DFT study of HOMO structural map of β-diketones and β-ketoesters; towards prediction of electrochemical oxidation

In this study, anodic oxidation potentials of a training set of the selected 14 β-diketones and β-diketoesters were measured by means of cyclic voltammetry on a glassy carbon electrode and correlated with the highest occupied molecular orbital (HOMO) calculated at the #B3LYP/6-311+G** level. HOMO energy level and HOMO structural map were used to make a powerful model to classify molecules which show similar chemical and electrochemical behaviours. The linear correlation between anodic peak potential, E_pA, and E_HOMO of classified compounds, certifies that they have the same mechanism of the electron transfer reaction.     

Antioxidant activities of carotenoids: quantitative relationships between theoretical calculations and experimental literature data

Quantitative structure activity relationships (QSARs) are described for the antioxidant activity of series of all-trans carotenoids. The antioxidant activity of the carotenoids is characterised by literature data for (i) their relative ability to scavenge the ABTS^·+ radical cation, reflected by the so-called trolox equivalent antioxidant capacity (TEAC) value, (ii) their relative rate of oxidation by a range of free radicals, or (iii) their capacity to inhibit lipid peroxidation in multilamellar liposomes, leading to a decrease in formation of thiobarbituric acid reactive substances (TBARS). All these antioxidant values for radical scavenging action correlate quantitatively with computer-calculated ionisation potentials of the carotenoids. These correlations are observed both when the ionisation potential is calculated as the negative of the energy of the highest occupied molecular orbital (-E(HOMO)) of the molecule, or as the relative change in heat of formation (ΔΔHF) upon the one-electron oxidation of the carotenoids.

The calculations provide a theoretical assay able to characterise the intrinsic electron donating capacity of an antioxidant, in hydrophilic, hydrophobic or artificial membrane environment.     

Poly –N = N– linked 1,5-dihydro-1,5-diazocine,the two-dimensional polymer with Dirac insulator properties: ab initio study

A new material – the monolayered, π-conjugated, poly dinitrogen-linked 1,5-dihydro-1,5-diazocine – is proposed. Density functional theory calculations reveal that the material is a two-dimensional Dirac insulator with a direct band gap. The generalised gradient approximation (GGA) and the local density approximation (LDA) give band gaps of E _gap ^GGA = 0.045 eV and E _gap ^LDA = 0.035 eV. Anisotropy of the Dirac cones results in variation of Fermi velocities of the charge carriers: [0.85–5.30] × 10⁶ m/s (GGA) and [0.86–5.38] × 10⁶ m/s (LDA). Corresponding effective masses are ～10³ lower than that of a free electron. G₀W₀ calculations confirm the topology of density functional theory band structure and show band gap of E _gap = 0.121 eV. Cohesive energies are estimated at E _c ^GGA = 5.30 eV/atom and E _c ^LDA = 6.15 eV/atom. Thus, the results provide the first evidence for the existence of Dirac insulators among organic polymers.     

Effect of functional groups on antioxidant properties of substituted selenoethers

Selenoethers attached to functional groups through propyl chain viz., bis(3-carboxypropyl)selenide (SeBA), bis(3-hydroxypropyl)selenide (SePOH) and bis(3-aminopropyl)selenide dihydrochloride (SePAm), have been examined for their ability to inhibit peroxyl radical mediated DNA damage, peroxyl radical scavenging ability and glutathione peroxidase (GPx) like activity. The DNA damage was monitored by gel electrophoresis, bimolecular rate constants for scavenging of model peroxyl radical were determined by pulse radiolysis and the GPx activity was followed by their ability to reduce hydrogen peroxide in the presence of glutathione utilizing NADPH decay and HPLC analysis. Among these compounds, SeBA showed maximum DNA protecting activity and it was also the most efficient in scavenging peroxyl radicals with the highest GPx mimicking activity. Quantum chemical calculations confirmed that SeBA with the highest energy level of HOMO (highest occupied molecular orbital) is the easiest to undergo oxidation and therefore exhibits better radical scavenging, GPx mimicking and DNA protecting activity than SePOH or SePAm.     

Synthesis of dimeric phenol derivatives and determination of in vitro antioxidant and radical scavenging activities

In this study, di(2,6-dimethylphenol) (Di-DMP), di(2,6-diisopropylphenol) (Di-DIP, dipropofol) and di(2,6-di-t-butylphenol) (Di-DTP) were synthesized by the reaction of monomeric phenol derivatives with catalytic CuCl(OH). TMEDA and Na₂S₂O₄. Their antioxidant capacity and radical scavenging activity were examined using different in vitro methodologies such as 1,1-diphenyl-2-picryl-hydrazyl (DPPH·) free radical scavenging, 2,2′-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS) radical scavenging activity, total antioxidant activity by ferric thiocyanate, total reducing power by potassium ferricyanide reduction method, superoxide anion radical scavenging, hydrogen peroxide scavenging and ferrous ions chelating activities.     

Relating Frontier Orbital Energies from Voltammetry and Photoelectron Spectroscopy to the Open‐Circuit Voltage of Organic Solar Cells

For 19 diketopyrrolopyrrole polymers, the highest occupied molecular orbital (HOMO) energies are determined from i) the oxidation potential with square‐wave voltammetry (SWV), ii) the ionization potential using ultraviolet photoelectron spectroscopy (UPS), and iii) density functional theory (DFT) calculations. The SWV HOMO energies show an excellent linear correlation with the open‐circuit voltage (V_oc) of optimized solar cells in which the polymers form blends with a fullerene acceptor ([6,6]‐phenyl‐C₆₁‐butyl acid methyl ester or [6,6]‐phenyl‐C₇₁‐butyl acid methyl ester). Remarkably, the slope of the best linear fit is 0.75 ± 0.04, i.e., significantly less than unity. A weaker correlation with V_oc is found for the HOMO energies obtained from UPS and DFT. Within the experimental error, the SWV and UPS data are correlated with a slope close to unity. The results show that electrochemically determined oxidation potentials provide an excellent method for predicting the V_oc of bulk heterojunction solar cells, with absolute deviations less than 0.1 V.     

Antioxidant activity of bisbenzylisoquinoline alkaloids from Stephania rotunda: cepharanthine and fangchinoline

In the present study, we determined the antioxidant activity of cepharanthine and fangchinoline from Stephania rotunda by performing different in vitro antioxidant assays, including 1,1-diphenyl-2-picryl-hydrazyl (DPPH) free radical scavenging, 2,2′-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS) radical scavenging, N,N- dimethyl-p-phenylenediamine dihydrochloride (DMPD) radical scavenging, superoxide anion (O₂^?–) radical scavenging, hydrogen peroxide scavenging, total antioxidant activity, reducing power, and ferrous ion (Fe²⁺) chelating activities. Cepharanthine and fangchinoline showed 94.6 and 93.3% inhibition on lipid peroxidation of linoleic acid emulsion at 30 μg/mL concentration, respectively. On the other hand, butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), α-tocopherol, and trolox indicated inhibitions of 83.3, 92.2, 72.4, and 81.3% on peroxidation of linoleic acid emulsion at the same concentration (30 μg/mL), respectively. According to the results, cepharanthine and fangchinoline have effective antioxidant and radical scavenging activity.     

Substituent effects on the electronic structures and nonlinear optical properties of Li-doped nano-carbon bowl

A series of Li-corannulene-(NH₂)_n and Li-corannulene-(NO₂)_n (n = 1, 2, 5) compounds have been theoretically designed and investigated using density functional theory. In this work, two models are systematically investigated to explore the important factors for enhancing the static first hyperpolarizability by introducing the substitution group. It is revealed that energy gaps (E_gap) between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of all compounds are in the range of 4.149–4.934 eV. Different DFT methods are adopted to calculate polarizabilities and the first hyperpolarizabilities of Li-corannulene-(NH₂)_n and Li-corannulene-(NO₂)_n (n = 1, 2, 5) compounds. It is revealed that polarizability values of the systems increase with increasing number of NH₂/NO₂ substitution group. Moreover, it is found that the first hyperpolarizabilities of Li-corannulene-(NO₂)_n are larger than those of Li-corannulene-(NH₂)_n, which can be attributed to the lower transition energies. In contrast to the NH₂ substitution group, NO₂ substitution group can be more powerful in increasing the first hyperpolarizability of Li-doped corannulene. We hope that this study can provide a new idea for designing nonlinear optical materials using the NH₂ and NO₂ groups.     

Quantitative structure activity relationships of some pyridine derivatives as corrosion inhibitors of steel in acidic medium

Quantum chemical calculations using the density functional theory (B3LYP/6-31G* DFT) and semi-empirical AM1 methods were performed on ten pyridine derivatives used as corrosion inhibitors for mild steel in acidic medium to determine the relationship between molecular structure and their inhibition efficiencies. Quantum chemical parameters such as total negative charge (TNC) on the molecule, energy of highest occupied molecular orbital (E _HOMO), energy of lowest unoccupied molecular orbital (E _LUMO) and dipole moment (μ) as well as linear solvation energy terms, molecular volume (Vi) and dipolar-polarization (π*) were correlated to corrosion inhibition efficiency of ten pyridine derivatives. A possible correlation between corrosion inhibition efficiencies and structural properties was searched to reduce the number of compounds to be selected for testing from a library of compounds. It was found that theoretical data support the experimental results. The results were used to predict the corrosion inhibition of 24 related pyridine derivatives.     

Theoretical insight into the conversion of xylose to furfural in the gas phase and water

The antioxidant properties of some phenolic Schiff bases in the presence of different reactive particles such as ^•OH, ^•OOH, (CH₂=CH−O−O^•), and ^-•O₂ were investigated. The thermodynamic values, ΔH _BDE, ΔH _IP, and ΔH _PA, were used for this purpose. Three possible mechanisms for transfer of hydrogen atom, concerted proton−electron transfer (CPET), single electron transfer followed by proton transfer (SET-PT), and sequential proton loss electron transfer (SPLET) were considered. These mechanisms were tested in solvents of different polarity. On the basis of the obtained results it was shown that SET-PT antioxidant mechanism can be the dominant mechanism when Schiff bases react with radical cation, while SPLET and CPET are competitive mechanisms for radical scavenging of hydroxy radical in all solvents under investigation. Examined Schiff bases react with the peroxy radicals via SPLET mechanism in polar and nonpolar solvents. The superoxide radical anion reacts with these Schiff bases very slowly.

     

Evaluation of antioxidant properties of medical plants using microbial test systems

The antioxidant activities of extracts from leaves of the medicinal plants growing in Siberia were examined. Total antioxidant activity was determined using in vitro methods including DPPH (2,2-diphenyl-1-picrylhydrazyl radical) free radical scavenging assay, chelating capacity assay with ferrozine, evaluation of capacity to protect plasmid DNA against oxidative damage, measurement of H₂O₂ production, and measurement of total flavonoid and tannin content as well. Using in vivo experiments, we also evaluated capacities of the plant extracts to protect bacteria Escherichia coli against bacteriostatic and bactericidal effects of H₂O₂, and influence of the plant extracts on expression of antioxidant gene katG, encoding catalase. The extracts from Chamerion angustifolium, Filipendula vulgaris and Pyrola rotundifolia indicated the highest levels of antioxidant activity both in vivo and in vitro. Our data suggest that the extracts of the tested plants may provide antioxidant effects on bacteria simultaneously through different pathways, including direct radical scavenging, iron chelation and induction of genes encoding antioxidant enzymes.     

3种百合鳞茎中多酚类物质的抗氧化活性分析

以野生百合渥丹、山丹和传统食用的兰州百合为研究对象,对其鳞茎中多酚类物质、11种单体酚的含量及抗氧化活性(ABTS自由基、超氧阴离子、羟自由基的清除能力,铜离子还原能力以及抑制脂质过氧化活性)进行了分析。结果表明:两种野生百合鳞茎中的多酚类物质含量及抗氧化活性均显著高于兰州百合。3种百合鳞茎中单体酚的种类也有所不同,但均含有没食子酸、矢车菊素-3-芸香糖苷、儿茶素、表儿茶素、杨梅酮、芦丁、对香豆酸、山奈酚。相关性分析显示,除对羟自由基的清除力外,各酚类物质总量与不同抗氧化指标之间呈显著正相关关系。试验结果认为,野生百合鳞茎可作为天然抗氧化资源应用于食品和医药业,具有一定的开发应用前景。     

Antioxidant and free radical scavenging properties of N-acetylcysteine amide (NACA) and comparison with N-acetylcysteine (NAC)

The antioxidant potential of N-acetylcysteine amide (NACA), also known as AD4, was assessed by employing different in vitro assays. These included reducing power, free radical scavenging capacities, peroxidation inhibiting activity through linoleic acid emulsion system and metal chelating capacity, as compared to NAC and three widely used antioxidants, α-tocopherol, ascorbic acid and butylated hydroxytoluene (BHT). Of the antioxidant properties that were investigated, NACA was shown to possess higher 2,2-diphenyl-1-picryl-hydrazyl-hydrate (DPPH) radical scavenging ability and reducing power than NAC, at all the concentrations, whereas the scavenging ability of H₂O₂ differed with concentration. While NACA had greater H₂O₂ scavenging capacity at the highest concentration, NAC was better than NACA at lower concentrations. NAC and NACA had a 60% and 55% higher ability to prevent β-carotene bleaching, respectively, as compared to control. The chelating activity of NACA was more than 50% that of the metal chelating capacity of EDTA and four and nine times that of BHT and α-tocopherol, respectively. When compared to NACA and NAC; α-tocopherol had higher DPPH scavenging abilities and BHT and α-tocopherol had better β-carotene bleaching power. These findings provide evidence that the novel antioxidant, NACA, has indeed enhanced the antioxidant properties of NAC.     

The influence of the cation of quaternized chitosans on antioxidant activity

Various quaternized chitosans (QCSs) were synthesized according to previous method. Their reducing power and antioxidant potency against hydroxyl radicals (OH) and hydrogen peroxide (H₂O₂) were explored by the established systems in vitro. The QCSs exhibited markedly antioxidant activity, especially TCEDMCS, whose IC₅₀ on hydroxyl radicals was 0.235 mg/mL. They showed 65–80% scavenging effect on hydrogen peroxide at a dose of 0.5 mg/mL. Generally, the antioxidant activity decreased in the order TCEDMCS > TBEDMCS > EDMCS > PDMCS > IBDMCS > Chitosan. Furthermore, the order of their OH and H₂O₂ scavenging activity was consistent with the electronegativity of different substituted groups in the QCSs. The QCSs showed much stronger antioxidant activity than that of chitosan may be due to the positive charge density of the nitrogen atoms in QCSs strengthened by the substituted groups.     

Kinetic and molecular orbital studies on the rate of oxidation of monosubstituted phenols and anilines by lactoperoxidase compound II in comparison with the case of horseradish peroxidase

The second-order rate constant (k4) for the oxidation of monosubstituted phenols and anilines by lactoperoxidase compound II was examined by Chance's method [B. Chance, Arch. Biochem. Biophys. 71 (1957), 130–136]. When the electronic states of these substrates were calculated by an ab initio molecular orbital method, it was found that the log k4 value correlates well with the highest occupied molecular orbital (HOMO) energy level but not with the net charge or frontier electron density. These results are essentially similar to those reported previously in the case of horseradish peroxidase [J. Sakurada, R. Sekiguchi, K. Sato, and T. Hosoya, Biochemistry 29 (1990), 4093–4098], showing some dissimilar features which are considered to reflect the structural difference between the two enzymes.Abbreviations HOMO highest occupied molecular orbital - HRP horseradish peroxidase - LPO lactoperoxidase (EC 1.11.1.7) - LUMO lowest unoccupied molecular orbital     

Molecular dynamics study of methane in water: diffusion and structure

In this study, theoretical analysis on the geometries and electronic properties of various conjugated oligomers based on thiophene (Th) or bicyclic non-classical Th units is reported. The dihedral angle, bond length, bond-length alternation, bond critical point (BCP) properties, nucleus-independent chemical shift (NICS) and Wiberg bond index (WBIs) are analysed and correlated with conduction properties. The changes of bond length, BCP properties, NICS and WBIs all show that the conjugational degree is increased systematically with main chain extension. As a result, the highest occupied molecular orbital–lowest unoccupied molecular orbital energy separation (E _g) is decreased upon chain elongation. The E _g of oligomers based on bicyclic non-classical Th unit is much lower than that of Th-based oligomers due to the narrower E _g of bicyclic non-classical Ths compared to Th, which indicates that the narrow E _g of the bicyclic non-classical Ths can be carried over to their polymers by using them as building blocks for the polymers. The band structures and density of states analysis show that the four polymers all have small band gap ( < 0.9 eV), wide highest occupied bandwidth and relatively small effective mass of hole, which indicate that those proposed polymers may be potential conductors.     

香桃木叶片粗多酚抗氧化活性的研究

以香桃木叶片为材料,60%乙醇提取获得粗多酚样品,并对样品的油脂抗氧化性和自由基清除能力进行研究。结果表明,未经纯化的香桃木粗多酚对油脂抗氧化效果优于BHT和V_C等人工合成抗氧化剂;在清除自由基的实验中,相同浓度的香桃木粗多酚对DPPH自由基的清除率为V_E的5倍,对羟自由基的清除效果也略优于V_C。由此可以预见香桃木粗多酚在精细加工、经纯化后抗氧化效果会更加显著。     

Antioxidant,metal-binding and DNA-damaging properties of flavonolignans: A joint experimental and computational highlight based on 7-O-galloylsilybin

Besides the well-known chemoprotective effects of polyphenols, their prooxidant activities via interactions with biomacromolecules as DNA and proteins are of the utmost importance. Current research focuses not only on natural polyphenols but also on synthetically prepared analogs with promising biological activities. In the present study, the antioxidant and prooxidant properties of a semi-synthetic flavonolignan 7-O-galloylsilybin (7-GSB) are described. The presence of the galloyl moiety significantly enhances the antioxidant capacity of 7-GSB compared to that of silybin (SB). These findings were supported by electrochemistry, DPPH (2,2-diphenyl-1-picrylhydrazyl) scavenging activity, total antioxidant capacity (CL-TAC) and DFT (density functional theory) calculations. A three-step oxidation mechanism of 7-GSB is proposed at pH 7.4, in which the galloyl moiety is first oxidized at E_p,1 = +0.20 V (vs. Ag/AgCl_{3M KCl}) followed by oxidation of the 20-OH (E_p,2 = +0.55 V) and most probably 5-OH (E_p,3 = +0.95 V) group of SB moiety. The molecular orbital analysis and the calculation of O–H bond dissociation enthalpies (BDE) fully rationalize the electrooxidation processes. The metal (Cu²⁺) complexation of 7-GSB was studied, which appeared to involve both the galloyl moiety and the 5-OH group. The prooxidant effects of the metal-complexes were then studied according to their capacity to oxidatively induce DNA modification and cleavage. These results paved the way towards the conclusion that 7-O-galloyl substitution to SB concomitantly (i) enhances antioxidant (ROS scavenging) capacity and (ii) decreases prooxidant effect/DNA damage after Cu complexation. This multidisciplinary approach provides a comprehensive mechanistic picture of the antioxidant vs. metal-induced prooxidant effects of flavonolignans at the molecular level, under ex vivo conditions.     

Isolation and antioxidant property of the extracellular polysaccharide from <Emphasis Type="Italic">Rhodella reticulata</Emphasis>

The extracellular polysaccharide from Rhodella reticulata was separated from the culture medium followed by concentration and ethanol precipitation, and purified by anion exchange chromatography on DEAE-Sepharose Fast Flow. This study compared the free radical-scavenging property and antioxidant activity with various treatments of crude extracellular polysaccharides of R. reticulata. The results showed that both the crude extracellular polysaccharide and deproteinized crude extracellular polysaccharide gave evidence of the free radical scavenging and antioxidant activity in a dose-dependent manner. The crude extracellular polysaccharide exhibited higher free radical scavenging capacity and better antioxidant activity than the various treatments of crude extracellular polysaccharide samples. The superoxide anion radical scavenging ability of various samples was significantly higher compared to standard antioxidant (α-tocopherol). These results indicate that the extracellular polysaccharide of R. reticulata is a potent natural antioxidant.     

In vitro antioxidant activity of silymarin

Silymarin, a known standardized extract obtained from seeds of Silybum marianum is widely used in treatment of several diseases of varying origin. In the present paper, we clarified the antioxidant activity of silymarin by employing various in vitro antioxidant assay such as 1,1-diphenyl-2-picryl-hydrazyl free radical (DPPH·) scavenging, 2,2′-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS) radical scavenging activity, total antioxidant activity determination by ferric thiocyanate, total reducing ability determination by Fe³⁺ ? Fe²⁺ transformation method and Cuprac assay, superoxide anion radical scavenging by riboflavin/methionine/illuminate system, hydrogen peroxide scavenging and ferrous ions (Fe²⁺) chelating activities. Silymarin inhibited 82.7% lipid peroxidation of linoleic acid emulsion at 30 μg/mL concentration; butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), α-tocopherol and trolox indicated inhibition of 83.3, 82.1, 68.1 and 81.3% on peroxidation of linoleic acid emulsion at the same concentration, respectively. In addition, silymarin had an effective DPPH· scavenging, ABTS^√+ scavenging, superoxide anion radical scavenging, hydrogen peroxide scavenging, ferric ions (Fe³⁺) reducing power by Fe³⁺ ? Fe²⁺ transformation, cupric ions (Cu²⁺) reducing ability by Cuprac method, and ferrous ions (Fe²⁺) chelating activities. Also, BHA, BHT, α-tocopherol and trolox, were used as the reference antioxidant and radical scavenger compounds. Moreover, this study, which clarifies antioxidant mechanism of silymarin, brings new information on the antioxidant properties of silymarin. According to the present study, silymarin had effective in vitro antioxidant and radical scavenging activity. It could be used in the pharmacological and food industry because of its antioxidant properties.