Jean-Luc Duroux

Antioxidant, anti-inflammatory and antiproliferative properties of sixteen water plant extracts used in the Limousin countryside as herbal teas

The present paper presents the antioxidant, anti-inflammatory and antiproliferative capabilities of 16 plants. These plants can be found in the Limousin countryside and most of them are used in popular medicine as herbal tea. The biological properties of the water-soluble fractions were measured. Antioxidant properties were evaluated by the ESR method in order to visualize the inhibition of the DPPH, superoxide and hydroxyl radicals. Some extracts were good antioxidants by comparison with reference molecules, e.g. vitamin E and quercetin. Antioxidant effects were correlated with the total amount of phenolic compounds contained in the extracts. Also measured were the anti-inflammatory activities of the 16 water-soluble fractions, by evaluating inhibition of lipoxygenase activity. Finally the effects of these plants on the proliferation of melanoma B16 cells were studied.

Mechanism of the antioxidant action of silybin and 2,3-dehydrosilybin flavonolignans: a joint experimental and theoretical study.

Flavonolignans from silymarin, the standardized plant extract obtained from thistle, exhibit various antioxidant activities, which correlate with the other biological and therapeutic properties of that extract. To highlight the mode of action of flavonolignans as free radical scavengers and antioxidants, 10 flavonolignans, selectively methylated at different positions, were tested in vitro for their capacity to scavenge radicals (DPPH and superoxide) and to inhibit the lipid peroxidation induced on microsome membranes. The results are rationalized on the basis of (i) the oxidation potentials experimentally obtained by cyclic voltammetry and (ii) the theoretical redox properties obtained by quantum-chemical calculations (using a polarizable continuum model (PCM)-density functional theory (DFT) approach) of the ionization potentials and the O-H bond dissociation enthalpies (BDEs) of each OH group of the 10 compounds. We clearly establish the importance of the 3-OH and 20-OH groups as H donors, in the presence of the 2,3 double bond and the catechol moiety in the E-ring, respectively. For silybin derivatives (i.e., in the absence of the 2,3 double bond), secondary mechanisms (i.e., electron transfer (ET) mechanism and adduct formation with radicals) could become more important (or predominant) as the active sites for H atom transfer (HAT) mechanism are much less effective (high BDEs).

Free radical scavenging by natural polyphenols: atom versus electron transfer.

Polyphenols (synthetically modified or directly provided by human diet) scavenge free radicals by H-atom transfer and may thus decrease noxious effects due to oxidative stress. Free radical scavenging by polyphenols has been widely theoretically studied from the thermodynamic point of view whereas the kinetic point of view has been much less addressed. The present study describes kinetic-based structure-activity relationship for quercetin. This compound is very characteristic of the wide flavonoid subclass of polyphenols. H-atom transfer is a mechanism based on either atom or electron transfer. This is analyzed here by quantum chemical calculations, which support the knowledge acquired from experimental studies. The competition between the different processes is discussed in terms of the nature of the prereaction complexes, the pH, the formation of activated-deprotonated forms, and the atom- and electron-transfer efficiency. The role of the catechol moiety and the 3-OH group of quercetin as scavengers of different types of free radicals (CH3OO(•), CH3O(•), (•)OH, and (•)CH2OH) is rationalized. Identifying the exact mechanism and accurately evaluating kinetics is of fundamental importance to understand antioxidant behavior in physiological environments.

Free Radical Scavenging Properties of Guaiacol Oligomers: A Combined Experimental and Quantum Study of the Guaiacyl-Moiety Role

Natural polyphenols are known to exhibit a lot of different biological properties, including antioxidant activity. For some polyphenols these activities are attributed to the presence of a guaiacol moiety. In the present paper we focus on the role of this moiety. For this purpose nine different compounds were enzymatically synthesized from guaiacol. To elucidate the structure-activity relationship of these polyphenols, DFT-(PCM)B3P86/6-311+G(2d,3pd)//(PCM)B3P86/6-31+G(d,p) calculations supported the experimental DPPH free radical scavenging activities. The antioxidant activities were correlated to (i) O-H BDEs (bond dissociation enthalpies), (ii) BDE(D) (BDE of a second H atom abstraction from the phenoxyradicals), (iii) spin density, (iv) HOMO (highest occupied molecular orbital) distribution, (v) IPs (ionization potentials), (vi) DeltaG and DeltaG(#) free energies of HAT (H atom transfer), and (vii) HAT rate constants. BDE(D) appeared to be the most important descriptor to understand the free radical scavenging ability of these compounds.

New aspects of the antioxidant properties of phenolic acids: a combined theoretical and experimental approach

Ferulic acid is widely distributed in the leaves and seeds of cereals as well as in coffee, apples, artichokes, peanuts, oranges and pineapples. Like numerous other natural polyphenols it exhibits antioxidant properties. It is known to act as a free radical scavenger by H atom transfer from the phenolic OH group. In the present joint experimental and theoretical studies we studied a new mechanism to explain such activities. Ferulic acid can indeed act by radical addition on the alpha,beta-double bond. On the basis of the identification of metabolites formed in an oxidative radiolytic solution and after DFT calculations, we studied the thermodynamic and kinetic aspects of this reaction. Addition and HAT reactions were treated as competitive reactions. The possibility of dimer formation was also investigated from a theoretical point of view; the high barriers we obtained contribute to explaining why we did not observe those compounds as major radiolytic compounds. The DPPH free radical scavenging capacity of ferulic acid and the oxidative products was measured and is discussed on the basis of DFT calculations (BDEs and spin densities).

Theoretical Investigation of the Formation of a New Series of Antioxidant Depsides from the Radiolysis of Flavonoid Compounds

Abstract Kozlowski, D., Marsal, P., Steel, M., Mokrini, R., Duroux, J-L., Lazzaroni, R. and Trouillas, P. Theoretical Investigation of the Formation of a New Series of Antioxidant Depsides from the Radiolysis of Flavonoid Compounds. Radiat. Res. 168, 243–252 (2007). This paper deals with the formation of a series of antioxidant depsides obtained from flavonoid solutions irradiated with γ rays. These reactions take place in radiolyzed alcohol solutions, a medium that is very rich in many different highly reactive species and that hosts specific reactions. We focus on the first step of those reactions, i.e., reactivity of the solute (flavonoid) with the alkoxy radicals CH3O· and CH3CH2O· formed in methanol and ethanol, respectively, and their carbon-centered isomers: the 1-hydroxy-methyl (·CH2OH) and the 1-hydroxy-ethyl (CH3·CHOH) radicals. Among the different flavonoid groups of molecules, only flavonols are transformed. To establish the structure–reactivity relationship that explains why the radiolytic transformation occurs only for those compounds, the process is rationalized theoretically, with Density Functional Theory calculations, taking into account the solvent effects by a Polarizable Continuum Model and a microhydrated environment (one or two water molecules surrounding the active center). The first redox reaction, occurring between the flavonol and the reactive species formed upon irradiation of the solvent, is studied in terms of (1) the O-H bond dissociation enthalpy of each OH group of the flavonoids and (2) electron abstraction from the molecule. We conclude that the reaction, initiated preferentially by the alkoxy radicals, first occurs at the 3-OH group of the flavonol. It is then followed by the formation of a peroxyl radical (after molecular oxygen or superoxide addition). The different cascades of reactions, which lead to the formation of depsides via C-ring opening, are discussed on the basis of the corresponding calculated energetic schemes.

Eight flavonoids and their potential as inhibitors of human cytomegalovirus replication

The drugs currently available for treatment of severe human cytomegalovirus (HCMV) infections suffer from many drawbacks, particularly toxicity, and potential teratogenicity contraindicating their use in target populations such as pregnant women. The emergence of drug-resistant strains is still a problem for disease management, particularly in immunosuppressed populations where antivirals are used for extended periods of time. The flavonoid family of drugs contains promising candidates as they have low toxicity and inhibit different targets to currently available antivirals. We report here that, unlike their chalcon homologs, four flavonoids (baicalein, quercetin, quercetagetin and naringenin) inhibit various stages of HCMV replication, the most active anti-HCMV compound being baicalein and the less active and less selective being quercetagetin. These drugs could provide potential inhibitors of virus replication alone or in combination, without increased toxicity.

Redox Reactions Obtained by γ Irradiation of Quercetin Methanol Solution are Similar to In Vivo Metabolism

Abstract Marfak, A., Trouillas, P., Allais, D. P., Calliste, C. A. and Duroux, J. L. Redox Reactions Obtained by γ Irradiation of Quercetin Methanol Solution are Similar to In Vivo Metabolism. Radiat. Res. 159, 218–227 (2003). The flavonol quercetin is one of the most well-known antioxidant flavonoids. Its antioxidant potential has been studied extensively during the last 10 years, but little is known about the metabolites formed in vivo that lead to the formation of depside and small molecules such as benzoic acids. In this study, γ irradiation of a quercetin methanol solution was used as a model of certain oxidative reactions that occur in vivo. Qercetin at concentrations ranging from 5 × 10−5 M to 5 × 10−3 M, was irradiated with γ rays at doses of 2–14 kGy. Quercetin degradation was evaluated by HPLC analysis. The major radiolytic metabolite was identified as a depside by NMR and LC-MS. Formation of 3,4-dihydroxybenzoic acid was also observed. The presence of CH3O· formed during methanol radiolysis is invoked to explain depside formation. Transformation of the 8-methoxy substituted depside (Q1) to the 8-hydroxyl substituted depside (Q2) is discussed. The antioxidant properties of quercetin metabolites are evaluated according to their capacity to decrease the EPR DPPH signal and to inhibit superoxide radical formed by the enzymatic reaction (xanthine + xanthine oxidase). For both assays, the IC50 of Q2 is twice as high as that of quercetin.

Mechanisms of Transformation of the Antioxidant Kaempferol into Depsides. Gamma-Radiolysis Study in Methanol and Ethanol

Abstract Marfak, A., Trouillas, P., Allais, D. P., Calliste, C. A., Cook-Moreau, J. and Duroux, J. L. Mechanisms of Transformation of the Antioxidant Kaempferol into Depsides. Gamma-Radiolysis Study in Methanol and Ethanol. Radiat. Res. 160, 355–365 (2003). In this study, we irradiated the antioxidant kaempferol in ethanol and methanol solutions with γ rays at doses ranging from 0.2–20 kGy. NMR and ES-MS spectroscopy were used to identify radiolysis products. Two depsides, {2-[(4′-hydroxybenzoyl)oxy]-4,6-dihydroxyphenyl}(oxo) methyl acetate and {2-[(4′-hydroxybenzoyl)oxy]-4,6-dihydroxyphenyl}(oxo) ethyl acetate, were the major compounds of kaempferol degradation in methanol and in ethanol, respectively. Other products formed in low concentrations were identified as [4-hydroxyphenyl](oxo) methyl acetate, [4-hydroxyphenyl](oxo) ethyl acetate, and depside {2-[(4′-hydroxybenzoyl)oxy]-4,6-dihydroxyphenyl}(oxo) acetic acid. The formation of the latter was observed in both solvents. We propose degradation mechanisms that suggest that ·CH2OH and CH3·CHOH, produced by solvent radiolysis, react with the 3-OH kaempferol group because of its high H-donor capacity. π-Electron delocalization in the flavonoxy formed after the first H-transfer leads to C-ring opening and consequently to the formation of depsides. G calculation of the degradation products and of ·CH2OH and CH3·CHOH radicals confirmed the proposed mechanism of kaempferol radiolysis. The rate constants for the reaction between kaempferol and these free radicals were also calculated. Formation of depside has also been observed in many studies of the oxidation of flavonoids; those studying human metabolism have suggested similar redox transformation of flavonols. The antioxidant activities of radiolysis products were evaluated and compared to those of kaempferol.

Dimerisation Process of Silybin-Type Flavonolignans: Insights from Theory

Natural polyphenols are known to be oxidized by free radicals, which partially explains the antioxidant properties of a number of these compounds. This oxidation may also be used to synthesise new compounds of biological interest, for example, dimers. The present theoretical study describes the existing experimental evidence showing that silybin and dehydrosilybin [natural polyphenols isolated from milk thistle (Silybum marianum)] form dimers regioselectively. Based on DFT calculations, thermodynamic and kinetic values were computed in order to better understand the physicochemical behaviour of these dimerisation processes. Calculations showed that after H-atom transfer (from polyphenol to radical), dimerisation could proceed in two steps: 1) bond formation and, when possible, 2) tautomerisation reorganisation. The former step is the limiting step, while the latter is crucial for the process to be thermodynamically favourable (ΔG<0). If this rearrangement is impossible then dimerisation is not feasible, or at least becomes a minor process (e.g., dehydrosilybin dimerisation after H-atom abstraction from the 3-OH group). This explains the regioselectivity of polyphenol dimerisation.