Vessela D. Kancheva

Antioxidant Activity of Chalcones: The Chemiluminescence Determination of the Reactivity and the Quantum Chemical Calculation of the Energies and Structures of Reagents and Intermediates

Six antioxidants from the class of chalcones (ArOH), compounds from which flavonoids are obtained in nature, were studied. The antiradical activity of chalcones and a number of related compounds was determined by a chemiluminescence method using the scavenging of peroxide radicals ROO· + ArOH → ROOH + OAr· (with the rate constant k7) in a model reaction of diphenylmethane (RH) oxidation. The structures and energies of the reagents and intermediates were determined by semiempirical quantum chemical (PM3, PM6) calculations. 3,4-Dihydroxychalcone and caffeic acid, which have a catechol structure, that is, two neighboring OH groups in phenyl ring B, exhibited high antioxidant activity (k7 ≈ 107 l mol−1 s−1); this is consistent with the lowest bond strengths D(ArO-H) of 79.2 and 76.6 kcal/mol, respectively. The abstraction of a hydrogen atom by the ROO· radical is the main reaction path of these compounds; however, the low stoichiometric coefficients of inhibition (f = 0.3–0.7) suggest a contribution of secondary and/or side reactions of ArOH and OAr·. In the other chalcones, the ArO-H bond is stronger (D(ArO-H) = 83–88 kcal/mol) and the antioxidant activity is lower (k7 = 104–105 l mol−1 s−1).

Antioxidant potential of curcumin-related compounds studied by chemiluminescence kinetics, chain-breaking efficiencies, scavenging activity (ORAC) and DFT calculations

Summary This study compares the ability to scavenge different peroxyl radicals and to act as chain-breaking antioxidants of monomers related to curcumin (1): dehydrozingerone (2), zingerone (3), (2Z,5E)-ethyl 2-hydroxy-6-(4-hydroxy-3-methoxyphenyl)-4-oxohexa-2,5-dienoate (4), ferulic acid (5) and their corresponding C 2-symmetric dimers 6–9. Four models were applied: model 1 – chemiluminescence (CL) of a hydrocarbon substrate used for determination of the rate constants (k A) of the reactions of the antioxidants with peroxyl radicals; model 2 – lipid autoxidation (lipidAO) used for assessing the chain-breaking antioxidant efficiency and reactivity; model 3 – oxygen radical absorbance capacity (ORAC), which yields the activity against peroxyl radicals generated by an azoinitiator; model 4 – density functional theory (DFT) calculations at UB3LYP/6-31+G(d,p) level, applied to explain the structure–activity relationship. Dimers showed 2–2.5-fold higher values of k A than their monomers. Model 2 gives information about the effects of the side chains and revealed much higher antioxidant activity for monomers and dimers with α,β-unsaturated side chains. Curcumin and 6 in fact are dimers of the same monomer 2. We conclude that the type of linkage between the two “halves” by which the molecule is made up does not exert influence on the antioxidant efficiency and reactivity of these two dimers. The dimers and the monomers demonstrated higher activity than Trolox (10) in aqueous medium (model 3). A comparison of the studied compounds with DL-α-tocopherol (11), Trolox and curcumin is made. All dimers are characterized through lower bond dissociation enthalpies (BDEs) than their monomers (model 4), which qualitatively supports the experimental results.

Protective effects of equimolar mixtures of monomer and dimer of dehydrozingerone with α-tocopherol and/or ascorbyl palmitate during bulk lipid autoxidation

Protective effects of recently synthesised dehydrozingerone, M1OH (which is one half of the molecule of curcumin) and dimer of dehydrozingerone, D1(OH)2, as individual compounds (1 mM) and as equimolar binary (1:1) and ternary (1:1:1) mixtures with α-tocopherol (TOH) and/or ascorbyl palmitate (AscPH), were studied during bulk lipid autoxidation at 80 °C. The highest oxidation stability of lipid substrate, in the presence of individual compounds, was found for TOH, followed by D1(OH)2 and M1OH, determined from the main kinetic parameters (antioxidant efficiency, reactivity and capacity). AscPH did not show any protective effect. Synergism was obtained for the binary mixtures of (TOH+AscPH) [42.4%], (M1OH+TOH) [32.4%] and (M1OH+AscPH) [35.6%] and for the ternary mixture of (M1OH+TOH+AscPH) [28.7%]. Different protective effects observed were explained on the basis (of results) of TOH regeneration and its content determined by HPLC. These antioxidant binary and ternary mixtures can be used as functional components of foods with health-promoting effects.

Antioxidant Activity of Flaxseed Extracts in Lipid Systems

The aim of this work was to compare the antioxidant activity of the extract of flaxseed and its alkaline hydrolysate in two model systems: lipid autoxidation of triacylglycerols of sunflower oil (TGSO)—in a homogeneous lipid media and during β-carotene-linoleate emulsion system. In addition, pure lignans were tested. The material was defatted with hexane and then phenolic compounds were extracted using dioxane-ethanol (50:50, v/v) mixture. Carbohydrates were removed from the crude extract using an Amberlite XAD-16 column chromatography. The content of total phenolic compounds in the crude extract and after alkaline hydrolysis was determined using a Folin-Ciocalteu’s phenol reagent. Individual phenolic compounds were determined by nordihydroguaiaretic acid (RP-HPLC) method in gradient system. The alkaline hydrolysis increased the content of total phenolics in the extract approximately by 10%. In the extracts of flaxseed, phenolic compounds were present in the form of macromolecular complex. In the alkaline hydrolysate, secoisolariciresinol diglucoside (SDG) was found as the main phenolic compound. Small amounts of p-coumaric and ferulic acids were also determined. SDG and both extracts were not able to inhibit effectively lipid autoxidation. The kinetics of TGSO autoxidation at 80 °C in absence and in presence of the extract before hydrolysis (EBH) and after hydrolysis (EAH) was monitored and compared with known standard antioxidants. Ferulic acid (FA) and butylated hydroxyl toluene (BHT) showed much higher antioxidant efficiency and reactivity than that of both extracts. Secoisolariciresinol (SECO) showed a higher activity in both model systems than SDG. However, the activity of SECO was much lower than that of nordihydroquaiaretic acid (NDGA).

The Structure–Antioxidant Activity Relationship of Ferulates

The antioxidant activity of ferulic acid (1), iso-ferulic acid (2), coniferyl aldehyde (3), methyl ferulate (4), and ethyl ferulate (5) were investigated using 2,2′-azinobis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) and ferric-reducing antioxidant power (FRAP) assays and autoxidation of triacylglycerols of commercially available sunflower oil (TGSO). The compounds tested for ability to scavenge ABTS radical cations was in the order of ferulic acid > coniferyl aldehyde ≈ iso-ferulic acid > ethyl ferulate ≈ methyl ferulate. The results of the FRAP assay for ferulic acid, iso-ferulic acid, and coniferyl aldehyde were similar to and higher than those of methyl ferulate and ethyl ferulate. In the lipid system, iso-ferulic acid showed weak antioxidant activity. The other ferulates exhibited much stronger, yet similar, activities.

Chapter 11 - Exogenous and Endogenous Mediators of Oxygen Metabolism: Alternatives for Chemical and Biological Activity

Abstract The present chapter provides the survey of the contemporary current state of art in studying the salient mechanistic alternatives of involving the bioactive mediators of oxygen metabolism in oxidative developments and draws particular attention to unresolved issues, which merit in-depth investigation. The forthcoming critical overview encompasses both chemical and biological issues, which are discussed in terms of oxidative stress. However, as the performed analysis reveals, the very term “oxidative stress” is still “ill defined,” and for comprehension of the latter phenomenon, first of all, the existing terminology needs to be revisited to reach a general agreement on definitions. The problems reviewed and analyzed herein refer to understanding the role of their numerous endogenous and exogenous mediators as prooxidant and antioxidant reactants involved in oxidation processes in vivo, merging and interconverting their chemical activities, and overlapping their functions with the other physiologically active compounds (most prominently, exhibiting hormonal activity).

Antiradical and Antioxidant Activities of New Natural-like Hydroxylated Biphenyls of Dehydrozingerone, Zingerone and Ferulic Acid

Selected synthesized natural and natural-like hydroxylated biphenyls (dimers) and their corresponding monomers (dehydrozingerone, zingerone and ferulic acid) were tested experimentally for their capacity both as radical scavengers and chain-breaking antioxidants in individual and binary mixture with α-tocopherol. Quantum-chemical calculations at the UB3LYP/6-13+G(d,p) level were used to get full geometry optimization of the compounds in neutral and radical forms. Good correlation between experimental data of radical scavenging and antioxidant activities and predicted activity was achieved.

Protective effects of new antioxidant compositions of 4-methylcoumarins and related compounds with dl-α-tocopherol and l-ascorbic acid: Protective effects of new antioxidant compositions of 4-methylcoumarins

BACKGROUND Coumarin derivatives possess a wide range of biological activities. By functionalization of the parent coumarin skeleton that has neither antioxidant nor biological activity, a series of new bio-antioxidants has been designed. RESULTS New antioxidant compositions (equimolar binary and ternary mixtures) of eight 4-methylcoumarins and three related compounds have been tested and different effects between individual components have been observed: synergism (positive effect), additivism (summary effect) and antagonism (negative effect). Higher oxidative stability of the lipid substrate was obtained in the presence of the new antioxidant compositions of the studied compounds with dl-α-tocopherol and l-ascorbic acid. The role of each component in the antioxidant compositions of ternary mixtures has been identified by using new equations composed by the authors. CONCLUSION All ternary mixtures demonstrate synergism as a result of continuous regeneration of dl-α-tocopherol from the studied antioxidants and l-ascorbic acid. Theoretical calculations have been probed as indicators of the expected effects between the individual components in a binary mixture.

Effects of γ-Irradiation of Wild Thyme (Thymus serpyllum L.) on the Phenolic Compounds Profile of Its Ethanolic Extract

Abstract The presented study revealed that there were changes in the phenolic compounds profile of extract of wild thyme (Thymus serpyllum L.) after γ-irradiation at the dose of 5 kGy. Ethanolic extracts of irradiated and non-irradiated herb were prepared and their compounds were analyzed by RP-HPLC-DAD technique. Between thirty two detected constituents, twelve phenolic compounds classified as hydroxybenzoic and hydroxycinnamic acids derivatives, flavones and flavanones were identified. Among them, caffeic acid derivatives and flavones predominated with the highest content of rosmarinic acid and luteolin-7-O-glucoside, respectively. Additionally, thymol was recognized in the analyzed extracts. γ-Irradiation slightly affected the quantitative profile of phenolic compounds of a wild thyme ethanolic extract. Only four constituents differed significantly (P<0.05) in terms of their content in the irradiated and non-irradiated samples. The content of phenolic acids (p-coumaric and caffeic acids) decreased and that of flavonoid aglycons (luteolin and eriodictyol) increased after the γ-ray treatment.