review article hydroxycinnamic acid antioxidants: an electrochemical overview - hindawi

Hindawi Publishing CorporationBioMed Research InternationalVolume 2013 Article ID 251754 11 pageshttpdxdoiorg1011552013251754

Review ArticleHydroxycinnamic Acid AntioxidantsAn Electrochemical Overview

Joseacute Teixeira1 Alexandra Gaspar1 E Manuela Garrido12

Jorge Garrido12 and Fernanda Borges1

1 CIQDepartamento de Quımica e Bioquımica Faculdade de Ciencias Universidade do Porto 4169-007 Porto Portugal2 Departamento de Engenharia Quımica Instituto Superior de Engenharia do Porto (ISEP) Instituto Politecnico do Porto4200-072 Porto Portugal

Correspondence should be addressed to Jorge Garrido jjgisepipppt

Received 30 April 2013 Accepted 18 June 2013

Academic Editor Kent M Reed

Copyright copy 2013 Jose Teixeira et al This is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

Hydroxycinnamic acids (such as ferulic caffeic sinapic and p-coumaric acids) are a group of compounds highly abundant in foodthat may account for about one-third of the phenolic compounds in our diet Hydroxycinnamic acids have gained an increasinginterest in health because they are known to be potent antioxidants These compounds have been described as chain-breakingantioxidants acting through radical scavenging activity that is related to their hydrogen or electron donating capacity and to theability to delocalizestabilize the resulting phenoxyl radical within their structureThe free radical scavenger ability of antioxidantscan be predicted from standard one-electron potentials Thus voltammetric methods have often been applied to characterize adiversity of natural and synthetic antioxidants essentially to get an insight into their mechanism and also as an important toolfor the rational design of new and potent antioxidants The structure-property-activity relationships (SPARs) correlations alreadyestablished for this type of compounds suggest that redox potentials could be considered a good measure of antioxidant activityand an accurate guideline on the drug discovery and development process Due to its magnitude in the antioxidant field theelectrochemistry of hydroxycinnamic acid-based antioxidants is reviewedhighlighting the structure-property-activity relationships(SPARs) obtained so far

1 Introduction

In the last decade dietary polyphenols which are the mostabundant antioxidants present in a human diet have receivedincreasing interest from researchers foodmanufacturers andalso consumers as one of the most promising groups ofdietary preventive agents They constitute a heterogeneousfamily of chemical compounds comprising phenolic acidsflavonoids tannins stilbenes coumarins and lignans amongothers

Phenolic acids are usually divided into two main groupsbenzoic acids containing seven carbon atoms (C6-C1) andcinnamic acids comprising nine carbon atoms (C6-C3)These compounds exist predominantly as hydroxybenzoicand hydroxycinnamic acids andmay occur either in their freeor conjugated forms

Cinnamic acids have been categorized as structural andfunctional constituents of plant cell walls and also as bioactive

ingredients of the diet [1 2] Recent data support theirbeneficial application as preventive andor therapeutic agentsin several oxidative stress related diseases such as atheroscle-rosis inflammatory injury and cancer [3 4] Data found inthe literature shows that hydroxycinnamic acids antioxidantefficacy is strongly dependent on their structural features andintrinsically related to the presence of hydroxyl function(s) inthe aromatic structure [4 5]

2 Hydroxycinnamic AcidsClassification and Occurrence

Hydroxycinnamic acids (HCAs) possess a simple chemicalbackbone consisting of a phenylpropanoid C6-C3 structureand are themajor subgroup of phenolic acids with ubiquitousdistribution in the plant kingdomThey are abundantly found

2 BioMed Research International

in tea leaves coffee red wine various fruits (especially redones) vegetables and whole grains

Hydroxycinnamic acids such as p-coumaric caffeicferulic and sinapic acids (Table 1) are known to play animportant role in nature In fact their wide distributionand high concentration provide them with a key role in thebiosynthesis of more complicated phenolic systems HCAsare secondary metabolites that are found also in severalconjugated forms including amides (conjugated with mono-or polyamines amino acids or peptides) estersmainly estersof hydroxy acids such as tartaric acid and sugar derivativesand glycosides Whereas cinnamate esters occur widely inhigher plants amides of cinnamic acids seem to be rare

The variety of cinnamic derivatives in plants is dependenton the species In general plants of the Solanaceae familyprovide both chlorogenic acid (5-caffeoylquinic acid) andfree hydroxycinnamic acids such as caffeic acid These com-pounds are themost predominant inmany fruits constitutingup to 75 of phenolic acids Actually derivatives of cinnamicacid exist in all fruit parts but concentrations are higher inthe outer parts of ripe fruit

3 Hydroxycinnamic AcidsAn Antioxidant Outlook

Antioxidants used to prevent or inhibit the natural phenom-ena of oxidation have a broad application in diverse fieldsas they have a huge importance either as industrial additivesor health agents In this context HCAs have been ascribedto act as powerful antioxidant compounds possessing diversephysiological roles in biological systems Research data haverevealed that HCAs can be used for preventive andortherapeutic purposes in several diseases related to oxidativestress (eg atherosclerosis inflammatory injury cancer andcardiovascular diseases)

The antioxidant foremost mechanism of action isassumed to be through its radical-scavenging activity that islinked to their hydrogen- or electron-donating ability andto the stability of the resulting phenoxyl radicals Howeverother mechanisms of action have been suggested such aschelation of transition metals like copper or iron whichare well-known catalysts of oxidative stress inhibition ofROSRNS-generating enzymes and modulation of geneexpression (ARENrf-2 pathway) [12ndash14]

Generally cinnamic acids work well in aqueous mediabeing their hydrophilic nature a restriction for lipophilicsystems protection (eg food or cosmetic matrices or bio-logical membranes) due mainly to the difficulty of theirincorporation into fat and oil mediums The hydrophobicityof HCAs can be enhanced by chemical modification namelyby esterification of the carboxyl group of phenolic acid withfor instance a fatty alcohol Using this type of approach onemust take in mind that the original functional propertiesmust be retained Therefore amplified sort of applications ofthesemodified natural antioxidants in lipophilicmedium canbe explored

4 Electrochemistry of Hydroxycinnamic Acidsand Derivatives

41 General Concepts Electrochemical techniques are versa-tile and powerful analytical tools which are able to providehigh sensitivity and low detection limits associated with theuse of inexpensive instrumentation In addition to their appli-cation in fundamental studies of oxidation and reductionprocesses to unravel reaction mechanisms these techniquesare also used in studying the kinetics and thermodynamics ofelectron and ion transfer processes [15 16]

The common characteristic of all voltammetric tech-niques is that they involve the application of a potential (E)to an electrode which causes the species to be determinedto react and a current (I) to pass In many cases theapplied potential is varied or the current is monitored overa period of time (t)Thus all voltammetric techniques can bedescribed as some function of E I and t They are consideredactive techniques (as opposed to passive techniques suchas potentiometry) because the applied potential forces achange in the concentration of an electroactive species at theelectrode surface by electrochemically reducing or oxidizingit [17]

Cyclic voltammetry (CV) has become an important andwidely used electroanalytical technique in many areas ofchemistry and biochemistry It is rarely used for quantitativedeterminations but it is widely used for the study of redoxprocesses namely for obtaining information on the nature ofintermediates and stability of reaction products

Advanced voltammetric techniques have been developedto increase the sensitivity and selectivity of the voltammetricsignal In most cases this requires minimizing the currentfrom nonfaradaic processes such as that due to the chargingand discharging of the double layer [16 18]These techniquesrely on combinations of sweeps and steps and have namessuch as normal pulse voltammetry differential pulse voltam-metry and square wave voltammetry [16 19]

Voltammograms show peaks and waves which representthe different redox processes occurring in the potentialwindow selected The positions of the peaks and wavesalong the potential axis reflect the species involved (via theirredox potential) but also whether the redox reactions arekinetically favorable or unfavorable The shape and heightof the peaks and waves also indicate the nature of theredox processes (eg whether a given redox process involvesan adsorptiondesorption or whether both reactants andproducts are soluble) A voltammogram is therefore a kindof finger print of the redox reactions [20]

42 Electrochemistry and Antioxidant Assays The termldquoantioxidantrdquo is increasingly popular nowadays as it is asso-ciated with a number of health benefits In fact antioxidantsare molecules that can be associated with the protection ofmacromolecules from oxidation [21] In biological systemsthe definition has been accepted to be ldquoany substance thatwhen present at low concentrations compared to those of anoxidisable substrate significantly delays or prevents oxidationof that substraterdquo [22 23]

BioMed Research International 3

Table 1 Hydroxycinnamic acids and ester derivatives

12

3

45

6120572

120573

R1

R2

HO

COOR3

R1 R2 R3

p-Coumaric H H HCaffeic acid OH H H5-Bromocaffeic acid OH Br HMethyl caffeate OH H CH3

Ethyl caffeate OH H CH2CH3

Ethyl 5-bromocaffeate OH Br CH2CH3

Propyl caffeate OH H CH2CH2CH3

Butyl caffeate OH H CH2(CH2)2CH3

Hexyl caffeate OH H CH2(CH2)4CH3

345-Trihydroxycinnamic acid OH OH HEthyl 345-trihydroxycinnamate OH OH CH2CH3

Ferulic acid OCH3 H H5-Bromoferulic acid OCH3 Br HMethyl ferulate OCH3 H CH3

Ethyl ferulate OCH3 H CH2CH3

Ethyl 5-bromoferulate OCH3 Br CH2CH3

Propyl ferulate OCH3 H CH2CH2CH3

Butyl ferulate OCH3 H CH2(CH2)2CH3

Hexyl ferulate OCH3 H CH2(CH2)4CH3

Sinapic acid OCH3 OCH3 HMethyl sinapate OCH3 OCH3 CH3

Ethyl sinapate OCH3 OCH3 CH2CH3

Propyl sinapate OCH3 OCH3 CH2CH2CH3

Butyl sinapate OCH3 OCH3 CH2(CH2)2CH3

All termed dietary and biological antioxidants arebelieved to be effective compounds and in general all of themexhibit a moderate-marked native electroactivity Thereforeelectrochemical techniques can play a relevant role in theevaluation of antioxidantsrsquo capacity

Electrochemistry is the conceptual base of several antiox-idant capacity assays In general the electron transfer (ET)based assays evaluate the capacity of an antioxidant toreduce an oxidant which usually change color when reduced[24] ET-based assays encompass one of the most popularantioxidant assays the DPPH radical scavenging capacityassay (Scheme 1)

Electrochemical approaches have been often used toevaluate the overall reducing power of antioxidant com-pounds present in food and biological samples They can beconsidered a direct test for the evaluation of total antioxidantcapacity of a diversity of compounds as they are based onlyon their chemical-physical properties simplifying notably theoverall analytical process [25ndash27] Actually being the oxida-tion potential conceptually correlated with the antioxidantcapacity one can conclude that low oxidation potentialsfound either in food or biological samples enlighten a high

antioxidant capacity Additionally the amperometric currentandor chargemeasured under fixed oxidation conditions canalso provide an idea about the extension of their antioxidantcapacity as well as the estimation of their total content [27]

43 Electrochemical Behaviour of Hydroxycinnamic Acidsand Derivatives Considering that hydroxycinnamic acidsare antioxidants compounds by excellence electrochemicaltechniques can be powerful tools for the study of reactionmechanisms involving electron transfer and afford comple-mentary informationThemain structural feature responsiblefor the antioxidant and free radical-scavenging activity ofhydroxycinnamic acid derivatives is the number and locationof hydroxyl groups present in the molecule Phenols are ableto donate the hydrogen atom of the phenolic group to thefree radicals stopping the chain propagation step during theoxidation process

Differential pulse (DP) and cyclic voltammetry were usedto investigate the oxidation of p-coumaric caffeic and ferulicacids (Table 1)


AH

HN

N N

O2N O2N

NO2NO2

NO2 NO2

DPPH (red)yellow

DPPH (ox)purple

N∙

A∙

Scheme 1 Principle of DPPH radical scavenging capacity assay

The first cyclic voltammetric scan of p-coumaric acidshowed the occurrence of one irreversible peak which wasassociated with the oxidation of the hydroxyl group on thearomatic ring of the molecule [6 28ndash31] After successiveoxidative scans a p-coumaric acid oxidation product isdeposited on the electrode surface forming a polymeric film[30] A DP voltammetric study of p-coumaric acid was alsoperformed over a wide pH range from 36 to 127 It hasbeen concluded that the oxidation of p-coumaric acid belowpH = pK

119886occurs with the transfer of one electron and

one proton [30] The voltammetric behavior of p-coumaricacid is in agreement with the general mechanism proposedfor oxidation of phenolic groups [32] The general stage ofoxidation of phenols leads to the formation of a phenolateion that can be further oxidized to a phenoxyl radical Theseproducts can undergo further chemical reactions such ascoupling (polymers) proton loss or nucleophilic attack [628ndash31]

The introduction of a second hydroxyl group at the orthoposition (catechol) leads to a significant decrease of the redoxpotentials (Tables 1 and 4) DP voltammogram of caffeic acidpresents only one well-defined anodic peak at physiologicalpH (Figure 1) [7ndash9]The oxidation peak observed was relatedto the oxidation of catechol group Cyclic voltammograms(Figure 2) obtained showed that caffeic acid is reversiblyoxidized in solutions of pHup to 55 [33]The electrochemicaloxidation follows a mechanism involving only one step withthe transfer of two electrons and two protons The oxidationproduct was identified as being the corresponding o-quinone[6ndash9 28 29 31 33] For pHs higher than 55 caffeic acid o-quinone becomes unstable with a chemically homogeneousirreversible reaction occurring [33]

Substitution of the 3-hydroxyl group of caffeic acid by amethoxy group (ferulic acid Table 1) considerably shifts theredox potential toward more positive values (Table 4) Thedifferential pulse voltammetric study of ferulic acid revealedthe presence of two convolved anodic peaks at physiologicalpH (Figure 1) The oxidation peaks were related to theoxidation of the phenolic group present in the structure Theresults have been interpreted by assuming that ferulic acidoxidation takes place by electron transfer for both free andadsorbed forms [7ndash9] The free form corresponds to the first

peakwhile the strongly adsorbed formwhich is consequentlystabilized is oxidized at a more anodic potential Cyclicvoltammograms obtained for ferulic acid have also showntwo convolved anodic peaks (Figure 2) [6ndash9 28 29 34] Theproposed mechanism is similar to that described previouslyto p-coumaric acid

It has been generally accepted that the introduction ofsubstituents in the aromatic ring of hydroxycinnamic acidscould have a positive influence on the formation and lifetimethrough a stabilizing effect of the corresponding phenoxylradical with a subsequent enhancement of their antioxidantactivity Therefore it would be expected that the introduc-tion of a third hydroxyl (345-trihydroxycinnamic acid) amethoxyl (sinapic acid) or a bromide (5-bromocaffeic and 5-bromoferulic acids) substituent in position-5 of the aromaticring of the cinnamic acid could affect the formation andorstabilization of the radical intermediate causing a decrease ofthe oxidation potential The differential pulse voltammetricstudy of 345-trihydroxycinnamic acid (Table 1) revealed theexistence of two well-defined anodic waves at physiologicalpH (Figure 1) [11] For 5-bromocaffeic acid one well-definedanodic peak was observed at physiological pH (Figure 1) [7]The oxidation waves of these hydroxycinnamic compoundshave been related to the oxidation of catechol group presentin their structure [7 11] Similar to caffeic acid it has beenassumed that 345-trihydroxycinnamic and 5-bromocaffeicacids are oxidized to quinone via semiquinone form Thevoltammetric behaviour of sinapic and 5-bromoferulic acids(Table 1) has been also studied at a glassy carbon workingelectrode using differential pulse voltammetry (Figure 1) [710 29] Both compounds present two convolved anodic peaksat physiological pH using differential pulse voltammetryThe oxidation peaks have been related to the oxidation ofthe phenolic group present in their structures As for ferulicacid these results may be interpreted by assuming that theoxidation of sinapic and 5-bromoferulic acids takes place byelectron transfer from both free and adsorbed forms Thepresence of a methoxyl group in position-5 causes a cathodicshift on the oxidation potential with respect to ferulic acid(Table 4)

In summary it was concluded that a considerabledecrease in the peak potential has only been observed for


00 02 04 06 08 10 12 14 16minus02

500nA

E (V) versus AgAgCl

(a)

00 02 04 06 08 10 12 14 16minus02

200nA

E (V) versus AgAgCl

(b)

Figure 1 Differential pulse voltammograms for 01mM solutions of (a) (black line) caffeic acid (blue line) 5-bromocaffeic acid (green line)345-trihydroxycinnamic acid and (red line) hydrocaffeic acid and (b) (black line) ferulic acid (blue line) 5-bromoferulic acid (green line)sinapic acid and (red line) hydroferulic acid in physiological pH 73 supporting electrolyte Scan rate 5mV sminus1

00 02 04 06 08 10 12minus02

200nA

E (V) versus AgAgCl

(a)

00 02 04 06 08 10 12minus02

500nA

E (V) versus AgAgCl

(b)

Figure 2 Cyclic voltammograms for 01mM solutions of (a) (black line) caffeic acid (blue line) 5-bromocaffeic acid (green line) 345-trihydroxycinnamic acid and (red line) hydrocaffeic acid and (b) (black line) ferulic acid (blue line) 5-bromoferulic acid (green line) sinapicacid and (red line) hydroferulic acid in physiological pH 73 supporting electrolyte Scan rate 50mV sminus1

345-trihydroxycinnamic acid and sinapic acid relative tocaffeic and ferulic acids respectively (Table 4) The introduc-tion of a bromide group in position-5 did not significantlychange the oxidation potentials when compared to caffeicand ferulic acids From the data acquired so far some rele-vant conclusions can be addressed the presence of electronwithdrawing groups of halogen type in an ortho position toa hydroxyl group does not influence noticeably the redoxpotential of hydroxycinnamic acids This type of substituentappears not to disturb the stability of the phenoxyl radicalformed as well as its stabilization through intramolecularinteractions On the other hand lower potential values canbe attained resulting from the increased stabilization of the

phenoxyl radical if electron-donating hydroxyl or methoxylsubstituents are present in the aromatic ring

Another structural feature that seems to have a positiveeffect on the reducing properties of hydroxycinnamic acidsis the ethylenic side chain that connects the aromatic ringto the carboxylic acid [4 5 35] In fact it is assumed thatan additional stabilization of the phenoxyl radical can occurdue to its conjugation effect Thus the study of hydrocaffeicand hydroferulic acids (Table 2) can contribute to shed a lighton this subject The DP voltammograms of hydrocaffeic andhydroferulic acids present only one well-defined anodic peakat physiological pH (Figure 1) [8] The cyclic voltammogramobtained for hydrocaffeic acid (Figure 2) is characteristic


Table 2 Hydroxyhydrocinnamic acids and ester derivatives

12

3

45

6120572

120573

R1

R2

HO

COOR3

R1 R2 R3

Hydrocaffeic acid OH H HHexyl hydrocaffeate OH H CH2(CH2)4CH3

Hydroferulic acid OCH3 H HHexyl hydroferulate OCH3 H CH2(CH2)4CH3

of an electrochemical reversible reaction showing only oneanodic peak and one cathodic peak on the reverse scan[8] The oxidation peak observed has been related to theoxidation of catechol group [8]The redox potential obtainedfor hydrocaffeic acid has been found to be lower than thatobtained for caffeic acid (Table 4) On the other hand the CVstudy of hydroferulic acid (Figure 2) showed the occurrenceof one irreversible peak at a potential slightly higher thanferulic acid (Table 4) So the information obtained for thesetwo hydroxycinnamic acids was not conclusive and thecontribution of conjugation remain unclear

Furthermore several studies have been conducted toevaluate the influence of the modification of carboxylicfunction by alkyl groups of different length and nature linkedby ester or amide moieties on the antioxidant efficiency ofhydroxycinnamic acidsThe voltammetric analysis of a seriesof caffeic hydrocaffeic ferulic hydroferulic and sinapic acidderivatives has been performed (Tables 1ndash3 Figure 3) [5 7ndash9 29 36 37] In general the voltammetric behaviour of thesederivatives was similar to that observed for the parent com-pounds The introduction of an alkylamide or an alkylestergroup in the side chain did not significantly change the redoxpotentials (Table 4) In fact from the voltammetric data it canbe stated that esterification or amidation of the carboxylicacid group of hydroxycinnamic acids by alkyl chains doesnot significantly change the oxidation potential Neverthelessthe slight changes observed could be satisfactory to explainthe antioxidant activity differences found between HCAs andtheir esters or amides

5 Relationship between Redox Potential andAntioxidant Properties

Total antioxidant capacity (TAC) assays have been often usedto determine the hierarchy of radical-scavenging abilities ofpotential phenolic antioxidant compounds that work eitherthrough electron- or H-donating mechanisms [24] Thusto evaluate the radical-scavenging ability of the phenolicacids and their derivatives total antioxidant capacity assaysnamely DPPH∙ (2 21015840-diphenyl-1-picrylhydrazyl radical) areoften used [8 38] DPPH∙ method has the advantage ofestablishing a ranking hierarchy of antioxidant activity assome factors that cause interference in other model systems

Table 3 Hydroxy(hydro)cinnamic amide derivatives

CONR212

3

45

6120572

120573

R1

HO

R1 R2

Caffeoylhexylamide OH CH2(CH2)4CH3

Feruloylhexylamide OCH3 CH2(CH2)4CH3

Hydrocaffeoylhexylamide OH CH2(CH2)4CH3

Hydroferuloylhexylamide OCH3 CH2(CH2)4CH3

such as metal chelation andor partitioning abilities areabsent [39 40]

The radical scavenging activity of hydroxycinnamic acidsand derivatives on DPPH radical are depicted in Table 4Trolox a water-soluble vitamin E analogue was used asreference standard In general all the compounds havebeen found to be radical scavengers in a dose-dependentmanner and present better radical-scavenging activity thanthe reference antioxidant (trolox)

The antioxidant activity of cinnamic acids and derivativesis dependent on their molecular structure The main struc-tural features responsible for the activity are the presence of aphenolic group and the capacity of the compound to stabilizethe resulting phenoxyl radicals

Taken together the results obtained reveal the existence ofa relation between redox potentials of the hydroxycinnamicacids and derivatives and antioxidant activity (DPPH assay)(Table 4) The correlation between the redox potential ofantioxidants especially phenolics and its antioxidant activityis already documented in the literature but still the subject ofintense research [8 25 26] The radical scavenging activitydata acquired for hydroxycinnamic acids and derivatives isin good agreement with the expected activities of this typeof phenolic systems it is higher when the redox potentialis lower (as seen in the caffeic series Table 4) The presenceof a catechol group leads to an increase of the activity duemainly to resonance stabilization of the phenoxyl radicalintermediate with subsequent ortho-quinone formation Theabsence of a double bond in the side chain (as seen inhydrocaffeic series Table 2) gives rise to compounds witha lower redox potential and an increase in the antioxidantactivity However the methoxylation of the meta-phenolicgroup (ferulic and hydroferulic series Tables 1 and 2) leadsto an increase of redox potential and a decrease of theantioxidant activity In this type of systems the ethylenicdouble bond displays a negative effect in both propertiesIt is important to notice that the introduction of anothermethoxyl group in an ortho position to a hydroxyl group(sinapic acid series Table 1) results in an increase of theantioxidant activity and a decrease of the redox potentialrelative to ferulic acid

In addition it was shown that the introduction of ahalogen substituent in an ortho position to a phenolic groupof the hydroxycinnamic acids (or their linear monoalkylesters) has no influence either in the redox potential or


00 02 04 06 08 10 12 14 16minus02

E (V) versus AgAgCl

250nA

(a)

00 02 04 06 08 10 12 14 16minus02

E (V) versus AgAgCl

250nA

(b)

00 02 04 06 08 10 12 14 16minus02

500nA

E (V) versus AgAgCl

(c)

00 02 04 06 08 10 12 14minus02

E (V) versus AgAgCl

150nA

(d)

Figure 3 Differential pulse voltammograms for 01mM solutions of (a) (black line) ethyl caffeate (blue line) ethyl 5-bromocaffeate(green line) ethyl 345-trihydroxycinnamate acid (b) (black line) ethyl ferulate (blue line) ethyl 5-bromoferulate (green line) ethylsinapate (c) (black line) caffeoylhexylamide (red line) hydrocaffeoylhexylamide and (d) (black line) feruloylhexylamide (red line)hydroferuloylhexylamide in physiological pH 73 supporting electrolyte Scan rate 5mV sminus1

the antioxidant activity (Tables 1 and 4) The presence ofthis electron withdrawing group (120590-acceptor and a weak 120587-donor) in an ortho position to a hydroxyl group appears notto disturb the stability of the phenoxyl radical formed as wellas its stabilization through intramolecular interactions

The insertion of an alkyl side chain either by using esteror amide as linkers has been found to have different effects onthe antioxidant activity on phenolic cinnamic acid systemsThe data obtained so far reveal that caffeic acid derivativeshave higher DPPH radical scavenging activities and lowerredox potentials than caffeic acid itself a result that seemsto be dependent on the extension or type of the ester oramide side chain (Tables 1 and 4) Opposite tendency wasobtained for the ferulic and sinapic alkyl esters In fact thealkyl derivatives have lower antioxidant activities a fact thatis in accordance with electrochemical data (Tables 1 and 4)

From the overall results one can assume that the effectof the alkyl ester side chain in hydroxycinnamic systems is

strongly related to the number of hydroxyl groups and thearomatic substitution pattern In fact it seems to be intimatelyconnected with the different oxidation mechanisms andcapacity of stabilization of intermediates Caffeic acid andits derivatives operate via quinone intermediate and ferulicand sinapic derivatives via semiquinone intermediates Thelatter are further stabilized by the inductive and mesomericeffects of the methoxyl functions These overall effects cansurpass the mesomericinductive influence of the ethylenicside chain

As concluding remarks we can endorse that a generalantioxidant relationship statement cannot be performed forhydroxycinnamic acids and derivatives each series of com-pounds is a case and the structural modification must becautiously analyzed as the activity is a consequence of the sumof steric mesomeric and inductive effects on the system

In summary one can say that redox potentials were foundto rule the antioxidant activities of this type of compounds


Table 4 Redox potentials and antioxidant activity of hydroxycinnamic acids ester and amide derivatives

Compound MW (gmolminus1) DPPH (TE)lowast DPPH (IC50)lowastdagger

119864119901(mV) Reference

p-Coumaric 16416 gt100 +736 [6]Caffeic acid 18016 129 166 +183 [7 8]Hydrocaffeic acid 18218 205 154 +139 [8]5-Bromocaffeic acid 25905 101 +182 [7]Methyl caffeate 19418 140 +165 [9]Ethyl caffeate 20821 135 +170 [9]Ethyl 5-bromocaffeate 28711 0938 +174 [7]Propyl caffeate 22224 145 +173 [9]Butyl caffeate 23626 141 +176 [9]Hexylcaffeate 26432 0970 +175 [8]Hexylhydrocaffeate 26634 0990 +125 [8]Caffeoylhexylamide 26334 111 +162 [8]Hydrocaffeoylhexylamide 26535 100 +125 [8]Ferulic acid 19419 0781 446 +335 +447 [7]Hydroferulic acid 19620 840 +410 [8]5-Bromoferulic acid 27308 0808 +335 +442 [7]Methyl ferulate 20821 747 +375 [9]Ethyl ferulate 22224 667 +370 [9]Ethyl 5-bromoferulate 30113 0558 +365 [7]Propyl ferulate 23626 641 +364 [9]Butyl ferulate 25029 563 +343 [9]Hexylferulate 27835 +328 [8]Hexylhydroferulate 28036 +434 [8]Feruloylhexylamide 27736 +322 [8]Hydroferuloylhexylamide 27938 +388 [8]Sinapic acid 22421 0862 322 +188 +295 [10]Methyl sinapate 23824 487 +219 [10]Ethyl sinapate 25226 519 +189 [10]Propyl sinapate 26629 506 +182 [10]Butyl sinapate 28032 501 +190 [10]345-Trihydroxycinnamic acid 19616 118 +11 +384 [11]Ethyl 345-trihydroxycinnamate 22421 151 +26 +133 [11]lowastThe results of DPPH assays are usually expressed as TEAC (trolox equivalent antioxidant capacity) or IC50 (concentration which is required to scavenge 50of DPPH free radicals) These values are not convertibledaggerIC50 is presented in 120583mol Lminus1Unpublished results

being an effective approach for the rational design of newantioxidant agents as well as for the understanding of theirmechanism of action

6 Electrochemistry in a Real World

Electrochemical methods are a very versatile tool that can beapplied not only in qualitative but also quantitative analysisHydroxycinnamic acids and derivatives need to be deter-mined in real samples at low (120583gL) or high concentrationlevels [41] Electrochemical methods using voltammetric oramperometric detection with different cell designs can offerexcellent sensitivity and selectivity by varying the workingpotential Although several methods have been developed tomonitor the levels of phenolic compounds in wine [42ndash45]

none of them is able to specify the reducing capacity of the(poly)phenols present or to distinguish between catechol andgalloyl-containing (poly)phenols which are easy to oxidize(eg caffeic acid) and those that are more difficult to oxidize(eg coumaric acid) Taking into account that (poly)phenolsare electrochemically active compounds and can be oxidizedat inert electrodes electrochemical methods can be usedto achieve this goal In fact cyclic voltammetry has beenextensively applied to characterize a variety of wine phenolics[28 46ndash54]This technique has been used to discriminate andto estimate the total polyphenolic content in wine [47ndash49]and to study the antioxidant activity providing a measure ofthe ldquoRedox spectra of Winesrdquo [50 51] Moreover this type ofmethods has been used to evaluate the ldquooxidation statusrdquo ofwhite wines [52] and perceived astringency of red wines [53]


Electrochemical methods appear as simple and sensitivemethods giving good estimations of the global content ofpolyphenols [54 55] As hydroxycinnamic acids and deriva-tives are of natural origin and are present in a diversity ofmatrices namely grapes and wine (red or white) beer greencoffee beans and coffee spices vegetables and blueberriesone can envision a flourishing application of electrochemicalmethods [56ndash61] These techniques will be capable of givingthe global amount of (poly)phenols and of characterizingnew compounds In the near future these methods will beapplied transversally in different areas such as food andpharmaceutical industries namely for the discovery andordetection of new antioxidants

References

[1] P A Kroon and G Williamson ldquoHydroxycinnamates in plantsand food current and future perspectivesrdquo Journal of the Scienceof Food and Agriculture vol 79 no 3 pp 355ndash361 1999

[2] F Shahidi and A Chandrasekara ldquoHydroxycinnamates andtheir in vitro and in vivo antioxidant activitiesrdquo PhytochemistryReviews vol 9 no 1 pp 147ndash170 2010

[3] P Fresco F Borges C Diniz and M P M Marques ldquoNewinsights on the anticancer properties of dietary polyphenolsrdquoMedicinal Research Reviews vol 26 no 6 pp 747ndash766 2006

[4] N Razzaghi-Asl J Garrido H Khazraei F Borges and OFiruzi ldquoAntioxidant properties of hydroxycinnamic acids areview of structure-activity relationshipsrdquo Current MedicinalChemistry In press

[5] C A Rice-Evans N J Miller and G Paganga ldquoStructure-antioxidant activity relationships of flavonoids and phenolicacidsrdquo Free Radical Biology andMedicine vol 20 no 7 pp 933ndash956 1996

[6] D Galato K Ckless M F Susin C Giacomelli R M Ribeiro-do-Valle and A Spinelli ldquoAntioxidant capacity of phenolic andrelated compounds correlation among electrochemical visi-ble spectroscopy methods and structure-antioxidant activityrdquoRedox Report vol 6 no 4 pp 243ndash250 2001

[7] A Gaspar E M Garrido M Esteves et al ldquoNew insights intothe antioxidant activity of hydroxycinnamic acids synthesisand physicochemical characterization of novel halogenatedderivativesrdquo European Journal of Medicinal Chemistry vol 44no 5 pp 2092ndash2099 2009

[8] F M F Roleira C Siquet E Orr et al ldquoLipophilic phenolicantioxidants correlation between antioxidant profile partitioncoefficients and redox propertiesrdquo Bioorganic and MedicinalChemistry vol 18 no 16 pp 5816ndash5825 2010

[9] J Garrido A Gaspar E M Garrido et al ldquoAlkyl estersof hydroxycinnamic acids with improved antioxidant activityand lipophilicity protect PC12 cells against oxidative stressrdquoBiochimie vol 94 no 4 pp 961ndash967 2012

[10] AGasparMMartins P Silva et al ldquoDietary phenolic acids andderivatives Evaluation of the antioxidant activity of sinapic acidand its alkyl estersrdquo Journal of Agricultural and Food Chemistryvol 58 no 21 pp 11273ndash11280 2010

[11] J Teixeira T Silva S Benfeito et al ldquoExploring nature profitsdevelopment of novel and potent lipophilic antioxidants basedon galloylmdashcinnamic hybridsrdquo European Journal of MedicinalChemistry vol 62 pp 289ndash296 2013

[12] T Nguyen P J Sherratt and C B Pickett ldquoRegulatory mecha-nisms controlling gene expression mediated by the antioxidantresponse elementrdquo Annual Review of Pharmacology and Toxi-cology vol 43 pp 233ndash260 2003

[13] R Rodrigo A Miranda and L Vergara ldquoModulation ofendogenous antioxidant system by wine polyphenols in humandiseaserdquo Clinica Chimica Acta vol 412 no 5-6 pp 410ndash4242011

[14] J K Jacob K Tiwari J Correa-Betanzo A Misran R Chan-drasekaran and G Paliyath ldquoBiochemical basis for functionalingredient design from fruitsrdquo Annual Review of Food ScienceTechnology vol 3 pp 79ndash104 2012

[15] C M A Brett and A M Oliveira Brett ElectrochemistryPrinciples Methods and Applications Oxford University PressOxford UK 1993

[16] A J Bard and L R Faulkner Electrochemical Methods Funda-mentals and Applications Wiley New York 2001

[17] S P Kounaves ldquoVoltammetric techniques rdquo in Handbook ofInstrumental Techniques for Analytical Chemistry F A SettleEd Prentice Hall Upper Saddle River NJ USA 1997

[18] R Greef R Peat L M Peter D Pletcher and J Robin-son Instrumental Methods in Electrochemistry Ellis HorwoodChichester UK 1985

[19] G Denuault M Sosna and K J Williams ldquoClassical exper-imentsrdquo in Handbook of Electrochemistry C G Zoski EdElsevier Amsterdam The Netherlands 2007

[20] G Denuault ldquoElectrochemical techniques and sensors forocean researchrdquo Ocean Science vol 5 no 4 pp 697ndash710 2009

[21] E M Becker L R Nissen and L H Skibsted ldquoAntioxidantevaluation protocols food quality or health effectsrdquo EuropeanFood Research and Technology vol 219 no 6 pp 561ndash571 2004

[22] B Halliwell ldquoHow to characterize a biological antioxidantrdquo FreeRadical Research Communications vol 9 no 1 pp 1ndash32 1990

[23] B Halliwell ldquoAntioxidants the basicsmdashwhat they are and howto evaluate themrdquo Advances in Pharmacology vol 38 pp 3ndash201997

[24] D Huang O U Boxin and R L Prior ldquoThe chemistry behindantioxidant capacity assaysrdquo Journal of Agricultural and FoodChemistry vol 53 no 6 pp 1841ndash1856 2005

[25] S Chevion and M Chevion ldquoAntioxidant status and humanhealth Use of cyclic voltammetry for the evaluation of theantioxidant capacity of plasma and of edible plantsrdquo Annals ofthe New York Academy of Sciences vol 899 pp 308ndash325 2000

[26] S Chevion M A Roberts and M Chevion ldquoThe use of cyclicvoltammetry for the evaluation of antioxidant capacityrdquo FreeRadical Biology and Medicine vol 28 no 6 pp 860ndash870 2000

[27] A J Blasco A G Crevillen M C Gonzalez and A EscarpaldquoDirect electrochemical sensing anddetection of natural antiox-idants and antioxidant capacity in vitro systemsrdquo Electroanaly-sis vol 19 no 22 pp 2275ndash2286 2007

[28] P Hapiot A Neudeck J Pinson H Fulcrand P Neta andC Rolando ldquoOxidation of caffeic acid and related hydroxycin-namic acidsrdquo Journal of Electroanalytical Chemistry vol 405no 1-2 pp 169ndash176 1996

[29] W R Sousa C da Rocha C L Cardoso D H S Silva andM V B Zanoni ldquoDetermination of the relative contributionof phenolic antioxidants in orange juice by voltammetricmethodsrdquo Journal of Food Composition and Analysis vol 17 no5 pp 619ndash633 2004


[30] P Janeiro I Novak M Seruga and A M Oliveira-BrettldquoElectroanalytical oxidation of p-coumaric acidrdquo AnalyticalLetters vol 40 no 17 pp 3309ndash3321 2007

[31] A Simic D Manojlovic D Segan and M Todorovic ldquoElec-trochemical behavior and antioxidant and prooxidant activityof natural phenolicsrdquo Molecules vol 12 no 10 pp 2327ndash23402007

[32] H Lund and M M Baizer Organic Electrochemistry MarcelDekker New York NY USA 3rd edition 1991

[33] C Giacomelli K Ckless D Galato F S Miranda and ASpinelli ldquoElectrochemistry of caffeic acid aqueous solutionswith pH 20 to 85rdquo Journal of the Brazilian Chemical Societyvol 13 no 3 pp 332ndash338 2002

[34] S K Trabelsi N B Tahar B Trabelsi and R AbdelhedildquoElectrochemical oxidation of ferulic acid in aqueous solutionsat gold oxide and lead dioxide electrodesrdquo Journal of AppliedElectrochemistry vol 35 no 10 pp 967ndash973 2005

[35] N Nenadis S Boyle E G Bakalbassis and M TsimidouldquoAn experimental approach to structure-activity relationshipsof caffeic and dihydrocaffeic acids and related monophenolsrdquoJournal of the American Oil Chemistsrsquo Society vol 80 no 5 pp451ndash458 2003

[36] M Born P-A Carrupt R Zini et al ldquoElectrochemicalbehaviour and antioxidant activity of some natural polyphe-nolsrdquoHelvetica Chimica Acta vol 79 no 4 pp 1147ndash1158 1996

[37] A Neudorffer D Bonnefont-Rousselot A Legrand M-BFleury and M Largeron ldquo4-Hydroxycinnamic ethyl esterderivatives and related dehydrodimers relationship betweenoxidation potential and protective effects against oxidationof low-density lipoproteinsrdquo Journal of Agricultural and FoodChemistry vol 52 no 7 pp 2084ndash2091 2004

[38] W Brand-Williams M E Cuvelier and C Berset ldquoUse of afree radical method to evaluate antioxidant activityrdquo LWT-FoodScience and Technology vol 28 no 1 pp 25ndash30 1995

[39] F A M Silva F Borges C Guimaraes J L F C Lima CMatos and S Reis ldquoPhenolic acids and derivatives studies onthe relationship among structure radical scavenging activityand physicochemical parametersrdquo Journal of Agricultural andFood Chemistry vol 48 no 6 pp 2122ndash2126 2000

[40] C Siquet F Paiva-Martins J L F C Lima S Reis andF Borges ldquoAntioxidant profile of dihydroxy- and trihydrox-yphenolic acidsmdasha structure-activity relationship studyrdquo FreeRadical Research vol 40 no 4 pp 433ndash442 2006

[41] C Bocchi M Careri F Groppi A Mangia P Manini andG Mori ldquoComparative investigation of UV electrochemicaland particle beam mass spectrometric detection for the high-performance liquid chromatographic determination of benzoicand cinnamic acids and of their corresponding phenolic acidsrdquoJournal of Chromatography A vol 753 no 2 pp 157ndash170 1996

[42] J A Kennedy and A L Waterhouse ldquoAnalysis of pigmentedhigh-molecular-mass grape phenolics using ion- pair normal-phase high-performance liquid chromatographyrdquo Journal ofChromatography A vol 866 no 1 pp 25ndash34 2000

[43] S A Lazarus G E Adamson J F Hammerstone and HH Schmitz ldquoHigh-performance liquid chromatographymassspectrometry analysis of proanthocyanidins in foods and bev-eragesrdquo Journal of Agricultural and Food Chemistry vol 47 no9 pp 3693ndash3701 1999

[44] S F Price P J Breen M Valladao and B T Watson ldquoClustersun exposure and quercetin in Pinot noir grapes and winerdquo

American Journal of Enology and Viticulture vol 46 no 2 pp187ndash194 1995

[45] M Ibern-Gomez C Andres-Lacueva R M Lamuela-Raventos and A L Waterhouse ldquoRapid HPLC analysis ofphenolic compounds in red winesrdquo American Journal ofEnology and Viticulture vol 53 no 3 pp 218ndash221 2002

[46] V Castaignede H Durliat and M Comtat ldquoAmperometricand potentiometric determination of catechin as model ofpolyphenols in winesrdquoAnalytical Letters vol 36 no 9 pp 1707ndash1720 2003

[47] V Parra T Hernando M L Rodrıguez-Mendez and J A DeSaja ldquoElectrochemical sensor array made from bisphthalocya-nine modified carbon paste electrodes for discrimination of redwinesrdquo Electrochimica Acta vol 49 no 28 pp 5177ndash5185 2004

[48] D De Beer J F Harbertson P A Kilmartin et al ldquoPhenolics acomparison of diverse analytical methodsrdquoAmerican Journal ofEnology and Viticulture vol 55 no 4 pp 389ndash400 2004

[49] J Piljac S Martinez T Stipcevic Z Petrovic and M Metikos-Hukovic ldquoCyclic voltammetry investigation of the phenoliccontent of croatian winesrdquo American Journal of Enology andViticulture vol 55 no 4 pp 417ndash422 2004

[50] V Roginsky D De Beer J F Harbertson P A KilmartinT Barsukova and D O Adams ldquoThe antioxidant activity ofCalifornian red wines does not correlate with wine agerdquo Journalof the Science of Food andAgriculture vol 86 no 5 pp 834ndash8402006

[51] O Makhotkina and P A Kilmartin ldquoThe use of cyclic voltam-metry for wine analysis determination of polyphenols and freesulfur dioxiderdquo Analytica Chimica Acta vol 668 no 2 pp 155ndash165 2010

[52] R C Martins R Oliveira F Bento et al ldquoOxidation manage-ment of white wines using cyclic voltammetry and multivariateprocess monitoringrdquo Journal of Agricultural and Food Chem-istry vol 56 no 24 pp 12092ndash12098 2008

[53] S C Petrovic ldquoCorrelation of perceived wine astringency tocyclic voltammetric responserdquoAmerican Journal of Enology andViticulture vol 60 no 3 pp 373ndash378 2009

[54] A S Arribas M Martınez-Fernandez and M Chicharro ldquoTherole of electroanalytical techniques in analysis of polyphenols inwinerdquo Trends in Analytical Chemistry vol 34 pp 78ndash96 2012

[55] M Hilgemann V C Bassetto and L T Kubota ldquoElectro-chemical approaches employed for sensing the antioxidantcapacity exhibited by vegetal extracts a reviewrdquo CombinatorialChemistry amp High Throughput Screen vol 16 no 2 pp 98ndash1082013

[56] E N Frankel A L Waterhouse and P L Teissedre ldquoPrincipalphenolic phytochemicals in selected California wines and theirantioxidant activity in inhibiting oxidation of human low-density lipoproteinsrdquo Journal of Agricultural and Food Chem-istry vol 43 no 4 pp 890ndash894 1995

[57] P A Kilmartin and C F Hsu ldquoCharacterisation of polyphenolsin green oolong and black teas and in coffee using cyclicvoltammetryrdquo Food Chemistry vol 82 no 4 pp 501ndash512 2003

[58] W J R Santos M Santhiago I V P Yoshida and L T KubotaldquoNovel electrochemical sensor for the selective recognition ofchlorogenic acidrdquo Analytica Chimica Acta vol 695 no 1-2 pp44ndash50 2011

[59] A M Alonso C Domınguez D A Guillen and C GBarroso ldquoDetermination of antioxidant power of red and whitewines by a new electrochemical method and its correlation


with polyphenolic contentrdquo Journal of Agricultural and FoodChemistry vol 50 no 11 pp 3112ndash3115 2002

[60] P A Kilmartin H Zou and A L Waterhouse ldquoA cyclicvoltammetry method suitable for characterizing antioxidantproperties of wine and wine phenolicsrdquo Journal of Agriculturaland Food Chemistry vol 49 no 4 pp 1957ndash1965 2001

[61] J Dobes O Zitka J Sochor et al ldquoElectrochemical tools fordetermination of phenolic compounds in plants a reviewrdquoInternational Journal of Electrochemical Science vol 8 pp 4520ndash4542 2013

Submit your manuscripts athttpwwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Anatomy Research International

PeptidesInternational Journal of


Hindawi Publishing Corporation httpwwwhindawicom

International Journal of

Volume 2014

Zoology


Molecular Biology International

GenomicsInternational Journal of


The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014


BioinformaticsAdvances in

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Signal TransductionJournal of


BioMed Research International

Evolutionary BiologyInternational Journal of



Biochemistry Research International

ArchaeaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014


Genetics Research International


Advances in

Virolog y

Hindawi Publishing Corporationhttpwwwhindawicom

Nucleic AcidsJournal of

Volume 2014

Stem CellsInternational



Enzyme Research



Microbiology


in tea leaves coffee red wine various fruits (especially redones) vegetables and whole grains

Hydroxycinnamic acids such as p-coumaric caffeicferulic and sinapic acids (Table 1) are known to play animportant role in nature In fact their wide distributionand high concentration provide them with a key role in thebiosynthesis of more complicated phenolic systems HCAsare secondary metabolites that are found also in severalconjugated forms including amides (conjugated with mono-or polyamines amino acids or peptides) estersmainly estersof hydroxy acids such as tartaric acid and sugar derivativesand glycosides Whereas cinnamate esters occur widely inhigher plants amides of cinnamic acids seem to be rare

The variety of cinnamic derivatives in plants is dependenton the species In general plants of the Solanaceae familyprovide both chlorogenic acid (5-caffeoylquinic acid) andfree hydroxycinnamic acids such as caffeic acid These com-pounds are themost predominant inmany fruits constitutingup to 75 of phenolic acids Actually derivatives of cinnamicacid exist in all fruit parts but concentrations are higher inthe outer parts of ripe fruit

3 Hydroxycinnamic AcidsAn Antioxidant Outlook

Antioxidants used to prevent or inhibit the natural phenom-ena of oxidation have a broad application in diverse fieldsas they have a huge importance either as industrial additivesor health agents In this context HCAs have been ascribedto act as powerful antioxidant compounds possessing diversephysiological roles in biological systems Research data haverevealed that HCAs can be used for preventive andortherapeutic purposes in several diseases related to oxidativestress (eg atherosclerosis inflammatory injury cancer andcardiovascular diseases)

The antioxidant foremost mechanism of action isassumed to be through its radical-scavenging activity that islinked to their hydrogen- or electron-donating ability andto the stability of the resulting phenoxyl radicals Howeverother mechanisms of action have been suggested such aschelation of transition metals like copper or iron whichare well-known catalysts of oxidative stress inhibition ofROSRNS-generating enzymes and modulation of geneexpression (ARENrf-2 pathway) [12ndash14]

Generally cinnamic acids work well in aqueous mediabeing their hydrophilic nature a restriction for lipophilicsystems protection (eg food or cosmetic matrices or bio-logical membranes) due mainly to the difficulty of theirincorporation into fat and oil mediums The hydrophobicityof HCAs can be enhanced by chemical modification namelyby esterification of the carboxyl group of phenolic acid withfor instance a fatty alcohol Using this type of approach onemust take in mind that the original functional propertiesmust be retained Therefore amplified sort of applications ofthesemodified natural antioxidants in lipophilicmedium canbe explored

4 Electrochemistry of Hydroxycinnamic Acidsand Derivatives

41 General Concepts Electrochemical techniques are versa-tile and powerful analytical tools which are able to providehigh sensitivity and low detection limits associated with theuse of inexpensive instrumentation In addition to their appli-cation in fundamental studies of oxidation and reductionprocesses to unravel reaction mechanisms these techniquesare also used in studying the kinetics and thermodynamics ofelectron and ion transfer processes [15 16]

The common characteristic of all voltammetric tech-niques is that they involve the application of a potential (E)to an electrode which causes the species to be determinedto react and a current (I) to pass In many cases theapplied potential is varied or the current is monitored overa period of time (t)Thus all voltammetric techniques can bedescribed as some function of E I and t They are consideredactive techniques (as opposed to passive techniques suchas potentiometry) because the applied potential forces achange in the concentration of an electroactive species at theelectrode surface by electrochemically reducing or oxidizingit [17]

Cyclic voltammetry (CV) has become an important andwidely used electroanalytical technique in many areas ofchemistry and biochemistry It is rarely used for quantitativedeterminations but it is widely used for the study of redoxprocesses namely for obtaining information on the nature ofintermediates and stability of reaction products

Advanced voltammetric techniques have been developedto increase the sensitivity and selectivity of the voltammetricsignal In most cases this requires minimizing the currentfrom nonfaradaic processes such as that due to the chargingand discharging of the double layer [16 18]These techniquesrely on combinations of sweeps and steps and have namessuch as normal pulse voltammetry differential pulse voltam-metry and square wave voltammetry [16 19]

Voltammograms show peaks and waves which representthe different redox processes occurring in the potentialwindow selected The positions of the peaks and wavesalong the potential axis reflect the species involved (via theirredox potential) but also whether the redox reactions arekinetically favorable or unfavorable The shape and heightof the peaks and waves also indicate the nature of theredox processes (eg whether a given redox process involvesan adsorptiondesorption or whether both reactants andproducts are soluble) A voltammogram is therefore a kindof finger print of the redox reactions [20]

42 Electrochemistry and Antioxidant Assays The termldquoantioxidantrdquo is increasingly popular nowadays as it is asso-ciated with a number of health benefits In fact antioxidantsare molecules that can be associated with the protection ofmacromolecules from oxidation [21] In biological systemsthe definition has been accepted to be ldquoany substance thatwhen present at low concentrations compared to those of anoxidisable substrate significantly delays or prevents oxidationof that substraterdquo [22 23]



12

3

45

6120572

120573

R1

R2

HO

COOR3

R1 R2 R3

























AH

HN

N N

O2N O2N

NO2NO2

NO2 NO2

DPPH (red)yellow

DPPH (ox)purple

N∙

A∙











00 02 04 06 08 10 12 14 16minus02

500nA

E (V) versus AgAgCl

(a)

00 02 04 06 08 10 12 14 16minus02

200nA

E (V) versus AgAgCl

(b)


00 02 04 06 08 10 12minus02

200nA

E (V) versus AgAgCl

(a)

00 02 04 06 08 10 12minus02

500nA

E (V) versus AgAgCl

(b)







12

3

45

6120572

120573

R1

R2

HO

COOR3

R1 R2 R3








CONR212

3

45

6120572

120573

R1

HO

R1 R2











00 02 04 06 08 10 12 14 16minus02

E (V) versus AgAgCl

250nA

(a)

00 02 04 06 08 10 12 14 16minus02

E (V) versus AgAgCl

250nA

(b)

00 02 04 06 08 10 12 14 16minus02

500nA

E (V) versus AgAgCl

(c)

00 02 04 06 08 10 12 14minus02

E (V) versus AgAgCl

150nA

(d)



















References









































































Volume 2014

Zoology






















Advances in

Virolog y



Volume 2014




Enzyme Research



Microbiology



12

3

45

6120572

120573

R1

R2

HO

COOR3

R1 R2 R3

























AH

HN

N N

O2N O2N

NO2NO2

NO2 NO2

DPPH (red)yellow

DPPH (ox)purple

N∙

A∙











00 02 04 06 08 10 12 14 16minus02

500nA

E (V) versus AgAgCl

(a)

00 02 04 06 08 10 12 14 16minus02

200nA

E (V) versus AgAgCl

(b)


00 02 04 06 08 10 12minus02

200nA

E (V) versus AgAgCl

(a)

00 02 04 06 08 10 12minus02

500nA

E (V) versus AgAgCl

(b)







12

3

45

6120572

120573

R1

R2

HO

COOR3

R1 R2 R3








CONR212

3

45

6120572

120573

R1

HO

R1 R2











00 02 04 06 08 10 12 14 16minus02

E (V) versus AgAgCl

250nA

(a)

00 02 04 06 08 10 12 14 16minus02

E (V) versus AgAgCl

250nA

(b)

00 02 04 06 08 10 12 14 16minus02

500nA

E (V) versus AgAgCl

(c)

00 02 04 06 08 10 12 14minus02

E (V) versus AgAgCl

150nA

(d)



















References









































































Volume 2014

Zoology






















Advances in

Virolog y



Volume 2014




Enzyme Research



Microbiology


AH

HN

N N

O2N O2N

NO2NO2

NO2 NO2

DPPH (red)yellow

DPPH (ox)purple

N∙

A∙











00 02 04 06 08 10 12 14 16minus02

500nA

E (V) versus AgAgCl

(a)

00 02 04 06 08 10 12 14 16minus02

200nA

E (V) versus AgAgCl

(b)


00 02 04 06 08 10 12minus02

200nA

E (V) versus AgAgCl

(a)

00 02 04 06 08 10 12minus02

500nA

E (V) versus AgAgCl

(b)







12

3

45

6120572

120573

R1

R2

HO

COOR3

R1 R2 R3








CONR212

3

45

6120572

120573

R1

HO

R1 R2











00 02 04 06 08 10 12 14 16minus02

E (V) versus AgAgCl

250nA

(a)

00 02 04 06 08 10 12 14 16minus02

E (V) versus AgAgCl

250nA

(b)

00 02 04 06 08 10 12 14 16minus02

500nA

E (V) versus AgAgCl

(c)

00 02 04 06 08 10 12 14minus02

E (V) versus AgAgCl

150nA

(d)



















References









































































Volume 2014

Zoology






















Advances in

Virolog y



Volume 2014




Enzyme Research



Microbiology


00 02 04 06 08 10 12 14 16minus02

500nA

E (V) versus AgAgCl

(a)

00 02 04 06 08 10 12 14 16minus02

200nA

E (V) versus AgAgCl

(b)


00 02 04 06 08 10 12minus02

200nA

E (V) versus AgAgCl

(a)

00 02 04 06 08 10 12minus02

500nA

E (V) versus AgAgCl

(b)







12

3

45

6120572

120573

R1

R2

HO

COOR3

R1 R2 R3








CONR212

3

45

6120572

120573

R1

HO

R1 R2











00 02 04 06 08 10 12 14 16minus02

E (V) versus AgAgCl

250nA

(a)

00 02 04 06 08 10 12 14 16minus02

E (V) versus AgAgCl

250nA

(b)

00 02 04 06 08 10 12 14 16minus02

500nA

E (V) versus AgAgCl

(c)

00 02 04 06 08 10 12 14minus02

E (V) versus AgAgCl

150nA

(d)



















References









































































Volume 2014

Zoology






















Advances in

Virolog y



Volume 2014




Enzyme Research



Microbiology



12

3

45

6120572

120573

R1

R2

HO

COOR3

R1 R2 R3








CONR212

3

45

6120572

120573

R1

HO

R1 R2











00 02 04 06 08 10 12 14 16minus02

E (V) versus AgAgCl

250nA

(a)

00 02 04 06 08 10 12 14 16minus02

E (V) versus AgAgCl

250nA

(b)

00 02 04 06 08 10 12 14 16minus02

500nA

E (V) versus AgAgCl

(c)

00 02 04 06 08 10 12 14minus02

E (V) versus AgAgCl

150nA

(d)



















References









































































Volume 2014

Zoology






















Advances in

Virolog y



Volume 2014




Enzyme Research



Microbiology


00 02 04 06 08 10 12 14 16minus02

E (V) versus AgAgCl

250nA

(a)

00 02 04 06 08 10 12 14 16minus02

E (V) versus AgAgCl

250nA

(b)

00 02 04 06 08 10 12 14 16minus02

500nA

E (V) versus AgAgCl

(c)

00 02 04 06 08 10 12 14minus02

E (V) versus AgAgCl

150nA

(d)



















References









































































Volume 2014

Zoology






















Advances in

Virolog y



Volume 2014




Enzyme Research



Microbiology












References









































































Volume 2014

Zoology






















Advances in

Virolog y



Volume 2014




Enzyme Research



Microbiology












































Volume 2014

Zoology






















Advances in

Virolog y



Volume 2014




Enzyme Research



Microbiology

review article hydroxycinnamic acid antioxidants: an electrochemical overview - hindawi

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