 3-O-β-isorhamninoside protect against DNA damage in human lymphoblastoid cell and enhance antioxidant activity

Kaempferol 3-O-β-isorhamninoside (K3O-ir) and rhamnocitrin 3-O-β-isorhamninoside (R3O-ir), were isolated from Rhamnus alaternus L leaves. The genotoxic and antigenotoxic properties of these compounds were investigated by assessing the induction and inhibition of the genotoxicity induced by the direct-acting mutagen, hydrogen peroxide (H₂O₂), using the “comet assay.” K3O-ir and R3O-ir exhibited a preventive effect against H₂O₂ induced DNA damages in human lymphoblastoid TK6 cells and its derivative the p53 deficient cell line NH32.These two flavonoids, also investigated for their antioxidant capacities, using different antioxidant tests, such as: Cuprac, Frap and reducing power assays, revealed significant activity through their capacities to transfer electrons.     

Exploring a possible way to synthesize novel better antioxidants based on vitamin E: a DFT study

The promoting effect of heterocyclic ring and heteroatom on the antioxidant properties of vitamin E has been investigated systemically by using density functional theory. The calculated results show that the heteroatom plays a more important role in promoting the antioxidant properties than the heterocyclic ring, indicating that replacing O atom by S or Se is impossible to synthesize the novel better antioxidants. Furthermore, the bond dissociation enthalpy (BDE) and ionization potential (IP) of the corresponding molecules are decreased dramatically when the O atom is replaced by NH. In addition, the calculated results also reveal that reducing the atom number of heterocyclic ring is an ideal way to synthesize novel antioxidants with better antioxidant activity than vitamin E.     

Mechanistic study of the structure–activity relationship for the free radical scavenging activity of baicalein

Density functional theory calculations were performed to evaluate the antioxidant activity of baicalein. The conformational behaviors of both the isolated and the aqueous-solvated species (simulated with the conductor-like polarizable continuum solvation model) were analyzed at the M052X/6-311 + G(d,p) level. The most stable tautomers of various forms of baicalein displayed three IHBs between O4 and OH5, O5 and OH6, and O6 and OH7. The most stable tautomer of the baicalein radical was obtained by dehydrogenating the hydroxyl at C6, while the most stable anion tautomer was obtained by deprotonating the C7 hydroxyl in gaseous and aqueous phases. The expected antioxidant activity of baicalein was explained by its ionization potentials (IPs) and homolytic O–H bond dissociation enthalpies (BDEs), which were obtained via the UM052X optimization level of the corresponding radical species. Heterolytic O–H bond cleavages (proton dissociation enthalpies, PDEs) were also computed. The calculated IP, BDE, and PDE values suggested that one-step H-atom transfer, rather than sequential proton loss–electron transfer or electron transfer–proton transfer, would be the most favorable mechanism for explaining the antioxidant activity of baicalein in the gas phase and in nonpolar solvents. In aqueous solution, the SPLET mechanism was more important.     

Benchmarking the DFT methodology for assessing antioxidant-related properties: quercetin and edaravone as case studies

The overall objective was to identify an accurate computational electronic method to virtually screen phenolic compounds through their antioxidant and free-radical scavenging activity. The impact of a key parameter of the density functional theory (DFT) approach was studied. Performances of the 21 most commonly used exchange-correlation functionals are thus detailed in the evaluation of the main energetic parameters related to the activities of two prototype antioxidants, namely quercetin and edaravone, is reported. These functionals have been chosen among those belonging to three different families of hybrid functionals, namely global, range separated, and double hybrids. Other computational parameters have also been considered, such as basis set and solvent effects. The selected parameters, namely bond dissociation enthalpy (BDE), ionization potential (IP), and proton dissociation enthalpy (PDE) allow a mechanistic evaluation of the antioxidant activities of free radical scavengers. Our results show that all the selected functionals provide a coherent picture of these properties, predicting the same order of BDEs and PDEs. However, with respect to the reference values, the errors found at CBS-Q3 level significantly vary with the functional. Although it is difficult to evidence a global trend from the reported data, it clearly appears that LC-ωPBE, M05-2X, and M06-2X are the most suitable approaches for the considered properties, giving the lowest cumulative mean absolute errors. These methods are therefore suggested for an accurate and fast evaluation of energetic parameters related to an antioxidant activity via free radical scavenging.     

The antioxidant system in Suaeda salsa under salt stress

ABSTRACTSuaeda salsaL. is a typical euhalophyte and is widely distributed throughout the world. Suaeda plants are important halophyte resources, and the physiological and biochemical characteristics of their various organsand their response to salt stress have been intensively studied. Leaf succulence, intracellular ion localization, increased osmotic regulation and enhanced antioxidant capacities are important responses for Suaeda plants to adapt to salt stress. Among these responses, scavenging of reactive oxygen species (ROS) is an important mechanism for plants to withstand oxidative stress and improve salt tolerance. The generation and scavenging pathways of ROS, as well as the expression of scavenging enzymes change under salt stress. This article reviews the antioxidant system constitute of S. salsa, and the mechanisms by which S. salsaantioxidant capacity is improved for salt tolerance. In addition, the differences between types of antioxidant mechanisms in S. salsaare reviewed, thereby revealing the adaptation mechanisms of Suaeda to different habitats. The review provides important clues for the comprehensive understanding of the salt tolerance mechanisms of halophytes.KEYWORDS: Suaeda salsa, halophyte, salt-tolerance mechanism, oxidative stress, antioxidant system     

Effect of different C3-aryl substituents on the antioxidant activity of 4-hydroxycoumarin derivatives

The antioxidant activity of 4-hydroxycoumarin synthetic derivatives and 4-methylumbelliferone were determined taking 4-hydroxycoumarin as the reference compound. Six 3-aryl-4-hydroxycoumarin derivatives were synthesized from 4-hydroxycoumarin as precursor in order to evaluate changes in their antioxidant properties due to C3-aryl substituent nature. Free radical scavenging capacities of these compounds against two different species DPPH(·) and ABTS(·+) and the protecting ability towards the β-carotene-linoleic acid co-oxidation enzymatically induced by lipoxygenase were measured. In addition, the relationship between the activities of these molecules against DPPH radical and the bond dissociation energy of O-H (BDE) calculated using methods of computational chemistry was evaluated.     

Differences in oxidative stress, antioxidant systems, and microscopic analysis between regenerating callus-derived protoplasts and recalcitrant leaf mesophyll-derived protoplasts of Citrus reticulata Blanco

It is known that protoplasts derived from either leaves or suspension cultures of a citrus genotype vary greatly in their regeneration capacities; however, the underlying physiological mechanisms are not well known. In this study, oxidative stress and antioxidant systems during in vitro culture of callus-derived protoplasts and leaf mesophyll-derived protoplasts of Ponkan (Citrus reticulata Blanco) were analyzed to gain insights into observed physiological differences. Morphological observations using light microscopy and scanning microscopy have shown that new cell wall materials appeared within 2–3 days, and the integrate cell walls were regenerated approximately after 6 days of culture of the callus protoplasts, whereas no cell wall formation was observed in the mesophyll protoplasts after culture. During the culture, higher levels of H₂O₂ and malondialdehyde were detected in the mesophyll protoplasts as compared with the callus ones. On the contrary, the callus protoplasts possessed higher activities of antioxidant enzymes (SOD, POD and CAT) and larger amount of glutathione and ascorbic acid (at one time point) than the mesophyll protoplasts during the culture process. The current data indicate that the mesophyll and callus protoplasts displayed remarkable difference in the degree of oxidative stress and the antioxidant systems, suggesting that high levels of antioxidant activities might play an important role in the regeneration of protoplasts.     

Analysis of the antioxidant activity of 4-(5-chloro-2-hydroxyphenylamino)-4-oxobut-2-enoic acid derivatives using quantum-chemistry descriptors and molecular docking

In the present work, the molecular structure and the antioxidant activity of 4-(5-chloro-2-hydroxyphenylamino)-4-oxobut-2-enoic acid (A) and its derivatives (B-E) have been studied at the B3LYP/6-31++G(2d,2p) computational level. The obtained results indicate that the hydrogen atom transfer (HAT mechanism) is thermodynamically more favored in gas phase; whereas, the sequential proton loss-electron transfer (SPLET mechanism) is more preferred in polar solvents. The antioxidant activity of compounds A-E is also analyzed by the calculation of atomic spin densities, chemical hardnesses, dipole moments, and lipophilicity indexes. It turns out that compound E (R?=?t-Bu) is predicted to be more antioxidant than ascorbic acid and other derivatives A-D in both gas phase and polar solvents. The high antioxidant activity of compound E compared to other derivatives A-D is also rationalized using the molecular docking technique.     

Developmental and physiological challenges of octopus (Octopus vulgaris) early life stages under ocean warming

The ability to understand and predict the effects of ocean warming (under realistic scenarios) on marine biota is of paramount importance, especially at the most vulnerable early life stages. Here we investigated the impact of predicted environmental warming (+3 °C) on the development, metabolism, heat shock response and antioxidant defense mechanisms of the early stages of the common octopus, Octopus vulgaris. As expected, warming shortened embryonic developmental time by 13 days, from 38 days at 18 °C to 25 days at 21 °C. Concomitantly, survival decreased significantly (~29.9 %). Size at hatching varied inversely with temperature, and the percentage of smaller premature paralarvae increased drastically, from 0 % at 18 °C to 17.8 % at 21 °C. The metabolic costs of the transition from an encapsulated embryo to a free planktonic form increased significantly with warming, and HSP70 concentrations and glutathione S-transferase activity levels were significantly magnified from late embryonic to paralarval stages. Yet, despite the presence of effective antioxidant defense mechanisms, ocean warming led to an augmentation of malondialdehyde levels (an indicative of enhanced ROS action), a process considered to be one of the most frequent cellular injury mechanisms. Thus, the present study provides clues about how the magnitude and rate of ocean warming will challenge the buffering capacities of octopus embryos and hatchlings’ physiology. The prediction and understanding of the biochemical and physiological responses to warmer temperatures (under realistic scenarios) is crucial for the management of highly commercial and ecologically important species, such as O. vulgaris.     

Chemical Composition and Biological Activities of Trans-Himalayan Alga Spirogyra porticalis (Muell.) Cleve

The freshwater alga Spirogyra porticalis (Muell.) Cleve, a filamentous charophyte, collected from the Indian trans-Himalayan cold desert, was identified on the basis of morpho-anatomical characters. Extracts of this alga were made using solvents of varying polarity viz. n-hexane, acetonitrile, methanol and water. The antioxidant capacities and phenolic profile of the extracts were estimated. The methanol extract showing highest antioxidant capacity and rich phenolic attributes was further investigated and phytochemical profiling was conducted by gas chromatography-mass spectrometry (GC/MS) hyphenated technique. The cytotoxic activity of methanol extract was evaluated on human hepatocellular carcinoma HepG2 and colon carcinoma RKO cell lines. The anti-hypoxic effect of methanol extract of the alga was tested on in vivo animal system to confirm its potential to ameliorate oxidative stress. The antioxidant assays viz. ferric reducing antioxidant power (FRAP), 2,2''-azinobis-(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS), 1,1-diphenyl-2-picrylhydrazyl (DPPH) and nitric oxide (NO) radical scavenging capacities, β-carotene-linoleic acid bleaching property and lipid peroxidation exhibited analogous results, wherein the algal extracts showed significantly high antioxidant potential. The extracts were also found to possess high content of total proanthocyanidin, flavonoid and polyphenol. GC/MS analysis revealed the presence of thirteen chemotypes in the methanol extract representing different phytochemical groups like fatty acid esters, sterols, unsaturated alcohols, alkynes etc. with substantial phyto-pharmaceutical importance. The methanol extract was observed to possess anticancer activity as revealed from studies on HepG2 and RKO cell lines. In the present study, S. porticalis methanol extract also provided protection from hypoxia-induced oxidative stress and accelerated the onset of adaptative changes in rats during exposure to hypobaric hypoxia. The bioactive phytochemicals present in this trans-Himalayan alga are of enormous interest and can be utilized sustainably for discovery of novel drugs against oxidative stress.