Evaluation of radical scavenging activity of antioxidants by means of electrogenerated HO radical

Hydroxyl radical is one of the most reactive radicals. The electrochemical generation of hydroxyl radical by oxidation of water can be achieved using different anode materials. The adsorption of HO radicals at the anode surface has a significant effect on their reactivity. Using materials such as BDD, electrogenerated hydroxyl radicals are weekly adsorbed whereas strongly adsorbed radicals are formed at Pt. Although most of the available studies deal with high oxidation power anodes for the degradation of pollutants, the use of anodes with low oxidation power can have important applications particularly when a certain degree of selectivity is required for oxidation. In this context electrochemically-generated hydroxyl radical was applied for the evaluation of antioxidant scavenging activity. A set of species with antioxidant activity was oxidized by galvanostatic electrolyses using a fairly oxidized Pt anode in conditions of O2 evolution. In such conditions the consumption of species was not limited by mass transport but by the oxidation kinetics of both charge transfer and reaction with HO radicals. Information regarding the rate constant of the reaction of antioxidants with electrogenerated HO radicals was obtained by means of the apparent rate constant variation with electrolysis current density. Scavenger activity of the analysed antioxidants estimated by the kinetic parameter kR,HO / kO2 was ordered as follows: gallic acid, trolox > ascorbic acid > caffeic acid. The method was applied successfully in synthetic solutions and in a green tea based beverage

Electrogenerated HO radical reactions: the role of competing reactions on the degradation kinetics of hydroxy-containing aromatic compounds

The rate of degradation of some aromatic compounds, benzoic acid, 2-hydroxybenzoic acid, 4-hydroxybenzoic acid and 2,3-hydroxybenzoic acid, by electrogenerated HO radicals is investigated. The effect of the addition of secondary species, that can either be a reaction product of the target compound or not is analysed. The presence of the actual reaction products is also considered. The action of these secondary species is integrated in a general model that accounts for the dependency of the anode coverage by HO radicals on the concentration and on the reactivity of HO radical scavengers. From the magnitude of the effect of competing reactions, the reactivity of the hydroxybenzoic acid derivatives was estimated as the product between the stoichiometric coefficients and the rate constants. A possible mechanistic interpretation is provided to explain the unexpected high values of the stoichiometric coefficients estimated that largely exceed the number of radicals required for the complete mineralization of the species. (C) 2014 Elsevier Ltd. All rights reserved.- We would like to acknowledge the contribution of Prof. Paula M.T. Ferreira on the mechanistic interpretation of the HO radical stoichiometric coefficients. Thanks are due to FCT (Fundacao para a Ciencia e Tecnologia) and FEDER (European Fund for Regional Development)-COMPETE-QREN-EU for financial support to the Research Centre, CQ/UM [PEst-C/QUI/UI0686/2011 (FCOMP-01-0124-FEDER-022716)]. Raquel Oliveira thanks to FCT, POPH (Programa Operacional Potencial Humano) and FSE (Fundo Social Europeu) for the PhD Grant (SFRH/BD/64189/2009).info:eu-repo/semantics/publishedVersio

Reactivity of hydroxy-containing aromatic compounds towards electrogenerated hydroxyl radicals

A kinetic study on the oxidation of hydroxy-containing aromatic compounds by electrogenerated HO radical and simultaneous by direct electron transfer is presented. First order kinetics are used to describe consumption rates of hydroquinone, benzoic acid and of hydroxybenzoic acid derivatives by galvanostatic electrolysis with simultaneous oxygen evolution at a Pt electrode. Linear correlations were established from the effect of electrolyses current density on kapp. The meaning of the intercept and of the slope is analysed. A good agreement is found between intercept values and the apparent rate constants from potentiostatic electrolysis without O2 evolution. Simultaneously, the slopes magnitude corroborate the relative reactivity order of species that was established considering the occurrence of positive charge densities on carbon atoms of the aromatic ring. Therefore, the present analysis provides kinetic information concerning both, the direct electron-transfer and the reaction with HO radical.Thanks are due to FCT (Fundação para a Ciência e Tecnologia) and FEDER (European Fund for Regional Development)-COMPETE-QREN-EU for financial support to the Research Centre, CQ/UM [PEst-C/QUI/UI0686/2011 (FCOMP-01-0124-FEDER- 022716)]. Raquel Oliveira thanks to FCT, POPH (Programa Operacional Potencial Humano) and FSE (Fundo Social Europeu) for the PhD Grant (SFRH/BD/64189/2009)

Reactivity of hydroxybenzoic acid derivatives towards electrogenerated HO radicals

In the last decade there has been a lot of interest in the study of reactive oxygen species (ROS) and their role in many areas such as medicine, biology and chemistry. Among the ROS, HO radicals are the most reactive species and the extension of their reaction with organics can lead to the formation of CO2 thus the generation of these radicals is an important process in applications such as detoxification of effluents.There are reports of severall methods used in this area to produce HO radicals such as Fenton, photo-­‐Fenton, electro-­‐Fenton and by the electrochemical oxidation of water. Electrochemically generated HO radicals have proved to be adequate for the destruction of organics in aqueous media. The reactivity and selectivity of the radicals generated at the surface of the electrode are related to the anode material. Although most of the available studies deal with high oxidation power anodes, like BDD, the use of anodes with low oxidation power, Pt, can have important applications particularly when a certain degree of selectivity is required for the oxidation. In this work we present a kinetic study on aromatic compounds oxidation by electrogenerated HO radical with simultaneous direct electron transfer at a Pt anode. Experimental conditions such as concentration, electrolysis times and electrochemical cell were selected in order to minimize the contribution from secondary reactions and from reactions taking place at the cathode. Kinetic data is analysed considering the presumed reactivity of these species suggested from charge density values on carbon atoms of the aromatic ring

In Medicago truncatula, water deficit modulates the transcript accumulation of components of small RNA pathways

<p>Abstract</p> <p>Background</p> <p>Small RNAs (sRNAs) are 20-24 nucleotide (nt) RNAs and are involved in plant development and response to abiotic stresses. Plants have several sRNA pathways implicated in the transcriptional and post-transcriptional silencing of gene expression. Two key enzyme families common to all pathways are the Dicer-like (DCL) proteins involved in sRNAs maturation and the Argonautes (AGOs) involved in the targeting and functional action of sRNAs. Post-transcriptional silencing mediated by AGOs may occur by cleavage or translational repression of target mRNA's, while transcriptional silencing may be controlled by DNA methylation and chromatin remodeling. Thus far, these gene families have not been characterized in legumes, nor has their involvement in adaptation to water deficit been studied.</p> <p>Results</p> <p>A bioinformatic search in <it>Medicago truncatula </it>genome databases, using <it>Arabidopsis thaliana </it>AGO and DCL cDNA and protein sequences, identified three sequences encoding for putative Dicer-like genes and twelve sequences encoding for putative Argonaute genes. Under water deficit conditions and mainly in roots, MtDCL1 and MtAGO1, two enzymes probably involved in the processing and activation of microRNAs (miRNAs), increased their transcript levels. mir162 which target DCL1 mRNA and mir168 which target AGO1 mRNA reduced their expression in the roots of plants subjected to water deficit. Three putative genes, MtDCL3, MtAGO4b and MtAGO4c probably involved in DNA methylation mechanisms, increased their mRNA levels. However, the mRNA levels of MtAGO6 reduced, which probably encodes a protein with functions similar to MtAGO4. MtAGO7 mRNA levels increased and possibly encodes a protein involved in the production of trans-acting small interfering RNAs. The transcript abundance of MtAGO12a, MtAGO12b and MtAGO12c reduced under water deprivation. Plants recovered from water deprivation reacquire the mRNA levels of the controls.</p> <p>Conclusions</p> <p>Our work demonstrates that in <it>M. truncatula </it>the transcript accumulation of the components of small RNA pathways is being modulated under water deficit. This shows that the transcriptional and post-transcriptional control of gene expression mediated by sRNAs is probably involved in plant adaptation to abiotic environmental changes. In the future this will allow the manipulation of these pathways providing a more efficient response of legumes towards water shortage.</p

Estudo da capacidade antioxidante de compostos fenólicos baseado em electrólises galvanostáticas

A caracterização da capacidade antioxidante de moléculas e de alimentos tem captado a atenção de investigadores em diferentes áreas dada a ação dos antioxidantes no retardamento de processos oxidativos que estão na origem do stresse oxidativo. Dentro dos métodos convencionais utilizados na avaliação da capacidade antioxidante destacam-se os métodos óticos baseados na análise da resposta do antioxidante ao ataque oxidativo por espécies reativas que podem ser adicionadas ou geradas durante o ensaio1, 2. A utilização de métodos eletroquímicos permite a geração de oxidantes fortes que são utilizados em titulações coulométricas de antioxidantes3. Alternativamente os métodos eletroquímicos permitem caracterizar diretamente os processos de transferência electrónica característicos da ação redutora do antioxidante por voltametria cíclica4, através do potencial e da intensidade de corrente. Mais recentemente foi proposto pelo nosso grupo um método alternativo para a avaliação do poder redutor dos antioxidantes denominado RACE5, que através de eletrólises potenciostáticas, permite caracterizar e quantificar a ação antioxidante através de um parâmetro absoluto que traduz a concentração da espécie reativa que este tem capacidade para neutralizar. Neste trabalho é apresentado um estudo envolvendo um conjunto de antioxidantes em que o seu comportamento é caraterizado em eletrólises galvanostáticas em condições em que ocorre a simultânea evolução de oxigénio. É sabido que a decomposição de água envolve a formação de radicais HO que ficam adsorvidos na superfície do ânodo6. Estes radicais altamente reativos podem participar em reações de oxidação com outras espécies presentes em solução. A sua aplicação tem sido amplamente estudada no âmbito da mineralização de materiais orgânicos em efluentes6. Assim, o consumo dos antioxidantes, bem como a sua suscetibilidade à variação da intensidade de corrente, são testados de modo a desenvolver um método alternativo para avaliar o carácter scavenger de antioxidantes. O método proposto foi testado utilizando compostos fenólicos da família do ácido gálico.Fundação para a Ciência e a Tecnologia (FCT) - SFRH/BD/64189/2009

Protein oxidation by electrogenerated hydroxyl radicals

Many chronic diseases are associated to oxidative stress that can result from the unbalance between the production of reactive oxygen species and the action of antioxidant defense systems. Proteins are important targets as they are major components of cells. Modification of proteins structure, function and stability can result from the attack by different radicals, e.g. O2 - or HO. In order to characterize the damages caused by these radicals it is important to analyze the proteins oxidation products obtained from assays where radicals are generated by clean processes [1]. In this context the electrochemical generation of hydroxyl radicals can provide an alternative method, where these radicals are formed as intermediate in the oxidation of water to produce oxygen. Anodes of different materials, such as of boron doped diamond electrodes (BDD) and platinum (Pt), can be used for this purpose [3]. In this work it is considered the use of high reactive HO radicals (weakly adsorbed at BDD) and of low reactive HO radicals (strongly adsorbed at Pt) to oxidize BSA. The extension of BSA oxidation is analyzed for different experimental conditions, such as current density and electrolyte nature. BSA oxidation is monitored by means of carbonyl groups concentration and by the extension of protein cleavage

Degradation of alizarin red promoted by electrogenerated HO radicals: a kinetic study

Alizarin Red (AR, 1,2-dihydroxyanthraquinone) is a dye used in textile industry since early antiquity. Nowadays it is used in several biochemical assays in studies involving bone growth, osteoporosis, bone marrow, calcium deposits, cellular signaling, gene expression, tissue engineering, and mesenchymal stem cells. Reactions of electrochemically generated hydroxyl radicals, by the oxidation of water, depend on the reactivity of these radicals according to their adsorption degree at the anode surface. At Pt HO radicals keep strongly adsorbed and display lower reactivity than those formed in anodes like BDD. This effect as been already demonstrated by means of a kinetic study of benzoic acid and p-hydroxybenzoic acid oxidation. The study of the AR oxidation by HO radicals can bring important insight on the reactivity of chemical functions such as aromatic rings, hydroxyl and carbonyl groups, that are common in antioxidant molecules. Therefore, using a single molecule it is possible to investigate the relative reactivity of these functional groups towards HO radical in identical conditions of surface coverage, what cannot be warranted when the different molecules are considered. The AR UV–Vis absorption spectrum displays three distinct peaks corresponding to the anthracene ring, the carbonyl groups and quinonyl. Therefore by means of the absorbance variation of the three UV-vis bands it was possible to evaluate the reactivity of the different functional groups