Didier Hauchard

Phenolic compounds in the brown seaweed Ascophyllum nodosum: distribution and radical-scavenging activities

INTRODUCTION Phenolic compounds are metabolites exhibited at high levels in Phaeophyceae. Although several studies have been conducted on total phenol contents, no one to our knowledge has dealt with the contents of phenolic compounds and antioxidant activities on purified fractions. OBJECTIVE The purpose of this study was the extraction and purification of phenolic compounds from the brown seaweed Ascophylllum nodosum, to determine both their distribution and their radical-scavenging activities, and to obtain a sufficiently purified oligophenolic fraction to perform an RP-HPLC analysis on molecules with a molecular weight (MW) < 2 kDa. METHODOLOGY Phenolic compounds were separated and purified by liquid-liquid extraction, tangential ultrafiltration and dialysis. Then, the contents of both phenolic compounds and radical-scavenging activities were measured by the Folin-Ciocalteu reagent, and DPPH and ABTS assays. NMR analysis was performed to validate the process. RP-HPLC with a C(18) column was performed on the oligophenolic fraction, using a novel method developed in this study. RESULTS Seven fractions were obtained as a function of polarity and molecular weight. Among them, the fraction containing phenolic compounds with a MW ≥ 50 kDa appeared to be the most active, correlated with the content of phenolic compounds. CONCLUSION This work constitutes a step forward in the separation and purification of bioactive phlorotannins and represents a prerequisite for further investigations into their structural characterisation and distribution in A. nodosum.

Superoxide Protonation by Weak Acids in Imidazolium Based Ionic Liquids

The reactivity of the superoxide anion versus a series of substituted phenols was investigated in a common ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BMIm][TFSI]) and for comparison in dimethylformamide (+0.1 mol x L(-1) of Bu4NPF6 as supporting electrolyte). On the whole, the mechanism of the reduction of O2 in the presence of the different phenols was found to be very similar in [BMIm][TFSI] and in DMF: A 2-electron mechanism involving a succession of electrochemical and protonation steps. These steps are accompanied by the production of the corresponding phenolate that was identified through its oxidation potential. The reactivities of the phenols were observed to slightly differ in the two media. A qualitative analysis of the voltammogram allows a classification of the reactivities of the superoxide as a function of the phenols. As previously found in organic solvents, the protonation of superoxide by phenol is an uphill reaction that is rendered possible thanks to a subsequent irreversible electron transfer. Its pK(a) is estimated to be around 4-5 units lower than that of unsubstituted phenol.

How do phenolic compounds react toward superoxide ion? A simple electrochemical method for evaluating antioxidant capacity.

The reactivities of different phenols and polyphenols versus superoxide ion (O₂(•-)) were investigated as an easy-to-handle electrochemical method for evaluating antioxidant capacities. In view of this application, the O₂/O₂(•-) couple and associated reactions between O₂(•-) and polyphenols (or phenols) were examined in an aprotic solvent [dimethylformamide (DMF)] by cyclic voltammetry. Comparisons based on simple criteria (reversibility of the O₂ reduction in the presence of the phenolic compound, electron stoichiometry, or apparent kinetic constants) allow discriminations between the possible mechanistic pathways (acid-base or radical reaction type). The results highlight that the proton-transfer and radical-transfer pathways are both present for monophenols and polyphenols, with the relative contributions of the two pathways depending on the phenol structure. In agreement with the literature, polyphenols containing an o-diphenol ring (as in flavonoids) were found to present the highest reactivities.

Validation of a new method using the reactivity of electrogenerated superoxide radical in the antioxidant capacity determination of flavonoids

This article lays out a new method to measure the antioxidant capacity of some flavonoids. The methodology developed is based on the kinetics of the reaction of the antioxidant substrate with the superoxide radical (O(2)(*-)). A cyclic voltammetric technique was used to generate O(2)(*-) by reduction of molecular oxygen in aprotic media. In the same experiment the consumption of the radical was directly measured by the anodic current decay of the superoxide radical oxidation in the presence of increasing concentrations of antioxidant substrate. The method was statistically validated on flavonoid monomers and on the standard antioxidants: trolox, ascorbic acid and phloroglucinol. The linear correlations between the anodic current of O(2)(*-) and the substrate concentration allowed the determination of antioxidant index values expressed by the substrate concentration needed to consume 30% (AI(30)) and 50% (AI(50)) of O(2)(*-) in given conditions of oxygen concentration and scanning rate. The fidelity of the method was examined intraday and interlaboratories.

Radical-scavenging capacity of phenol fractions in the brown seaweed Ascophyllum nodosum: An electrochemical approach

In this article, the radical-scavenging capacity of phenol fractions extracted from the brown seaweed Ascophyllum nodosum was assessed using in parallel colorimetric methods (ABTS and DPPH) and electrochemistry (cyclic voltammetry). Results obtained by the three methods correlated in the case of global fractions, whereas only ABTS and DPPH correlated when activities were expressed on a phenol basis. The successive fractions separated by both their average molecular size and their polarity exhibited activities largely dependent on their phenol content, suggesting that phlorotannins are the main anti-oxidant molecules in hydro-alcoholic extracts of A. nodosum. In addition, phenol fractions of relative low molecular weight were clearly more active than others. This work opens new opportunities to better evaluate the radical-scavenging potential of phenol pools in algae using both bi-parametric fractionating and electrochemistry.

A DFT and experimental investigation of the electron affinity of the triscyclopentadienyl uranium complexes Cp3UX

Relativistic Density Functional Theory (DFT) based methods coupled with the Conductor-like Screening Model (COSMO) for a realistic solvation approach are used to investigate the electron affinity (EA) of a series of triscyclopentadienyl uranium complexes Cp(3)UX (X = Cl, BH(4), SPh, S(i)Pr and O(i)Pr) related to the U(iv)/U(iii) redox system. E(1/2) half-wave potentials have been measured in solution (THF) under the same rigorous conditions for all the species under consideration. A good correlation (r(2) = 0.99) is found between the computed EA values, either in the gas phase or in solution, and the experimental half-wave potentials; the study brings to light the importance of spin-orbit coupling effects which must be taken into account in order to achieve the observed agreement between theory and experiment. The influence of the electron donating character of the X ligand on the orbital involved in the reduction process, namely the lowest unoccupied molecular orbital (LUMO) of the neutral U(iv) complexes, and on the EAs is discussed.

Electrochemical sensors using modified electrodes based on copper complexes formed with Algerian humic acid modified with ethylenediamine or triethylenetetramine for determination of nitrite in water

The response and efficiency of new sensors for nitrite ions analysis have been studied electrochemically. These sensors were developed by modifying a carbon paste electrode (CPE) with copper (II) complexes formed with commercial (PFHA) and Algerian (YHA) humic acids and their modified compounds with ethylenediamine (EDA) or triethylenetetramine (TETA). The developed mechanism is based on the electrochemical oxidation of NO2(-) on the modified CPE for different nitrite concentrations. The obtained results showed that the carbon paste electrode modified with copper (II)-modified humic acids complexes (Cu-MHA) exhibited substantial electrocatalytic effect on the oxidation of nitrite anions compared with carbon paste electrode. The sensitivity of the modified CPE towards nitrite concentrations depends on the nature of the humic acid and its modified compounds. The measurements performed by using CPE/Cu-YHA-EDA and CPE/Cu-YHA-TETA gave the best sensitivity and a good linear response of current versus nitrite concentrations. The oxidation peak current of nitrite at CPE/Cu-YHA-TETA and CPE/Cu-YHA-EDA electrodes in weak acid solution is proportional to the concentration of nitrite over the range 0-1.38 × 10(-2) mol L(-1) with a limit of detection (LOD) of 1.46 μmol L(-1) (Sensitivity=41.06 μA(mmol L(-1))(-1) and 2.17 μmol L(-1) (Sensitivity=27.63μA(mmol L(-1))(-1), respectively. Compared to the sensors published in the literature, our CPE/Cu-YHA-TETA and CPE/Cu-YHA-EDA electrodes exhibit a good catalytic activity towards nitrite oxidation and a low limit of detection over a wide nitrite concentrations range.

Evaluation of adsorption capacities of humic acids extracted from Algerian soil on polyaniline for application to remove pollutants such as Cd(II), Zn(II) and Ni(II) and characterization with cavity microelectrode

The adsorption capacities of new humic acids isolated from Yakouren forest (YHA) and Sahara (Tamenrasset: THA) soils (Algeria) and commercial humic acid (PFHA) on polyaniline emeraldine base (PEB) were studied at pH 6.6. Also the adsorption of heavy metals such as Cd2+, Zn2+ and Ni2+ on humic acid-polyaniline systems (HA-PEB) was investigated at the same conditions. HA-PEB compounds were characterized by scanning electron microscopy (SEM), infrared spectrometry and cavity microelectrode. In addition, batch adsorption and cavity microelectrode were used in the adsorption study of Cd2+, Zn2+ and Ni2+ on HA-PEB. To develop biocaptors of polluting metals using a cavity microelectrode modified by HA-PEB systems, the adsorption kinetic and adsorption capacity were investigated. The SEM analysis showed that the presence of humic acid affected the PEB surface and caused the formation of a granular morphology. The maximum adsorption capacities (q(max)) of PFHA, THA and YHA determined by adsorption isotherms were 91.31, 132.1 and 151.0 mg/g, respectively. Batch adsorption results showed that q(max) of Cd2+, Zn2+ and Ni2+ on HA-PEB followed the order: THA-PEB > YHA-PEB > PFHA-PEB. The voltammograms obtained with HA-PEB modified cavity microelectrode showed the appearance of new redox couples reflecting the adsorption of HA on PEB. Metal-humic acid-polyaniline voltammograms were characterized by appearance of oxidation-reduction couples or reduction wave corresponding to metal. Finally, the result may be exploited to develop a biocaptor based on the cavity microelectrode amended by THA-PEB and YHA-PEB.

Acceleration of zinc corrosion in alkaline suspensions containing iron oxides or iron hydroxides

Fast zinc dissolution is of industrial interest in recycling galvanised steel scraps. An acceleration of zinc corrosion in alkaline solutions was observed in the presence of various iron oxides or iron hydroxides. This corrosion was investigated by weight loss, measurements of hydrogen evolution and variation of current in a galvanic cell. The mechanism of this fast zinc corrosion was investigated by electrochemical means and by X-ray diffraction and scanning electron microscopy observations of zinc surface after immersion in alkaline suspensions of iron oxides or iron hydroxides. These insoluble iron compounds were involved in a reduction step leading to iron containing microparticles characterised by a low hydrogen overpotential and which acted as cathodic areas in a galvanic corrosion of zinc.

Anionic cyclopentadienyluranium(III) complexes

Abstract The Na/Hg reduction of Cp3UX (X  Me, n-Bu, BH4) and Cp2U(BH4)2 in the presence of 18-crown-6 ether has given the anionic uranium(III) complexes [Cp3UX][Na(18-crown-6)] and [Cp2U(BH4)2][Na(18-crown-6)]; in agreement with cyclic voltammetry experiments, the borohydride anions were found to be reoxidized by TlBH4 into the corresponding uranium(IV) complexes.