Research Article Voltammetry Study of an Anti-HIV...

Hindawi Publishing CorporationInternational Journal of ElectrochemistryVolume 2013 Article ID 902872 8 pageshttpdxdoiorg1011552013902872

Research ArticleVoltammetry Study of an Anti-HIV Compoundby means of a Thin Organic Membrane

Achille Nassi12 Thiery Christophe Ebelle12 Joseph M Dika Manga2

Jules-Blaise Mabou Leuna1 Joel Donkeng Dazie1 and Emmanuel Ngameni1

1 Laboratoire de Chimie Analytique Faculte des Sciences Universite de Yaounde 1 BP 812 Yaounde Cameroon2 Laboratoire de Chimie Minerale Faculte des Sciences Universite de Douala BP 24157 Douala Cameroon

Correspondence should be addressed toThiery Christophe Ebelle thiercebelyahoofr

Received 20 June 2013 Revised 17 August 2013 Accepted 20 August 2013

Academic Editor Angela Molina

Copyright copy 2013 Achille Nassi 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

Cyclic and square wave voltammetries have been used to study electrochemical behaviour of an anti-HIV agent (Guttiferone A)at the liquid-liquid interface The thin organic membrane is formed by an organic solvent containing redox probe Guttiferone Aa benzophenone (BP) with appropriate electrolyte It is demonstrated that BP possesses three reduction systems due to the redoxtransformation of the three tautomeric forms that lead to the migration of proton between the hydroxyl group in position 4 and thecarbonyl group in positions 2 and 10 The transfer of proton from the aqueous solution to the organic phase is crucial for the redoxtransformation of BP into the organic membrane The voltammograms obtained are strongly influenced by the pH of the aqueousphase The electrochemical mechanism consists of 2eminus2H+ exchange to form the separate redox compound BPH

2

1 Introduction

Numerous benzophenones reported in the literature areknown to possess various biological activities [1] Their anti-microbial activities are due to their ability to act as (i) potentinhibitors in electron transport [2] (ii) amodel for the reduc-tion of aromatic ketones especially in an aqueous solution [34] where the main product is benzopinacole and (iii) media-tor in the biosynthesis of a variety of polyisoprenylated ben-zophenones [5ndash8] a class of compounds which is not onlychemically interesting due to their structurally complex fea-tures but also pharmacologically valuable

More specifically Guttiferone A (Figure 1) and its ana-logues are known to possess antioxidant [9] cytotoxic [1011] and cancer chemopreventive [12] properties Recentresults revealed that Guttiferone A possesses different bio-logical properties such as the cytoprotection against HIV-1 invitro [6 13] Most of the studies concerning Guttiferone aredevoted to only biological aspects The electrochemistry ofGuttiferone is less described [14] Electrochemistry is devotedto understanding the reactivity of molecules through thestudy of the changes in their properties during electron

transfer processes Inmany other research areas such as phar-macology these relationships are important since they pro-vide the basis for intelligent design of new drugs or treat-ments for specific diseases The areas of research related tomolecular biochemical and analytical electrochemistry forthe analysis of reaction mechanisms and structure-reactivityrelationships are of great importance in elucidating thehuman bodyrsquos metabolic processing drugs and of the effect ofdrugs on the body [15ndash17]

In this work our purpose is to describe redox propertiesof Guttiferone A using thinfilm electrodes in connectionwith voltammetric techniques Modified electrodes are oftenconstituted of a solid electrode covered with an organic phasewhich contains a compound having redox properties Suchelectrodes are used usually in contact with aqueous phasesfor various reasons such as the study of the redox substancepresent in the organic medium when its insolubility in waterdoes not allow a direct study in an aqueous environmentThinfilm has same properties as an artificial organic-filmmembrane Electrochemical techniques are very well suitedfor characterizing benzophenone-like compounds As mostof the physiologically active benzophenones are lipophilic

2 International Journal of Electrochemistry

HO

OH

O OH

O O

R43

21

8 7

6

5

9

101113

1415

R =

Figure 1 Molecular structure of Guttiferone A (BP)

electrochemical methods in nonaqueous medium have beendeveloped Particularly important are the biomimetic studiesin which lipophilic benzophenones are embedded in a lipidmembrane support on the electrode surface Liposomes arealso suitable for membrane immobilization of lipophilic ben-zophenone In this context we recently incorporated Lapa-chol to build an artificial light-driven transmembrane cal-cium pump and explain a new reduction mechanism ofLapachol in thinfilm electrode [15] The aim of the work is toprovide an insight in the redox chemistry of GutifferoneA animportant natural product embedded in an artificial mem-brane using the model developed by Shi and Anson [18 19]

The device used by Shi is derived from the classical three-electrode configuration depicted in Figure 2 the workingelectrode is covered with a thin film of organic solvent previ-ously saturatedwithwater inwhich themodel compound andan appropriate electrolyte are dissolvedThe electrode is thenimmersed in an aqueous electrolyte The resulting devicebehaves like a stable liquid interface consisting of an organicmembrane in contact with an aqueous phase

The electrochemical experiments have been conductedwith conventional cyclic (CV) and square wave voltammetry(SWV) techniques The advantages of square wave voltam-metry are a higher speed of analysis a lower consumptionof electroactive species and reduction of problems related tothe inhibition of the electrode surface However SWV is oneof the most advanced voltammetry methods unifying theadvantages of CV and pulse voltammetry techniques [20]

2 Experimental

21 Chemicals and Reagent Guttiferone A (BP) was obtainedfrom CH

2

Cl2

ndashMeOH (1 1) extract of Symphonia globuliferausing a procedure described by Ngouela and coworkers [21]The other chemicals were of high purity (99) and wereobtained from Reidel de Haen and Merck BP was dissolvedin a water-saturated nitrobenzene (NB) mixture containing01M tetrabuthylammonium perchlorate (Bu

4

NClO4

) as anorganic electrolyte

Aqueous electrolyte

Edge plane pyrolytic graphite electrode

BP

Organic solvent

BPH2

Counter electrodeReference electrode

2H+

2H+

+2e

Figure 2 Plane edge pyrolytic graphite electrode covered with amicro film of electroinactive water immiscible organic solvent con-taining a neutral redox probe and an organic electrolyte immersedin an aqueous phase-buffered solution containing a common ionwith the organic electrolyte The modified electrode is used in aconventional three-electrode configuration

22 Preparation of the Film The modification and pretreat-ment of the working electrode were described elsewhere [15]as follow the organic solution (1 120583L) was deposited on thegraphite electrode with a micropipette the organic solutionspreads spontaneously over the electrode surface forming astable film The so-modified electrode was immersed in aphosphate buffer solution (01MK

2

HPO4

+ 01MKH2

PO4

)When necessary pH values were adjusted by the addition ofcitric acid or hydroxide sodiumTheaqueous phase ismade ofa 01MLiClO

4

solution NB-saturated water (purities select)was used throughout

23 Electrochemical Cell and Measurements As shown inFigure 2 a three-electrode system was used with a diskelectrode (032 cm2) of edge plane highly oriented pyrolyticgraphite as a working electrodeThe reference electrode was aSaturated Calomel Electrode (SCE) and the counter elec-trode was a platinum wire Prior to each experiment solu-tions were deaerated thoroughly for at least 20min with purenitrogen A positive pressure of this gas was maintained dur-ing subsequentwork Electrochemical data (cyclic voltamme-try and SWV) were obtained using an Autolab Potentiostat(Eco-Chemie Netherlands) driven by a PC with GPES elec-trochemical analysis software

3 Results and Discussion

Before starting SWV measurements the response of theorganic film electrodes was thoroughly inspected by cyclicvoltammetry in order to check the reproducibility and sta-bility of the different forms of BP during successive poten-tial cycling Figure 3(b) shows typical cyclic voltammogramobtained between 00 and minus19 V and corresponding to theelectrolysis of the BP present in the organic filmmembrane incontact with an aqueous buffer solution at pH 1 During suc-cessive cycles figure not shown these CVs reflect the stabilityof the membrane and transformation of BP

The CV consists of two distinct peaks (I-I1015840 and II) whichare not very sensitive to the potential scan rateThe peak pairII1015840 is well developed and quasireversible whereas the secondII located at a more negative potential (Δ119864 = minus13V)

International Journal of Electrochemistry 3

EVSCE

minus30

minus20

minus10

minus40

00

minus12 minus08 minus04minus16 00

(a)

(b)

I(m

A)

Figure 3 Cyclic voltammograms recorded with a (a) thin film-electrode without BP (b) the thinfilm electrode containing 20mMof BP in contact with 01MLiClO

4

aqueous solution buffered witha 01molL phosphate buffer at pH = 1 Besides the redox probethe organic phase contains 01M tetrabuthylammonium perchlorate(Bu4

NClO4

) as an organic electrolyte Scan rate was V = 20mVs

is irreversible Both redox systems have previously beenobserved by classic cyclic voltammetry in aprotic medium orionic liquid for other benzophenone [22ndash24] The reductionof benzophenone in ionic liquid presents two reversible well-resolved one-electron reductions which were observed in dryliquid ionic which did not contain any readily available pro-ton source Upon addition of water or proton the second pro-cess became chemically irreversible and shifted to amore pos-itive potential by approximately 600mV moreover the tworeduction processes merged into a single two-electron pro-ton-coupled process when a small among of proton is avail-able in the media This large dependence of potential on pro-ton content which was not observed in molecular solventswas explained by a reaction mechanism that incorporatedprotonation and hydrogen-bonding interaction of the ben-zophenone dianion with as many water molecules as possible[23]

It is well known that the scan rate can have a dramaticinfluence on peak separationThis is the case with couple I-I1015840At low scan rate V = 10mVsminus1 the peak potential separationsare 80mV and 90mVwhen the pHs of aqueous solution are 1and 2 respectively these values are not so far from the theo-retical value of 60mV tabulated for a reversible one-electronprocess An increase of the scan rate up to V = 30mVsminus1 wasaccompanied by an increase of the separation of the coupleI-I1015840 which is typical for a quasireversible electrode processAs a matter of fact the transfer energy of the proton from theaqueous phase to the organic phase is too high (325 kJsdotmolminus1)[25] so that a slow scanning allows enough time to the pro-tons to diffuse towards the surface of the electrode

The evolution of CVs as functions of the scan rate whenthe pH is maintained constant is shown in Figure 4 In addi-tion the cathodic peaks I and II shifted towards more nega-tive potentials with increasing scan rateThe inset of Figure 4shows the variation of the scan rate-normalised cathodic peakcurrent 119868

119901119888

V versus V for cathodic peak IIThe ratio 119868119901

V forcathodic peak decreases severely with an increasing scan rateindicating a CEmechanism in which the electrode reaction is

minus20 minus18 minus16 minus14 minus12 minus10 minus08 minus06 minus04 minus02 00

minus50

minus40

minus30

minus20

minus10

00

10

60 mVs

10

II

I

I(m

A)

EVSCE

10 20 30 40 50

30405060708090

(mVmiddotSminus1)

I pcVminus1

(120583A

mVminus

1middotS

)

I998400

Figure 4 Effect of the scan rate on the cyclic voltammogramsrecorded with a thinfilm electrode in contact with an aqueousmedium at pH = 1 The scan rate was V = 10 to 60mVsdotsminus1 The insetshows the variation of the ratio 119868

119901119888

V on the scan rate for the peaksII

gated by a slow preceding chemical step [15 26]These resultssuggest that the two systems are not similar At this level wecannot establish the fact that the system II1015840 exchanges oneelectron In order to shed more light on these systems andgather more information (eg the number of the electronsexchanged) some few experiments were done using squarewave voltammetry a more sensitive method especially whenone deals with membrane processes [26ndash30] The SWVtechnique is one of themost advanced voltammetricmethodsunifying the advantages of CV and pulse voltammetric tech-niques In the course of a SW voltammetric experiment thepotential is repeatedly changed to an oxidative and reductivemode in a form of square-shaped potential pulses [31 32]Figure 5(a) compares typical CV and SWV recorded in anNB membrane in contact with an aqueous solution of pH 1Unlike the CV SWV reveals a third system noted as III TheSWV response of BP in the NB in contact with an aqueoussolution at pH 1 consists of three well-defined and separatedreductive peaks (Figure 5(b)) The three reductive peaks arewell developed and separated The processes II1015840 and IIIIII1015840are clearly quasireversible while the peak II reflects a totallyirreversible electrochemical process in both electrochemicalmethods as indicated in Figure 5 The relative heights varyproportionally to the number of repetitive scans It can beobserved that in the course of these repetitive scans the mag-nitude of peak II decreases over time whereas peaks I and IIIincrease in the same proportion Figure 6 shows that theintensities of both systems evolve in opposite directions Ifthe electrode was kept for a certain period in contact with theaqueous solution under open circuit conditions or at the ini-tial potential value the duration of contact seems to have noeffect on the shape of voltammograms as the position of thetautomerism equilibriumwas predominantly fixed by the pHof the medium Table 1 clearly shows that when the electrodeis in contact with the aqueous phase the intensities of peaks Iand III increase with resting time The decrease in the peak


minus22 minus20 minus18 minus16 minus14 minus12 minus10 minus08 minus04 minus02 00 12minus50

minus40

minus30

minus20

minus10

00

I

II

III

EVSCE

SWV

CV

I(m

A)

Inet

(a)

minus18 minus16 minus14 minus12 minus10 minus08 minus06 minus04 minus02 00 02minus80

minus70

minus60

minus50

minus40

minus30

minus20

minus10

00

III

II

I

I(m

A)

EVSCE

Inet

Iback

Ifor

(b)

Figure 5 (a) Comparison of typical CV and SW voltammograms of 20mM of BP recorded in NB membrane in contact with an aqueousphase consisting of a buffer at pH 1 and (b) Net SW voltammograms recorded with a the thinfilm electrode in contact with aqueous mediumat pH = 1 For CV the sweep rate is V = 20mVsdotsminus1 and for SWV the frequency is 119891 = 12Hz SW amplitude 119864sw = 50mV and step potentialdE = 1mV

minus20 minus16 minus12 minus08 minus04 00

minus30

minus20

minus10

00

I

II

III

I(m

A)

EVSCE

Figure 6 Consecutive net SWV voltammograms recorded with athinfilm electrode in contact with aqueous medium at pH = 1 Theother conditions are the same as for Figure 5

current of system II reveals that the species that gave riseto this system is unstable Contrary two this the increaseof intensities of systems II1015840 and IIIIII1015840 show that thesespecies are stable and similar This indeed tends to showthat the concentration of molecular forms 1 and 3 (Scheme 1)increases in the membrane By contrast peak II decreases inthe same time showing that molecular form 2 is more stablein the membrane than molecular forms 1 and 3 If the elec-trode was scanned starting at a potential located between theredox systems I and II (ie 119864rest = minus120V) the magnitudeof peak II remains unchanged no matter the duration of thecontact between the electrode and the aqueous solution Inaddition it appears that the length of the electrode at thispotential does not affect the signal of the peak II (Figure 7)

The behaviour of this systemdoes not changewith the restric-tion of the potential Indicating that the electroactive speciesthat generates the system II is not obtained after the pro-cessing of the process I All these systems are affected by therepetition of the scan and the immersion time of the electrodein the aqueous phase It thus appears that the electroactivespecies that gives rise to process II is not generated duringthe electrode process I (Figure 7) On the other hand it isobserved that the response of BP in the NB membrane isinfluenced by the pH of the aqueous phase Figure 8 showstypical SWVs at different pHs The relative heights andpositions of the voltammetric peaks are strongly sensitive tothe pHThe evolution of the potential of each of the cathodicpeaks I II and III as function of the pH varies linearly (datanot shown) with a slope of about 60mVpH it is then clearthat the redox processes I II and III involve an overall2eminus2H+ exchange which is typical for BPBPH

2

redoxcouple The process II exhibits typical characteristics of a CEmechanism in which the electroactive reactant is supplied bya preceding chemical reaction This is confirmed when thescan rate in CV and frequency SWV are varied for experi-mental arrangements The overall voltammetric behavior ofthe studied compound strongly deviates from the commonbehavior of the BPBPH

2

redox couple [7 32] During thethinfilm experiment the reduction of the BPmust be accom-pagnied by an ion-transfer reaction across the organic sol-vent-water interface to maintain the chargersquos neutrality in thethin film The proton transfer is a charge-compensating ion-transfer reaction The partition concentration of protons inthe organic phase depends on Δ120601SW as follows

[H+]S = [H+

]W exp [minus 119865119877119879(Δ120601

SW minus Δ

Wrarr S120601SH+)] (1)

Δ120601SW is the Galvani potential difference at the membrane-

water interface We see that reduction the of BP under theseconditions will be made at a less reductive potential


OHO O

O

HO

OHR

O

OH

O

OHO

OH

R

O O

HO

O

HO

OHR

K3

K2

K1

3

1

2

Scheme 1

minus15 minus14 minus13 minus12 minus11 minus12

minus40

minus30

minus20

minus10

00

II

I(m

A)

EVSCE

Figure 7 Effect of the resting potential on the net SW peak IIrecorded with a thinfilm electrode in contact with an aqueoussolution at pH = 1 The resting potential was minus03 V The timeincreases in the direction of the arrow from 0 25 50 75 to 100 sThe other conditions are the same as for Figure 5

Moreover the results presented in Figure 7 show that theelectroactive reactant for the reductive process II is not gener-ated in the previous electrochemical processes I and III Thisexcludes the possibility to explain the three reductive pro-cesses on the basis of three one-electron consecutive pro-cesses with a formation of stable radical-anion intermediate(BP∙minus) Furthermore a careful inspection of the voltammetriccurves in Figure 7 reveals that the current due to the secondreductive process II slightly decreases with time Thereforethe three electrochemical processes originate from redoxtransformations of three distinctive forms of BP molecule

Table 1 Variation of intensity with the time of immersion of theelectrode in aqueous phase at pH 15 varied from 0 to 120 s with astep of 30 s Each voltammogram was recorded by imposing a newfilm on the electrode surface

1198681

(120583A) 1198682

(120583A) 1198683

(120583A)t = 0 s 342 2440 414t = 30 s 400 2430 617t = 60 s 408 2420 658t = 90 s 436 2350 680t = 120 s 461 2280 686

The interrelation of the peak magnitudes implies that thethree redox forms are interconnected by equilibrium reac-tion In addition the stability of the systems I and III showsthat they have a comparable nature To check this assertionby repetition of the scan we recorded the curve of Figure 9in the range potentials minus02 and minus10 VThe repetition of scanshows that the intensity of system III grows while system Idecreases This suggests that the species which produce theseprocesses are independent The stability of process III resultsin the split of system I and confirms the presence of anothertautomeric form of BP

In the light of the preceding observation and consideringthe molecular structure of BP (Figure 1) it is reasonable topostulate that the intriguing voltammetric characteristic ofthe present compound originates from the influence of thehydroxyl and carbonyl groups The carbonyl group in posi-tions 2 and 10 can establish tautomerism equilibriumwith theadjacent hydroxyl group in position 4 (Scheme 1) This wasdescribed by Martins et al [33] who say that the tautomersof Gutifferone in general exist in a solution in a condition ofequilibrium


OHO O O

OOHR

3

H OHO O O

OOHR

H OHO O

O

O

OHR

H

∙∙

∙∙

minus

+

minus

+

minus ∙minus+

minus

+

1

O

OH

O

O

OOH

R

H

OOH

O O

O OHR

H OOH

O O

O OHR

H

Scheme 2

minus12 minus04 minus02

minus50

minus40

minus30

minus20

minus10

00

(c)

(a)

(b)I(m

A)

EVSCE

Figure 8 SquareWave Voltammetry at thinfilm electrode recordedin contact with aqueous medium at pH = 046 (a) 076 (b) and 100(c) The parameters of the potential modulation are the same as forFigure 5

The position of the tautomerism equilibrium K1

dependscritically on the pH of the medium This equilibrium isshifted toward the form 2 when the pH decreases which isin agreement with the known acidic catalysis of the keton-ol transformation The slow establishment of the tautomerequilibrium explains the evolution of the squarewave voltam-mograms under the repetitive cycling of the SWV poten-tial modulation (Figure 6) Mesomerrism and intramolecu-lar hydrogen-bonding can explain the stability of differenttautomeric forms (Scheme 2) It is well known that intramo-lecular hydrogen-bonding causes the shift of the reductionpeak potential towards less negative values [23 34]Then thereduction of the compounds 1 and 3 yields the same reductionproduct

We see from Scheme 2 that the mesomeric form 3 islonger than mesomeric form 1 It is well know that the mole-cule is more stable when its mesomeric is too long This con-firms the different tautomeric forms obtained

minus04 minus08 minus06 minus04 minus02 00 02

minus07

minus06

minus05

minus04

minus03

minus02

IIII

I(m

A)

EVSCE

Figure 9 Consecutive net SWV voltammograms recorded with athinfilm electrode in contact with aqueous medium at pH = 1 Theparameters of the potential modulation are the same as for Figure 5

The three tautomeric forms can undergo independentredox transformations to yield a hydroxyl form according toScheme 3

Reaction (K4

) is the common redox transformation ofthe ketonalcohol redox couple involving an overall 2eminus2H+exchange assigned to the reduction process I The chemicalequilibriums K

2

and K1

coupled to the redox reactions K5

and K6

complete the CE mechanism scheme thus beingattributed to the reduction processes II and III For thethinfilm experiment the situation is particularly complex asthe 2eminus2H+ redox transformation in the membrane must beaccompanied by a corresponding ion-transfer reaction acrossthe membranewater interface to maintain the charge neu-trality of the membrane If the proton transfer is the chargecompensating ion-transfer reaction the reaction K

4

shouldbe written as follows

BP(NB) + 2H

+

(W) + 2eminus

999448999471 BPH2(NB) (2)


HO

HO

HOHO HO

HO

OH

OH

OH

OH

O

O O

OH

O

O

O

O

O

O

R

R

R

HO

HO

OH

OH

OH

OH

OH

OH

OH

OH

O

O

O

O

O

O

R

R

R

+ 2eminus + 2H+

+ 2eminus + 2H+

+ 2eminus + 2H+

(K4)

(K5)

(K6)

Scheme 3

This overall reaction comprises the 2eminus2H+ redox transfor-mation in the membrane along with the transfer of protonsinto the membrane

4 Conclusion

The electrochemical study of a Gutifferone A (BP) revealedon the voltammogram the presence of three successive pro-cesses attributed to tautomerism reactions Detailed voltam-metric analysis of these signals reveals the structure of thechemical compound bearing the hydroxyl-moiety in thereduction mechanism These differences are determined bythe stability of intermolecular hydrogen-bondingThe resultspresented here are the first example of the use of the highlysensitive square wave voltammetry technique for the electro-chemical study of Gutifferone A and it is shown that thismethod can be used for the analytical determination of thisclass of natural compoundsWe found that carbonyl group inBP can be reduced to a hydroxyl form through 2eminus2H+ redoxpathway coupled with the transfer of protons BP undergoesthree distinct reduction processes with each yielding a differ-ent form of BPH

2

The three redox processes are assigned tothree tautomer forms of BP formed by migration of a proton

between the hydroxyl group in the position 3 and the adjacentcarbonyl group in positions 1 and 10

Acknowledgments

This work was done with the support of AIRES-Sud a pro-gramme of the French Ministry of Foreign and EuropeanAffairs implemented by the Institut de Recherche pour leDeveloppement (IRD-DSF) The authors also acknowledgethe support of the Academy of Science for the DevelopingWorld (Grant no 07-052-LDCCHEAFAC allowed to ENgamenIrsquos TWAS Research Unit) We thank Bruno Lenta(Departement de Chimie Ecole Normale Superieure deYaounde (Cameroon)) for the kind gift of Gutifferone A

References

[1] J Lokvam J F Braddock P B Reichardt and T P ClausenldquoTwo polyisoprenylated benzophenones from the trunk latex ofClusia grandiflora (Clusiaceae)rdquo Phytochemistry vol 55 no 1pp 29ndash34 2000

[2] B Batanero C M Sanchez-Sanchez V Montiel A Aldaz andF Barba ldquoElectrochemical synthesis of 3-phenylcinnamonitrileby reduction of benzophenone in acetonitrilerdquo ElectrochemistryCommunications vol 5 no 4 pp 349ndash353 2003


[3] P J Elving and J T Leone ldquoMechanism of the electrochemicalreduction of phenyl ketonesrdquo Journal of the American ChemicalSociety vol 80 no 5 pp 1021ndash1029 1958

[4] M Suzuki and P J Elving ldquoKinetics and mechanism for theelectrochemical reduction of benzophenone in acidic mediardquoJournal of Physical Chemistry vol 65 no 3 pp 391ndash398 1961

[5] W Hamed S Brajeul F Mahuteau-Betzer et al ldquoOblongifolinsA-D polyprenylated benzoylphloroglucinol derivatives fromGarcinia oblongifoliardquo Journal of Natural Products vol 69 no 5pp 774ndash777 2006

[6] K R Gustafson J W Blunt M H G Munro et al ldquoThe guttif-erones HIV-inhibitory benzophenones from Symphonia globu-lifera Garcinia livingstonei Garcinia ovalifolia andClusia roseardquoTetrahedron vol 48 no 46 pp 10093ndash10102 1992

[7] N G Tsierkezos ldquoInvestigation of the electrochemical reduc-tion of benzophenone in aprotic solvents using the method ofcyclic voltammetryrdquo Journal of Solution Chemistry vol 36 no10 pp 1301ndash1310 2007

[8] J J Magadula M C Kapingu M Bezabih and B M AbegazldquoPolyisoprenylated benzophenones fromGarcinia semseii (Clu-siaceae)rdquo Phytochemistry Letters vol 1 no 4 pp 215ndash218 2008

[9] S Sang C-H Liao M-H Pan et al ldquoChemical studies onantioxidant mechanism of garcinol analysis of radical reactionproducts of garcinol with peroxyl radicals and their antitumoractivitiesrdquo Tetrahedron vol 58 no 51 pp 10095ndash10102 2002

[10] K Matsumoto Y Akao E Kobayashi et al ldquoCytotoxic ben-zophenone derivatives from Garcinia species display a strongapoptosis-inducing effect against human leukemia cell linesrdquoBiological amp Pharmaceutical Bulletin vol 26 no 4 pp 569ndash5712003

[11] S Baggett P Protiva E P Mazzola et al ldquoBioactive benzophe-nones from Garcinia xanthochymus fruitsrdquo Journal of NaturalProducts vol 68 no 3 pp 354ndash360 2005

[12] C Ito Y Miyamoto M Nakayama Y Kawai K S Rao and HFurukawa ldquoA novel depsidone and some new xanthones fromGarcinia speciesrdquo Chemical and Pharmaceutical Bulletin vol45 no 9 pp 1403ndash1413 1997

[13] H RW Dharmaratne G T Tan G P K Marasinghe and J MPezzuto ldquoInhibition of HIV-1 reverse transcriptase and HIV-1replication by Calophyllum coumarins and xanthonesrdquo PlantaMedica vol 68 no 1 pp 86ndash87 2002

[14] A A Isse A Galia C Belfiore G Silvestri and A GennaroldquoElectrochemical reduction and carboxylation of haloben-zophenonesrdquo Journal of Electroanalytical Chemistry vol 526no 1-2 pp 41ndash52 2002

[15] C T Ebelle A Nassi E Njanja and E Ngameni ldquoCharacteriza-tion of lapachol in artificial organic-filmmembrane applicationfor the trans-membrane transport of Mg2+rdquo Journal of Electro-analytical Chemistry vol 642 no 1 pp 61ndash68 2010

[16] D CM FerreiraMO F Goulart I Tapsoba S Arbault andCAmatore ldquoElectrochemicalstudy of pharmacological activity atsingle cells beta-lapachone effect on oxidative stress of macro-phagesrdquo ECS Transactions vol 3 pp 3ndash11 2007

[17] C Frontana and I Gonzalez ldquoStructural factors affecting thereactivity of natural a-hydroxyquinones an electrochemicaland ESR studyrdquo ECS Transactions vol 3 pp 13ndash23 2007

[18] C Shi and F C Anson ldquoSimple electrochemical procedure formeasuring the rates of electron transfer across liquidliquidinterfaces formed by coating graphite electrodeswith thin layersof nitrobenzenerdquo Journal of Physical Chemistry B vol 102 no49 pp 9850ndash9854 1998

[19] C Shi and F C Anson ldquoElectron transfer between reactantslocated on opposite sides of liquidliquid interfacesrdquo Journal ofPhysical Chemistry B vol 103 no 30 pp 6283ndash6289 1999

[20] A M O Brett and M-E Ghica ldquoElectrochemical oxidation ofquercetinrdquo Electroanalysis vol 15 no 22 pp 1745ndash1750 2003

[21] S Ngouela B N Lenta D T Noungoue et al ldquoAnti-plasmodialand antioxidant activities of constituents of the seed shells ofSymphonia globulifera Linn frdquo Phytochemistry vol 67 no 3 pp302ndash306 2006

[22] S-F Zhao J-X Lu A M Bond and J Zhang ldquoVoltammet-ric studies in ldquowetrdquo 1-butyl-1-methylpyrrolidinium bis(trifluor-omethylsulfonyl)imide ionic liquid using electrodes withadheredmicroparticlesrdquo Electrochemistry Communications vol16 no 1 pp 14ndash18 2012

[23] S-F Zhao J-X Lu A M Bond and J Zhang ldquoRemarkablesensitivity of the electrochemical reduction of benzophenone toproton availability in ionic liquidsrdquoChemistry vol 18 no 17 pp5290ndash5301 2012

[24] H Li F Gao C Wang J Wang and S Zhang ldquoMoleculargeometry optimization two-photon absorption and electro-chemistry of new diphenylethylene derivatives linking withbenzophenone moiety through ether covalent bondrdquo Journal ofFluorescence vol 21 no 1 pp 327ndash338 2011

[25] H H Girault and D J Schiffrin ldquoElectrochemistry of liquid-liquid interfacesrdquo in Electroanalytical Chemistry A J Bard Edvol 15 of A series of Advances pp 1ndash132 Marcel Dekker NewYork NY USA 1989

[26] V Mirceski R Gulaboski I Bogeski and M Hoth ldquoRedoxchemistry of Ca-transporter 2-palmitoylhydroquinone in anartificial thin organic film membranerdquo Journal of PhysicalChemistry C vol 111 no 16 pp 6068ndash6076 2007

[27] F Marken A Neudeck and A M Bond ldquoCyclic voltammetryrdquoin Electroanalytical Methods F Scholz Ed pp 50ndash97 SpringerBerlin Germany 2002

[28] VMirceski ldquoCharge transfer kinetics in thin-film voltammetryTheoretical study under conditions of square-wave voltamme-tryrdquo Journal of Physical Chemistry B vol 108 no 36 pp 13719ndash13725 2004

[29] A Nassi C T Ebelle E Njanja and E Ngameni ldquoThin-film voltammetry of a lutetium bisphthalocyanine at ionic liq-uidwater interfacerdquo Electroanalysis vol 23 no 2 pp 424ndash4322011

[30] A Sivakumar S J Reddy and V R Krishnan ldquoVoltammetricstudy of p-methylbenzophenone in aqueous mediardquo Journal ofthe Electrochemical Society of India vol 33 no 2 pp 121ndash1231984

[31] M Lovric ldquoSquare wave voltammetryrdquo in ElectroanalyticalMethods F Scholz Ed Springer Berlin Germany 2001

[32] V Mirceski S Komorsky-Lovric Lovric and M Scholz FSquare-Wave Voltammetry Theory and Application SpringerBerlin Germany 2008

[33] F T Martins J W Cruz Jr P B M C Derogis et al ldquoNaturalpolyprenylated benzophenones keto-enol tautomerism andstereochemistryrdquo Journal of the Brazilian Chemical Society vol18 no 8 pp 1515ndash1523 2007

[34] A Ashnagar J M Bruce P L Dutton and R C Prince ldquoOne-and two-electron reduction of hydroxy-14-naphthoquinonesand hydroxy-910-anthraquinones The role of internal hydro-gen bonding and its bearing on the redox chemistry of theanthracycline antitumour quinonesrdquo Biochimica et BiophysicaActa vol 801 no 3 pp 351ndash359 1984

Submit your manuscripts athttpwwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Inorganic ChemistryInternational Journal of

Hindawi Publishing Corporation httpwwwhindawicom Volume 2014

International Journal ofPhotoenergy


Carbohydrate Chemistry

International Journal of


Journal of

Chemistry


Advances in

Physical Chemistry

Hindawi Publishing Corporationhttpwwwhindawicom

Analytical Methods in Chemistry

Journal of

Volume 2014

Bioinorganic Chemistry and ApplicationsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

SpectroscopyInternational Journal of


The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Medicinal ChemistryInternational Journal of


Chromatography Research International


Applied ChemistryJournal of



Theoretical ChemistryJournal of


Journal of

Spectroscopy

Analytical ChemistryInternational Journal of


Journal of


Quantum Chemistry


Organic Chemistry International

ElectrochemistryInternational Journal of



CatalystsJournal of


HO

OH

O OH

O O

R43

21

8 7

6

5

9

101113

1415

R =

Figure 1 Molecular structure of Guttiferone A (BP)

electrochemical methods in nonaqueous medium have beendeveloped Particularly important are the biomimetic studiesin which lipophilic benzophenones are embedded in a lipidmembrane support on the electrode surface Liposomes arealso suitable for membrane immobilization of lipophilic ben-zophenone In this context we recently incorporated Lapa-chol to build an artificial light-driven transmembrane cal-cium pump and explain a new reduction mechanism ofLapachol in thinfilm electrode [15] The aim of the work is toprovide an insight in the redox chemistry of GutifferoneA animportant natural product embedded in an artificial mem-brane using the model developed by Shi and Anson [18 19]

The device used by Shi is derived from the classical three-electrode configuration depicted in Figure 2 the workingelectrode is covered with a thin film of organic solvent previ-ously saturatedwithwater inwhich themodel compound andan appropriate electrolyte are dissolvedThe electrode is thenimmersed in an aqueous electrolyte The resulting devicebehaves like a stable liquid interface consisting of an organicmembrane in contact with an aqueous phase

The electrochemical experiments have been conductedwith conventional cyclic (CV) and square wave voltammetry(SWV) techniques The advantages of square wave voltam-metry are a higher speed of analysis a lower consumptionof electroactive species and reduction of problems related tothe inhibition of the electrode surface However SWV is oneof the most advanced voltammetry methods unifying theadvantages of CV and pulse voltammetry techniques [20]

2 Experimental

21 Chemicals and Reagent Guttiferone A (BP) was obtainedfrom CH

2

Cl2

ndashMeOH (1 1) extract of Symphonia globuliferausing a procedure described by Ngouela and coworkers [21]The other chemicals were of high purity (99) and wereobtained from Reidel de Haen and Merck BP was dissolvedin a water-saturated nitrobenzene (NB) mixture containing01M tetrabuthylammonium perchlorate (Bu

4

NClO4

) as anorganic electrolyte

Aqueous electrolyte

Edge plane pyrolytic graphite electrode

BP

Organic solvent

BPH2

Counter electrodeReference electrode

2H+

2H+

+2e

Figure 2 Plane edge pyrolytic graphite electrode covered with amicro film of electroinactive water immiscible organic solvent con-taining a neutral redox probe and an organic electrolyte immersedin an aqueous phase-buffered solution containing a common ionwith the organic electrolyte The modified electrode is used in aconventional three-electrode configuration

22 Preparation of the Film The modification and pretreat-ment of the working electrode were described elsewhere [15]as follow the organic solution (1 120583L) was deposited on thegraphite electrode with a micropipette the organic solutionspreads spontaneously over the electrode surface forming astable film The so-modified electrode was immersed in aphosphate buffer solution (01MK

2

HPO4

+ 01MKH2

PO4

)When necessary pH values were adjusted by the addition ofcitric acid or hydroxide sodiumTheaqueous phase ismade ofa 01MLiClO

4

solution NB-saturated water (purities select)was used throughout

23 Electrochemical Cell and Measurements As shown inFigure 2 a three-electrode system was used with a diskelectrode (032 cm2) of edge plane highly oriented pyrolyticgraphite as a working electrodeThe reference electrode was aSaturated Calomel Electrode (SCE) and the counter elec-trode was a platinum wire Prior to each experiment solu-tions were deaerated thoroughly for at least 20min with purenitrogen A positive pressure of this gas was maintained dur-ing subsequentwork Electrochemical data (cyclic voltamme-try and SWV) were obtained using an Autolab Potentiostat(Eco-Chemie Netherlands) driven by a PC with GPES elec-trochemical analysis software

3 Results and Discussion

Before starting SWV measurements the response of theorganic film electrodes was thoroughly inspected by cyclicvoltammetry in order to check the reproducibility and sta-bility of the different forms of BP during successive poten-tial cycling Figure 3(b) shows typical cyclic voltammogramobtained between 00 and minus19 V and corresponding to theelectrolysis of the BP present in the organic filmmembrane incontact with an aqueous buffer solution at pH 1 During suc-cessive cycles figure not shown these CVs reflect the stabilityof the membrane and transformation of BP

The CV consists of two distinct peaks (I-I1015840 and II) whichare not very sensitive to the potential scan rateThe peak pairII1015840 is well developed and quasireversible whereas the secondII located at a more negative potential (Δ119864 = minus13V)


EVSCE

minus30

minus20

minus10

minus40

00


(a)

(b)

I(m

A)


4


NClO4





119901119888




minus50

minus40

minus30

minus20

minus10

00

10

60 mVs

10

II

I

I(m

A)

EVSCE

10 20 30 40 50

30405060708090

(mVmiddotSminus1)

I pcVminus1

(120583A

mVminus

1middotS

)

I998400


119901119888





minus40

minus30

minus20

minus10

00

I

II

III

EVSCE

SWV

CV

I(m

A)

Inet

(a)


minus70

minus60

minus50

minus40

minus30

minus20

minus10

00

III

II

I

I(m

A)

EVSCE

Inet

Iback

Ifor

(b)



minus30

minus20

minus10

00

I

II

III

I(m

A)

EVSCE




2


2


[H+]S = [H+

]W exp [minus 119865119877119879(Δ120601

SW minus Δ

Wrarr S120601SH+)] (1)




OHO O

O

HO

OHR

O

OH

O

OHO

OH

R

O O

HO

O

HO

OHR

K3

K2

K1

3

1

2

Scheme 1


minus40

minus30

minus20

minus10

00

II

I(m

A)

EVSCE




1198681

(120583A) 1198682

(120583A) 1198683

(120583A)t = 0 s 342 2440 414t = 30 s 400 2430 617t = 60 s 408 2420 658t = 90 s 436 2350 680t = 120 s 461 2280 686




OHO O O

OOHR

3

H OHO O O

OOHR

H OHO O

O

O

OHR

H

∙∙

∙∙

minus

+

minus

+

minus ∙minus+

minus

+

1

O

OH

O

O

OOH

R

H

OOH

O O

O OHR

H OOH

O O

O OHR

H

Scheme 2


minus50

minus40

minus30

minus20

minus10

00

(c)

(a)

(b)I(m

A)

EVSCE






minus07

minus06

minus05

minus04

minus03

minus02

IIII

I(m

A)

EVSCE



Reaction (K4


2

and K1


and K6


4


BP(NB) + 2H

+

(W) + 2eminus

999448999471 BPH2(NB) (2)


HO

HO

HOHO HO

HO

OH

OH

OH

OH

O

O O

OH

O

O

O

O

O

O

R

R

R

HO

HO

OH

OH

OH

OH

OH

OH

OH

OH

O

O

O

O

O

O

R

R

R

+ 2eminus + 2H+

+ 2eminus + 2H+

+ 2eminus + 2H+

(K4)

(K5)

(K6)

Scheme 3


4 Conclusion


2



Acknowledgments


References













































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of


EVSCE

minus30

minus20

minus10

minus40

00


(a)

(b)

I(m

A)


4


NClO4





119901119888




minus50

minus40

minus30

minus20

minus10

00

10

60 mVs

10

II

I

I(m

A)

EVSCE

10 20 30 40 50

30405060708090

(mVmiddotSminus1)

I pcVminus1

(120583A

mVminus

1middotS

)

I998400


119901119888





minus40

minus30

minus20

minus10

00

I

II

III

EVSCE

SWV

CV

I(m

A)

Inet

(a)


minus70

minus60

minus50

minus40

minus30

minus20

minus10

00

III

II

I

I(m

A)

EVSCE

Inet

Iback

Ifor

(b)



minus30

minus20

minus10

00

I

II

III

I(m

A)

EVSCE




2


2


[H+]S = [H+

]W exp [minus 119865119877119879(Δ120601

SW minus Δ

Wrarr S120601SH+)] (1)




OHO O

O

HO

OHR

O

OH

O

OHO

OH

R

O O

HO

O

HO

OHR

K3

K2

K1

3

1

2

Scheme 1


minus40

minus30

minus20

minus10

00

II

I(m

A)

EVSCE




1198681

(120583A) 1198682

(120583A) 1198683

(120583A)t = 0 s 342 2440 414t = 30 s 400 2430 617t = 60 s 408 2420 658t = 90 s 436 2350 680t = 120 s 461 2280 686




OHO O O

OOHR

3

H OHO O O

OOHR

H OHO O

O

O

OHR

H

∙∙

∙∙

minus

+

minus

+

minus ∙minus+

minus

+

1

O

OH

O

O

OOH

R

H

OOH

O O

O OHR

H OOH

O O

O OHR

H

Scheme 2


minus50

minus40

minus30

minus20

minus10

00

(c)

(a)

(b)I(m

A)

EVSCE






minus07

minus06

minus05

minus04

minus03

minus02

IIII

I(m

A)

EVSCE



Reaction (K4


2

and K1


and K6


4


BP(NB) + 2H

+

(W) + 2eminus

999448999471 BPH2(NB) (2)


HO

HO

HOHO HO

HO

OH

OH

OH

OH

O

O O

OH

O

O

O

O

O

O

R

R

R

HO

HO

OH

OH

OH

OH

OH

OH

OH

OH

O

O

O

O

O

O

R

R

R

+ 2eminus + 2H+

+ 2eminus + 2H+

+ 2eminus + 2H+

(K4)

(K5)

(K6)

Scheme 3


4 Conclusion


2



Acknowledgments


References













































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of



minus40

minus30

minus20

minus10

00

I

II

III

EVSCE

SWV

CV

I(m

A)

Inet

(a)


minus70

minus60

minus50

minus40

minus30

minus20

minus10

00

III

II

I

I(m

A)

EVSCE

Inet

Iback

Ifor

(b)



minus30

minus20

minus10

00

I

II

III

I(m

A)

EVSCE




2


2


[H+]S = [H+

]W exp [minus 119865119877119879(Δ120601

SW minus Δ

Wrarr S120601SH+)] (1)




OHO O

O

HO

OHR

O

OH

O

OHO

OH

R

O O

HO

O

HO

OHR

K3

K2

K1

3

1

2

Scheme 1


minus40

minus30

minus20

minus10

00

II

I(m

A)

EVSCE




1198681

(120583A) 1198682

(120583A) 1198683

(120583A)t = 0 s 342 2440 414t = 30 s 400 2430 617t = 60 s 408 2420 658t = 90 s 436 2350 680t = 120 s 461 2280 686




OHO O O

OOHR

3

H OHO O O

OOHR

H OHO O

O

O

OHR

H

∙∙

∙∙

minus

+

minus

+

minus ∙minus+

minus

+

1

O

OH

O

O

OOH

R

H

OOH

O O

O OHR

H OOH

O O

O OHR

H

Scheme 2


minus50

minus40

minus30

minus20

minus10

00

(c)

(a)

(b)I(m

A)

EVSCE






minus07

minus06

minus05

minus04

minus03

minus02

IIII

I(m

A)

EVSCE



Reaction (K4


2

and K1


and K6


4


BP(NB) + 2H

+

(W) + 2eminus

999448999471 BPH2(NB) (2)


HO

HO

HOHO HO

HO

OH

OH

OH

OH

O

O O

OH

O

O

O

O

O

O

R

R

R

HO

HO

OH

OH

OH

OH

OH

OH

OH

OH

O

O

O

O

O

O

R

R

R

+ 2eminus + 2H+

+ 2eminus + 2H+

+ 2eminus + 2H+

(K4)

(K5)

(K6)

Scheme 3


4 Conclusion


2



Acknowledgments


References













































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of


OHO O

O

HO

OHR

O

OH

O

OHO

OH

R

O O

HO

O

HO

OHR

K3

K2

K1

3

1

2

Scheme 1


minus40

minus30

minus20

minus10

00

II

I(m

A)

EVSCE




1198681

(120583A) 1198682

(120583A) 1198683

(120583A)t = 0 s 342 2440 414t = 30 s 400 2430 617t = 60 s 408 2420 658t = 90 s 436 2350 680t = 120 s 461 2280 686




OHO O O

OOHR

3

H OHO O O

OOHR

H OHO O

O

O

OHR

H

∙∙

∙∙

minus

+

minus

+

minus ∙minus+

minus

+

1

O

OH

O

O

OOH

R

H

OOH

O O

O OHR

H OOH

O O

O OHR

H

Scheme 2


minus50

minus40

minus30

minus20

minus10

00

(c)

(a)

(b)I(m

A)

EVSCE






minus07

minus06

minus05

minus04

minus03

minus02

IIII

I(m

A)

EVSCE



Reaction (K4


2

and K1


and K6


4


BP(NB) + 2H

+

(W) + 2eminus

999448999471 BPH2(NB) (2)


HO

HO

HOHO HO

HO

OH

OH

OH

OH

O

O O

OH

O

O

O

O

O

O

R

R

R

HO

HO

OH

OH

OH

OH

OH

OH

OH

OH

O

O

O

O

O

O

R

R

R

+ 2eminus + 2H+

+ 2eminus + 2H+

+ 2eminus + 2H+

(K4)

(K5)

(K6)

Scheme 3


4 Conclusion


2



Acknowledgments


References













































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of


OHO O O

OOHR

3

H OHO O O

OOHR

H OHO O

O

O

OHR

H

∙∙

∙∙

minus

+

minus

+

minus ∙minus+

minus

+

1

O

OH

O

O

OOH

R

H

OOH

O O

O OHR

H OOH

O O

O OHR

H

Scheme 2


minus50

minus40

minus30

minus20

minus10

00

(c)

(a)

(b)I(m

A)

EVSCE






minus07

minus06

minus05

minus04

minus03

minus02

IIII

I(m

A)

EVSCE



Reaction (K4


2

and K1


and K6


4


BP(NB) + 2H

+

(W) + 2eminus

999448999471 BPH2(NB) (2)


HO

HO

HOHO HO

HO

OH

OH

OH

OH

O

O O

OH

O

O

O

O

O

O

R

R

R

HO

HO

OH

OH

OH

OH

OH

OH

OH

OH

O

O

O

O

O

O

R

R

R

+ 2eminus + 2H+

+ 2eminus + 2H+

+ 2eminus + 2H+

(K4)

(K5)

(K6)

Scheme 3


4 Conclusion


2



Acknowledgments


References













































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of


HO

HO

HOHO HO

HO

OH

OH

OH

OH

O

O O

OH

O

O

O

O

O

O

R

R

R

HO

HO

OH

OH

OH

OH

OH

OH

OH

OH

O

O

O

O

O

O

R

R

R

+ 2eminus + 2H+

+ 2eminus + 2H+

+ 2eminus + 2H+

(K4)

(K5)

(K6)

Scheme 3


4 Conclusion


2



Acknowledgments


References













































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of











































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of

Research Article Voltammetry Study of an Anti-HIV...

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