Electroanalytical Oxidation of p ‐Coumaric Acid

This article was downloaded by:[B-on Consortium - 2007]On: 6 February 2008Access Details: [subscription number 778384761]Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK

Analytical LettersPublication details, including instructions for authors and subscription information:http://www.informaworld.com/smpp/title~content=t713597227

Electroanalytical Oxidation of p-Coumaric AcidPatricia Janeiro a; Ivana Novak ab; Marijan Seruga b; Ana Maria Oliveira-Brett aa Faculdade de Ciências e Tecnologia, Departamento de Química, Universidade deCoimbra, Coimbra, Portugalb Faculty of Food Technology, Department of Applied Chemistry and Ecology, J. J.Strossmayer University of Osijek, Osijek, Croatia

Online Publication Date: 01 January 2007To cite this Article: Janeiro, Patricia, Novak, Ivana, Seruga, Marijan andOliveira-Brett, Ana Maria (2007) 'Electroanalytical Oxidation of p-Coumaric Acid',Analytical Letters, 40:17, 3309 - 3321To link to this article: DOI: 10.1080/00032710701672822URL: http://dx.doi.org/10.1080/00032710701672822

PLEASE SCROLL DOWN FOR ARTICLE

Full terms and conditions of use: http://www.informaworld.com/terms-and-conditions-of-access.pdf

This article maybe used for research, teaching and private study purposes. Any substantial or systematic reproduction,re-distribution, re-selling, loan or sub-licensing, systematic supply or distribution in any form to anyone is expresslyforbidden.

The publisher does not give any warranty express or implied or make any representation that the contents will becomplete or accurate or up to date. The accuracy of any instructions, formulae and drug doses should beindependently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings,demand or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with orarising out of the use of this material.

Dow

nloa

ded

By:

[B-o

n C

onso

rtium

- 20

07] A

t: 10

:34

6 Fe

brua

ry 2

008

ELECTROCHEMISTRY

Electroanalytical Oxidation of p-CoumaricAcid

Patricia Janeiro

Faculdade de Ciencias e Tecnologia, Departamento de Quımica,

Universidade de Coimbra, Coimbra, Portugal

Ivana Novak

Faculdade de Ciencias e Tecnologia, Departamento de Quımica,

Universidade de Coimbra, Coimbra, Portugal and Faculty of Food

Technology, Department of Applied Chemistry and Ecology, J. J.

Strossmayer University of Osijek, Osijek, Croatia

Marijan Seruga

Faculty of Food Technology, Department of Applied Chemistry and

Ecology, J. J. Strossmayer University of Osijek, Osijek, Croatia

Ana Maria Oliveira-Brett

Faculdade de Ciencias e Tecnologia, Departamento de Quımica,

Universidade de Coimbra, Coimbra, Portugal

Abstract: The mechanism of the electrochemical oxidation of p-coumaric acid on a

glassy carbon electrode was investigated using cyclic, differential pulse, and square

wave voltammetry at different pHs. The oxidation of p-coumaric acid is irreversible

over the whole pH range. After successive scans, the p-coumaric acid oxidation

product deposits on the electrode surface, forming a polymeric film that undergoes

reversible oxidation at a lower potential than p-coumaric acid. This polymeric film

increases in thickness with the number of scans, covering the electrode surface, and

impeding the diffusion of the p-coumaric acid and its oxidation on the electrode.

The oxidation of p-coumaric acid is pH dependent up until values close to the pKa.

For pHs higher than pKa, the p-coumaric acid oxidation process is pH independent.

Received 16 August 2007; accepted 10 September 2007

Address correspondence to Ana Maria Oliveira-Brett, Faculdade de Ciencias e

Tecnologia, Departamento de Quımica, Universidade de Coimbra, Coimbra 3004-

535, Portugal. E-mail: [email protected]

Analytical Letters, 40: 3309–3321, 2007

Copyright # Taylor & Francis Group, LLC

ISSN 0003-2719 print/1532-236X online

DOI: 10.1080/00032710701672822

3309

Dow

nloa

ded

By:

[B-o

n C

onso

rtium

- 20

07] A

t: 10

:34

6 Fe

brua

ry 2

008

An electroanalytical determination procedure of p-coumaric in pH 8.7 0.2 M

ammonium buffer was developed, and a detection limit, LOD ¼ 83 nM, and the

limit of quantification, LOQ ¼ 250 nM, were obtained.

Keywords: p-Coumaric acid, oxidation, radical scavenging activity, hydroxycinnamic

acids, free radicals

INTRODUCTION

Polyphenols (e.g., phenolic acids, flavonoids, tannins) are a diverse group of

secondary metabolites which are naturally present in grapes and wines.

Distinction among polyphenol classes are drawn first on the basis of the

number of constitutive carbon atoms and in light of the structure of their

basic skeleton.

Phenolic compounds are known to act as antioxidants (Rice-Evants et al.

1997). Hydroxycinnamic acids (HCA), as well as hydroxybenzoic acids

(HBA), belong to the phenolic acids group. The antioxidant activity of

phenolic acids depends on the number of hydroxyl groups that are strength-

ened by steric hindrance. In contrast to other phenolic compounds, HBA

and HCA present an acidic character because of the presence of one

carboxyl group on the molecule. The insertion of an ethylenic group

between a phenyl ring carrying a p-hydroxyl group and the carboxylate

group, as in p-coumaric acid, has a highly favourable effect on the reducing

properties of the OH group (Rice-Evans et al. 1996).

HCAs are among the most widely distributed phenylpropanoids in the

plant kingdom, with fruits, vegetables, beverages, and cereal being

abundant dietary sources (Nielsen and Sandstrom 2003; Herrmann 1989).

The interest in HCAs is also related to their pharmacological, anti-

mutagenic and anti-carcinogenic activity. In wines, hydroxycinnamic acids

can affect the color, astringency, bitterness, oxidation level, and clarity of

the beverages. The antioxidant properties of HCAs have been demonstrated

with regard to their ability to scavenge radicals generated in aqueous media

to increase the resistance of low density lipoprotein to lipid peroxidation

(Nardini et al. 1995). However, since they occur in very low concentrations,

they require sensitive and selective analytical methods (Santos et al. 2005).

The activity of phenols toward free radical protection is important due to

their scavenging role, related to their ability to react with radicals much more

rapidly than with other organic substrates. The presence of -CH55CH-COOH

groups in cinnamic acids ensures a greater H-donating ability, and subsequent

greater radical stabilization then the carboxylate group in benzoic acids.

(Rice-Evans et al. 1997). Oxidation of some HCAs was studied using

several electrochemical techniques, at different pHs, and with different

electrode materials (Santos et al. 2005; Giacomelli et al. 2002; Trabelsi

et al. 2004; Hapiot et al. 1996). The redox properties of polyphenols have

P. Janeiro et al.3310

Dow

nloa

ded

By:

[B-o

n C

onso

rtium

- 20

07] A

t: 10

:34

6 Fe

brua

ry 2

008

been utilised as a measure of the antioxidant properties of wines on the basis of

the measurement of the oxidation current at a constant potential by HPLC with

electrochemical detection (Mannino et al. 1998). So far, few attempts have

been made to pursue a possible correlation between the oxidation potentials

of antioxidants and their antioxidant activities.

Most phenolic compounds can be electrochemically oxidized due to the

hydroxyl groups attached to the aromatic rings. The dependence of the

oxidation potential, E1/2, on solution pH of phenol derivatives has been

studied thoroughly for different classes of polyphenols using a glassy

carbon electrode (Janeiro and Brett 2004; Janeiro and Brett 2005; Janeiro

et al. 2005; Corduneanu et al. 2006; Janeiro and Brett 2007; Brett and

Ghica 2003; Ghica and Brett 2005). Electrochemical measurements enabled

the determination of the physicochemical parameters of several polyphenols

(e.g., redox potential, number of electrons, etc.). These parameters gave infor-

mation not only for evaluating the antioxidant potentialities of polyphenols

but also for understanding their reaction mechanisms (Hotta et al. 2001).

Cyclic voltammetry has also been used for the evaluation of the antioxidant

capacity of several polyphenols and their mixtures (Chevion et al. 2000).

The p-coumaric acid molecule contains a reducible olefinic bond and oxi-

dizable phenolic substituents on the aromatic ring that could provide the basis

for its voltammetric determination, Scheme 1.

The reduction was studied and is due to the pre-protonated conjugated

ethylene bond and occurs at very negative potenital, 21.5 V (Sousa et al.

2004). The absence of o-hydroxyl substituents in the chemical structure of

p-coumaric acid leads to irreversibility, since the semiquinone radical is not

stabilized in the electrochemical mechanism (Santos et al. 2005).

In this study the mechanism of electrochemical oxidation of p-coumaric

acid was investigated for a wide range of solution conditions, using cyclic. differ-

ential, and square wave voltammetry. The information on the mechanism of

p-coumaric acid oxidation was obtained from results at different pH and is

shown to play a crucial role in understanding its antioxidant activity.

EXPERIMENTAL

Materials and Reagents

The p-coumaric acid was from Sigma–Aldrich, Madrid, Spain and all the

other reagents were Merck analytical grade. The p-coumaric stock solution

Scheme 1. Chemical structure of p-coumaric acid.

Electroanalytical Oxidation of p-Coumaric Acid 3311

Dow

nloa

ded

By:

[B-o

n C

onso

rtium

- 20

07] A

t: 10

:34

6 Fe

brua

ry 2

008

(1.0 � 1022 M) was prepared from the dry pure substance by dissolution in

water. The pH was varied from 3.6 to 12.7 in order to determine the effect

of pH on the peak potential of the main anodic wave for p-coumaric acid.

Solutions of buffer supporting electrolyte of ionic strength 0.2 were used in

all experiments, Table 1. All solutions were made using deionised water

obtained from a Millipore Milli-Q purification system (resistivity � MV cm).

All experiments were carried out a room temperature (ca. 22 + 18C) and in

the presence of dissolved oxygen.

Voltammetric Parameters and Electrochemical Cells

Electrochemical experiments were done using an Autolab PGSTAT 10 runing

with General-Purpose Electrochemical System (GPES) version 4.9 software

(Eco-Chemie, Utrecht, The Netherlands). Voltammetric curves were

recorded using a three-electrode system in a small-volume electrochemical

cell of capacity 2 mL (Cypress System Inc., USA). The working electrode

was a glassy carbon mini-electrode of 1.5 mm diameter; Ag/AgCl

(saturated KCl) was used as a reference electrode, and a platinum wire as

counter electrode. In this work, all potentials are reported versus Ag/AgCl

KCl (sat) electrode.

The glassy carbon working electrode was polished with diamond spray (6

and 1 mm). Cyclic voltammograms were recorded at scan rates of 25, 50, 100

and 500 mV s21. Differential pulse voltammetry conditions used were pulse

amplitude 50 mV, pulse width 70 ms, and scan rate of 5 mV s21. Square

wave voltammetry conditions were frequency 13, 25 and 50 Hz, amplitude

50 mV, and potential increment 2 mV, The effective scan rates were 25, 50

and 100 mV s21 respectively. Voltammetric scans were carried out in the

potential range 0 to þ1.4 V versus Ag/Agcl.

The pH measurements were carried out with a CRISON GLP 21 pH-

meter.

Table 1. Supporting electrolyte buffer solutions

Supporting electrolyte solutions pH

0.2 M NaOAc þ 0.2 M HOAc 3.6

0.2 M NaOAc þ 0.2 M HOAc 4.3

0.2 M NaOAc þ 0.2 M HOAc 5.3

0.2 M Na2HPO4 þ 0.2 M NaH2PO4 7.0

0.2 M Na2HPO4 þ 0.2 M NaH2PO4 8.0

0.2 M NH3 þ 0.2 M NH4Cl 8.7

0.2 M NH3 þ 0.2 M NH4Cl 9.8

0.2 M NH3 þ 0.2 M NH4Cl 10.5

0.2 M NH3 þ 0.2 M NH4Cl 11.0

0.2 M KCl þ 0.2 M NaOH 12.7

P. Janeiro et al.3312

Dow

nloa

ded

By:

[B-o

n C

onso

rtium

- 20

07] A

t: 10

:34

6 Fe

brua

ry 2

008

Acquisition and Presentation of Voltammetric Data

All the voltammograms presented were background-subtracted and baseline-

corrected using a moving average algorithm with a step window of 5 mV

included in GPES version 4.9 software. This mathematical treatment

improves the visualization and identification of peaks over the baseline

without introducing any artifact, although the peak height is in some cases

reduced (,10%) relative to that of the untreated curve. Nevertheless, this

mathematical treatment of the original voltammograms was used in the pres-

entation of all experimental voltammograms for a better and clearer identifi-

cation of the peaks. The values for peak current presented in all graphs

were determined from the original untreated voltammograms after subtraction

of the baseline.

RESULTS AND DISCUSSION

The p-coumaric acid has a functional OH group attached to the ring structure

in a para position to the -CH55CH-COOH group that can be electrochemically

oxidised, Scheme 1. The electrochemical mechanism of oxidation of

p-coumaric acid was investigated in different pH conditions using cyclic,

differential, and square wave voltammetry.

Cyclic Voltammetry

The first cyclic voltammetric scan of p-coumaric acid at pH 3.6 showed the

occurrence of one peak Pl, at Epa ¼ þ0.86 V, which is associated with the

oxidation of the hydroxyl group on the aromatic ring of the molecule,

Fig. 1. The peak Pl is irreversible. The irreversibility of Pl was verified over

the whole pH range investigated in this work.

After successive oxidation scans the p-coumaric acid oxidation product,

Pox, is deposited on the electrode surface forming a polymeric film, as can

be seen in the second scan, which undergoes reversible oxidation at a lower

potential then p-coumaric acid. This polymeric film increases in thickness

with the number of scans covering the electrode surface, and impeding the

diffusion of the p-coumaric acid and its oxidation on the electrode surface.

The oxidation peak potential of Pox occurs at Epa ¼ þ0.46 V, and the

reduction peak at Epc ¼ þ0.38 V, confirming the reversibility of the

oxidation product. The strong adsorption of the oxidation product at all

pHs, covering the electrode surface, causing the p-coumaric acid oxidation

peak to decrease drastically in the second scan for all pH values, was also

observed.

It is known that oxidation product peak, with an oxidation potential lower

than those of the starting material, are commonly due to organic polymers

Electroanalytical Oxidation of p-Coumaric Acid 3313

Dow

nloa

ded

By:

[B-o

n C

onso

rtium

- 20

07] A

t: 10

:34

6 Fe

brua

ry 2

008

formed by oxidation of the respective monomers. The oxidised p-coumaric

acid promotes further film growth, as was confirmed by the increase in the

size of the peak upon consecutive scans. It was found that the Pox polymer

film was a highly reversible, electroactive film.

Differential Pulse Voltammetry

Differential pulse voltammetry of the polymeric film clearly show it increas-

ing with the number of scans, and covering the electrode surface at pH 7.0,

Figure 2 and impeding the diffusion of the p-coumaric acid and its

oxidation on the electrode surface. These results are similar to those

obtained for caffeic and ferulic acid (Hapiot et al. 1996; Kilmartin 2001).

A differential pulse voltammetric study of p-coumaric acid was

performed over a wide pH range from 3.6 to 12.7. Comparing the oxidation

potentials at different pH values provides very important information on the

mechanism of p-coumaric acid oxidation. The oxidation peak P1 was

always observed, and an increase of pH is associated with a decrease of the

oxidation potential until the pH ¼ pKa in phenols pKa ¼ 9.9, and pH indepen-

dent afterwards, Fig. 3A.

The peak P1 potential dependence with pH is linear and follows the

relationship EpaP1 (V) ¼ 0.82 2 0.059 pH, Figure 3B. The slop of the line,

59 mV per pH unit, showed that the oxidation of p-coumaric acid involves

the same number of electrons and protons. The number of electrons trans-

ferred, n, was determined by the peak width at half height, W1/2 � 95 mV,

and is close to the theoretical value of 90 mV, corresponding to an

Figure 1. Cyclic voltammograms of 0.5 mM pcoumaric acid in pH 3.6 0.2 M acetate

buffer: (– – –)1st, (†††) 2nd, and (—) 3rd scans. Scan rate 50 mV s21.

P. Janeiro et al.3314

Dow

nloa

ded

By:

[B-o

n C

onso

rtium

- 20

07] A

t: 10

:34

6 Fe

brua

ry 2

008

electrochemical reaction involving the transfer of one electron (Brett et al.

1993). Thus, it can be concluded that the oxidation of p-coumaric acid

below pH ¼ pKa occurs with the transfer of one electron and one proton.

The same behavior was found for the peak Pox, the peak potential

showing a linear dependence with pH and following the relationship. Epapox

(V) ¼ 0.50 2 0.059 pH, Figure 3B, suggesting that also one electron and

one proton are implicated in this redox process.

It was also found that at pH . pKa � 9.9, the oxidation product peak Pox,

disappeared.

The electrochemical oxidation of phenols, Ep/2, which at pH lower than

pKa are in the molecular form ArOH, depends on the pH of the medium.

An increase of one pH unit causes a decrease of Ep/2 by 59 + 3 mV

(Zuman and Holthuis 1988; Strandis and Hasanli 1993). This slope has

been usually determined for phenolic systems in acqueous media and

involves the loss of one electron followed by deprotonation.

Upon reaching the pH where deprotonation of the phenol group takes

place, Ep/2 no longer depends on pH, and the undissociated phenol form pre-

dominates in the bulk of the solution in this pH range. This is shown in Fig. 4,

by the cyclic voltammograms of p-coumaric acid at different pH values, equal

or higher than the pKa corresponding to an oxidation process characterized by

the absence of a proton transfer. The oxidation peak of p-coumaric acid occurs

at EpaP1 ¼ þ0.48 V, confirming that the oxidation potential is independent of

pH at values higher than pKa. A value of Ep 2 Ep/2 ¼ 0.046 V was found.

Figure 2. Successive differential pulse voltammograms of 0.1 mM of p-coumaric

acid in pH 7.0 phosphate buffer: (—) 1st, (–††–) 3rd, (†††), 5th, (–†–) 7th, and

(– – –)10th scans. Scan rate 5 mV s21.

Electroanalytical Oxidation of p-Coumaric Acid 3315

Dow

nloa

ded

By:

[B-o

n C

onso

rtium

- 20

07] A

t: 10

:34

6 Fe

brua

ry 2

008

This value indicates that one electron is involved in the oxidation reaction.

Such an approach is valid only for irreversible systems; modifications for

reversible and quasi-reversible systems have been described in detail (Brett

and Brett 1993; Brett and Brett 1998).

Square Wave Voltammetry

Square-wave voltammetry has the advantage of a greater speed of analysis,

lower consumption of electroactive species in relation to differential pulse

Figure 3. (A) 3D plot of first scan differential pulse voltammograms of p-coumaric

acid as a function of pH. (B) Plot of Epa vs. pH, († P1) peak 1 of p-coumaric acid and

(O Pox) p-coumaric acid oxidation product peak. Scan rate 5 V s21.

P. Janeiro et al.3316

Dow

nloa

ded

By:

[B-o

n C

onso

rtium

- 20

07] A

t: 10

:34

6 Fe

brua

ry 2

008

voltammetry, and reduced problems regarding the blocking of the electrode

surface.

This technique showed similar results as differential pulse and cyclic vol-

tammetry. The oxidation peak P1 occurred on the first scan, and the oxidation

product peak Pox only appeared after a few scans in p-coumaric acid solution,

Fig. 5. The great advantage of the square wave method is the possibility to see

during one scan if the electron transfer reaction is or is not reversible. Since the

current is sampled in both the positive and the negative-going pulses, peaks

corresponding to the oxidation or reduction of the electroactive species at

the electrode surface can be obtained in the same experiment. The good rever-

sibility of the oxidation product peak Pox, is shown in Fig 5a at pH 3.6 after

four scans. Forward and backward currents are equal and the oxidation and

reduction peaks occur at the same potential, confirming the formation of the

adsorbed Pox polymer film. The irreversibility presented by P1 was

observed over the whole range of pH studied. The square wave voltammetry

conditions chosen, corresponding to an effective scan rate of 50 mV s22, led to

well-defined voltammograms for all pH studied, Fig. 5b, 5c and 5d.

Analytical Determination

An electroanalytical procedure was developed for the determination

p-coumaric acid using differential pulse voltammetry. A supporting

Figure 4. First cyclic voltammograms of 0.5 mM p-coumaric acid in 0.2 M

ammonium chloride buffer pH: (—) 9.8, (– – –) 10.5, and (†††) 11.0 Scan rate

50 mV s21.

Electroanalytical Oxidation of p-Coumaric Acid 3317

Dow

nloa

ded

By:

[B-o

n C

onso

rtium

- 20

07] A

t: 10

:34

6 Fe

brua

ry 2

008

Figure 5. Square wave voltammograms of 0.1 mM p-coumaric acid at: (a) pH 3.6,

scan 4. Frequency 8 Hz, effective scan rate 16 mV s21; (b) pH 4.3, scan 3. Frequency

25 Hz, effective scan rate 50 mV s21; (c) pH 7.0, scan 3. Frequency 25 Hz, effective

scan rate 50 mV s21, (d) pH 8.7, scan 3. Frequency 25 Hz, effective scan rate

50 mV s21.

Figure 6. Differential pulse voltammograms in pH 8.7 0.2 M ammonium buffer of

p-coumaric acid: 3, 8, 10, 30, 50, and 100 mM. Scan rate 5 mV s21.

P. Janeiro et al.3318

Dow

nloa

ded

By:

[B-o

n C

onso

rtium

- 20

07] A

t: 10

:34

6 Fe

brua

ry 2

008

electrolyte, of pH 8.7 0.2 M ammonium buffer, was preferred, since higher

oxidation peaks are obtained, as shown in Figure 3A.

Differential pulse voltammograms of p-coumaric acid, with three

measurements at each concentration, are shown in Fig. 6. A good linearity

was found between peak current (Ipc) vs. and concentration of p-coumaric

acid from 3.0 � 1026 M to 1.0 � 1024 M, described by the equation:

Ipc=mA ¼ �4:38 � 10�3 þ 3:64 � 10�3½ p-coumaric acid�=mM

where R ¼ 0.998, N ¼ 7, P , 0.0001, and SD ¼ 0.091 nM. The parameters

to define the sensitivity were calculated and the value obtained for the limit

of detection, LOD ¼ 83 nM, was based on three times the noise level, and

the limit of quantification was LOQ ¼ 250 nM, based on ten times the noise

level.

CONCLUSIONS

The mechanism of the electrochemical oxidation of p-coumaric acid on a

glassy carbon electrode was investigated using cyclic, differential pulse, and

square wave voltammetry at different pHs. The oxidised p-coumaric acid

forms a polymeric film that can be reversibly oxidised. The p-coumaric acid

oxidation showed a dependence on pH until values close to pKa, with the

same number of electrons and protons involved. At pH values higher than

pKa the oxidation potential value was independent of pH, and the oxidised

p-coumaric acid was not electroactive.

An electroanalytical determination method for p-coumaric acid in pH 8.7

0.2 M ammonium buffer was developed using differential pulse voltammetry.

The detection limit was 83 nM, and the limit of quantification 250 nM.

ACKNOWLEDGMENTS

Financial support from fundacao para a Ciencia e Tecnologia (FCT), Ph.D.

Grant SFRH/BD/28333/2006 (P. Janeiro), POCI 2010 (co-financed by the

European Community Fund FEDER), ICEMS (Research Unit 103), and

Ph.D. Grant (I. Novak) from National Foundation for Science, Education

and Technological Development of the Republic of Croatia, is gratefully

acknowledged.

REFERENCES

Brett, C.M.A. and Oliveira Brett, A.M. 1993. Electrochemistry: Principles, Methodsand Applications; Oxford Science Publications: Oxford.

Electroanalytical Oxidation of p-Coumaric Acid 3319

Dow

nloa

ded

By:

[B-o

n C

onso

rtium

- 20

07] A

t: 10

:34

6 Fe

brua

ry 2

008

Brett, C.M.A. and Oliveira Brett, A.M. 1998. Electroanalysis; Oxford Chemistry

Primers: Oxford.

Brett, A.M.O. and Ghica, M.E. 2003. Electrochemical oxidation of quercetin. Electro-

analysis, 15: 1745–1750.

Chevion, S., Roberts, M.A., and Chevion, M. 2000. The use of cyclic voltammetry for

the evaluation of antioxidant capacity. Free Radical Biol. Med., 6: 860–870.Corduneanu, O., Janeiro, P., and Oliveira Brett, A.M. 2006. Electrochemical study of

resveratrol. Electroanalysis, 18: 757–762.

Ghica, M.E. and Oliveira Brett, A.M. 2005. Electrochemical oxidation of rutin. Elec-

troanalysis, 17: 313–318.

Giacomelli, C., Ckless, K., Galato, D., Miranda, F.S., and Spinelli, A. 2002. Electro-

chemistry of caffeic acid aqueous solutions with pH 2.0 to 8.5. J. Braz. Chem.

Soc., 13: 332–338.

Hapiot, P., Neudeck, A., Pinson, J., Fulcrand, H., Neta, P., and Rolando, C. 1996.

Oxidation of caffeic acid and related hydroxycinnamics acids. J. Electroanal.Chem., 405: 169–176.

Hermann, K. 1989. Occurence and content of hydroxycinnamic and hydroxybenzoic

acid compounds in foods. Crit. Rev. Food Sci. Nutr., 28: 315–347.

Hotta, H., Sakamoto, H., Nagano, S., Osakai, T., and Tsujino, Y. 2001. Unusually large

numbers of electrons for the oxidation of polyphenolic antioxidants. Biochim.

Biophys. Acta, 1526: 159–167.

Janeiro, P. and Oliveira Brett, A.M. 2004. Catechin electrochemical oxidation mechan-

isms. Anal. Chim. Acta, 518: 109–115.

Janeiro, P. and Oliveira Brett, A.M. 2005. Solid state electrochemical oxidationmechanism of morin in aqueous media. Electroanalysis, 17: 733–738.

Janeiro, P. and Oliveira Brett, A.M. 2007. Redox behaviour of anthocyanins. Electro-

analysis, 19: 1779–1786.

Janeiro, P., Corduneanu, O., and Oliveira Brett, A.M. 2005. Chrysin and (+)-taxifolin

electrochemical oxidation mechanisms. Electroanalysis, 17: 1059–1064.

Kilmartin, P.A. 2001. Electrochemical detection o natural antioxidants: priniciples and

protocols. Antioxidants & Redox Signalling, 3: 941–955.

Mannino, S., Brenna, O., Buratti, S., and Cosio, M.S. 1998. A new method for the

evaluation of the “antioxidant power” of wines. Electroanalysis, 10: 908–912.Nardini, M., D’Aquino, M., Tomassi, G., Gentili, V., Di Felice, M., and Scaccini, C.

1995. Inhibition of human low density lipoprotein oxidation by caffeic acid and

other hydroxycinnamic derivatives. Free Radic. Biol. Med., 19: 541–552.

Nielsen, S.E. and Sandstrom, B. 2003. Simultaneous determination of hydroxycinna-

mates and catechins in human urine samples by column switching liquid chromato-

graphy coupled to atmospheric pressure chemical ionzation mass spectometry.

J. Chromat. B, 787: 369–379.

Rice-Evans, C.A., Miller, N.J., and Paganaga, G. 1996. Structure-antioxidant activity

relationships of flavonoids and phenolic acids. Free Radical Biol. Med., 20:

933–956.Rice-Evans, C.A., Miller, N.J., and Paganaga, G. 1997. Antioxidant properties of

phenolic compounds. Trends Plant Sci., 2: 152–159.

Santos, D.P., Bergamini, M.F., Fogg, A.G., and Zanoni, M.V.B. 2005. Application of a

glassy carbon electrode modified with poly (glutamic acid) in caffeic acid determi-

nation. Microchim. Acta, 151: 127–134.

Sousa, W.R., Rocha, C., Cardoso, C.L., Silva, D.H.S., and Zanoni, M.V.B. 2004.

Determination of the relative contribution of phenolic antioxidants in orange juice

by voltammetric methods. J. Food Composition and Anal., 17: 619–633.

P. Janeiro et al.3320

Dow

nloa

ded

By:

[B-o

n C

onso

rtium

- 20

07] A

t: 10

:34

6 Fe

brua

ry 2

008

Stradins, J. and Hasanli, B. 1993. Part 1. Influence of pH on electro-oxidation potentialsof substituted phenols and evaluation of pKa from anodic voltammetry data.J. Electroanal. Chem., 353: 57–69.

Trabelsi, S.K., Tahar, N.B., and Abdelhedi, R. 2004. Electrochemical behaviour ofcaffeic acid. Electrochim. Acta, 49: 1647–1654.

Zuman, P. and Holthuis, J.J.M. 1988. Mechanism of electrooxidation of substitutedphenols in aqueous solutions: some podophyllotoxin derivatives as models. Recl.Chim. Pays-Bas, 107: 403–406.

Electroanalytical Oxidation of p-Coumaric Acid 3321