inclusion complexation of β-cyclodextrin and 6-o-α-maltosyl- and...

Inclusion complexation of bb-cyclodextrin and 6-O-a-maltosyl- and 2-O-(2-hydroxypropyl)-bb-cyclodextrinswith some ¯uorescent dyes

Yu Liu* and Chang-Cheng You

Department of Chemistry, Nankai University, Tianjin 300071, China

Received 30 April 2000; revised 28 June 2000; accepted 25 August 2000

ABSTRACT: Spectrofluorimetric titrations were performed in aqueous phosphate buffer (pH 7.20, 0.1 mol dmÿ3) todetermine the binding constants ofb-cyclodextrin (1), mono(6-O-a-maltosyl)-b-cyclodextrin (2) and mono[2-O-(2-hydroxypropyl)]-b-cyclodextrin (3) with four fluorescent dyes, ammonium 8-anilino-1-naphthalenesulfonate (ANS),sodium 2-(p-toluidino)naphthalene-6-sulfonate (TNS), Acridine Red (AR) and Rhodamine B (RhB). Thefluorescence of ANS, TNS and AR were enhanced, whereas that of RhB was quenched, by the inclusioncomplexation withb-cyclodextrin hosts1–3. It was found that the binding ability of the three cyclodextrin hostsgenerally decreases in the order1> 3> 2, which indicates that the hydrophobicity of the substituent affects theoriginal binding ability of parentb-cyclodextrin to some extent. On the other hand, the size, shape and charge of theguest are the crucial factors which dominate the stability of the supramolecular complex formed. Copyright 2000John Wiley & Sons, Ltd.

KEYWORDS: cyclodextrins; fluorescent dyes; inclusion complexation; molecular recognition

INTRODUCTION

Cyclodextrins are torus-shaped cyclic oligosaccharidescomposed of six, seven or eightD-glucopyranose units,which are nameda-, b- andg-cyclodextrin, respectively.Their exterior, bristling with hydroxy groups, is fairlypolar, whereas the interior of the cavity is non-polar.These structural features enable cyclodextrins to accom-modate diverse hydrophobic organic and biologicalmolecules to form host–guest or supramolecular com-plexes in aqueous solution.1–4 Cyclodextrins have beenused extensively as molecular receptors, chemicalsensors and enzyme mimics in science and technology.5,6

Among the three common cyclodextrins,b-cyclodextrinis readily available and bears an appropriate cavity toaccommodate bicyclic aliphatic and aromatic com-pounds, and therefore it has received much moreattention than the other analogues. In the past two

decades, numerous chemically modifiedb-cyclodextrinshave been designed and synthesized in order either toinvestigate the mechanisms of enzyme-catalyzed reac-tions or to achieve solubility in a desired solvent.7

Although many studies on the molecular recognition ofchemically modified cyclodextrins with various guestmolecules have been reported and considerable signifi-cant results have been achieved,8–10it is still of interest toinvestigate the mechanism and influence factors of thiskind of interaction since it may provide a simple model ofenzyme–substrate, antibody–antigen and protein–DNAinteractions in biological systems.

In this paper, we report the inclusion complexation ofb-cyclodextrin (1), mono(6-O-a-maltosyl)-b-cyclodex-trin (2) and mono[2-O-(2-hydroxypropyl)]-b-cyclodex-trin (3) (Scheme 1) with four structurally relatedfluorescent dyes, ammonium 8-anilino-1-naphthalenesul-fonate (ANS), sodium 2-(p-toluidino)naphthalene-6-sul-fonate (TNS), Acridine Red (AR) and Rhodamine B(RhB). (Scheme 2). One important reason for choosingtheseb-cyclodextrin derivatives as hosts is that glucose-branchedb-cyclodextrins and hydroxypropyl-modifiedb-cyclodextrins can significantly enhance the solubilityof hydrophobic guest molecules compared with nativeb-cyclodextrin and have been used successfully aslipophilic drug carriers and stabilizers in pharmacy.11,12

The results obtained indicate that the substituent ofb-cyclodextrin greatly affects its original molecular bindingability upon inclusion complexation with ANS, TNS, AR

JOURNAL OF PHYSICAL ORGANIC CHEMISTRYJ. Phys. Org. Chem.2001;14: 11–16

Copyright 2000 John Wiley & Sons, Ltd. J. Phys. Org. Chem.2001;14: 11–16

*Correspondence to:Y. Liu, Department of Chemistry, NankaiUniversity, Tianjin 300071, China.E-mail: [email protected]/grant sponsor:National Outstanding Youth Fund;Contract/grant number:29625203.Contract/grant sponsor:Natural Science Foundation of China;Contract/grant number: 29992590-8; Contract/grant number:29972029.Contract/grant sponsor:Trans-Century Qualified Personnel Fund(Sun-Light Plan) of Tianjin Municipality.Contract/grant sponsor:Tianjin Natural Science Fund;Contract/grantnumber:993601311.

andRhB.On theotherhand,thestability of thecomplexformed apparently dependson the size, shape andelectrostaticdensityof theguestmolecules.

Experimental

Materials. b-Cyclodextrin (1) and mono(6-O-a-malto-syl)-b-cyclodextrin (2) were purchasedfrom EnsuikoSeito. Mono[2-O-(2-hydroxypropyl)]-b-cyclodextrin (3)wassynthesizedby thealkylationof b-cyclodextrinwithepoxypropane in aqueous alkali according to theproceduesreported by Pitha et al.13. Ammonium 8-anilino-1-naphthalenesulfonate(ANS) andsodium2-(p-toluidino) naphthalene-6-sulfonate(TNS) were pur-chasedfrom Tokyo Kasei and Acridine Red (AR) and

RhodamineB (RhB) from Chroma-GesellschaftSchmid,andwereusedasreceived.Sodiumdihydrogenphosphateand disodium hydrogenphosphatewere dissolved indoubly distilled, deionized water to make a0.10mol dmÿ3 buffer solution of pH 7.20, which wasusedassolventthroughoutthemeasurements.

Spectral measurements. Fluorescencespectra weremeasured in a conventional quartz cell(10� 10� 40mm) on a JASCO FP-750spectrofluori-meter.Theexcitationandemissionslitswere5 nmfor allthe fluorescentdyes. The excitation wavelengthsforANS, TNS, AR, and RhB were 350, 350, 490 and520nm, respectively.The sample solution containingfluorescentdyes (1.0� 10ÿ5 mol dmÿ3 for ANS, TNSandAR and5.0� 10ÿ6 mol dmÿ3 for RhB) andvariousconcentrationsof hosts (0–2� 10ÿ3 mol dmÿ3) werekept at 25.0� 0.1°C for spectralmeasurementsby acirculatingthermostatedwater-jacket.

Results and Discussion

Fluorescence spectra

ANS andTNS arebarelyfluorescentin aqueoussolutionbut arevery sensitiveto environmentalchanges,14 whichenablesusto usethesefluorescentdyesasspectralprobesto investigatetheinclusioncomplexationwith cyclodex-trins 1–3. The relative intensity and the fluorescenceemissionmaximaof thefluorescentdyesexaminedin theabsenceand presenceof b-cyclodextrinhosts(1–3) arelistedin Table1.As expected,therelativeintensityof thefluorescenceof ANS andTNS is dramaticallyenhanceduponbindingwith cyclodextrinhosts.At thesametime,the additionof the cyclodextrinhostscausedsignificanthypochromicshifts(10–25nm)of thefluorescencepeak.

Scheme 1

Scheme 2

Copyright 2000JohnWiley & Sons,Ltd. J. Phys.Org. Chem.2001;14: 11–16

12 Y. LIU AND C.-C.YOU

Theseobservationsclearly indicate that the residueofANS or TNS wasembeddedinto thehydrophobiccavityof b-cyclodextrinapartfrom bulk water.

Although both AR and RhB possessa xantheneresidue,they exhibit dramaticallydifferent fluorescencebehavioruponinclusioncomplexationwith thehosts.Ascanbe seenfrom Fig. 1, with the additionof host3, thefluorescenceof AR graduallyincreases,accompaniedbyslight hypochromic shifts (8 nm, 561–553 nm). Thisphenomenonis consistentwith theobservationsfor ANSand TNS. In contrast, as illustrated in Fig. 2, thefluorescenceof RhB is quenchedby the addition of acyclodextrin host. Politzer et al. examinedthe fluores-cencechangesof RhB at different concentrationswithandwithout theadditionof b-cyclodextrinandfoundthatb-cyclodextrin enhancesthe fluorescenceof RhB inconcentrated(10ÿ3 mol dmÿ3) aqueousdye solutionbut

quenchesit in dilute (10ÿ4–10ÿ8 mol dmÿ3) dyeaqueoussolution.15 These phenomenawere ascribed to theexistenceof dyeaggregatesin the concentratedaqueoussolution. From the equilibrium constant of 2100dm3 molÿ1 for the dimer monomertransition of RhBin aqueoussolution determinedby Lopez Arbeloa etal.,16 it seemslikely thatmonomersarethepredominantspecies(>99%) at the dye concentration(5� 10ÿ6 moldmÿ3) we used.

We areespeciallyconcernedasto whetheror not thefluorescencequenchingwasinducedby theformationofan RhB–CD complex. In Fig. 3(a), a typical Stern–Volmer plot for thequenchingof RhB by 2 is illustrated.This downward-curvingplot may indicatethat therearetwo kinds of speciesin the RhB dye aqueoussolution,oneaccessibleandtheotherinaccessibleto cyclodextrin.Then,a modified Stern–Volmerequationfor the above

Table 1. Fluorescent properties of the dye guests in the absence and presence of b-cyclodextrin hosts 1±3(1.0� 10ÿ3 mol dmÿ3)

Fluorescentdye Concentration(mM) Host �ex(nm) �em(nm) Relativeintensity

ANS 10 None 350 524 11 510 2.42 509 2.13 499 5.8

TNS 10 None 350 496 11 483 162 481 283 480 22.7

AR 10 None 490 561 11 553 42 556 3.63 553 4.2

RhB 10 None 520 575 11 573 0.712 573 0.813 572 0.70

Figure 1. Fluorescence spectral changes of AR(1.0� 10ÿ5 mol dmÿ3) on addition of mono[2-O-(2-hydro-xypropyl)]-b-cyclodextrin (3) in aqueous buffer solution atpH 7.20. The concentration of 3 increases in the range0±1.8� 10ÿ3 mol dmÿ3 from a to l. Excitation wavelength,490 nm

Figure 2. Fluorescence spectral changes of RhB(5.0� 10ÿ6 mol dmÿ3) on addition of mono(6-O-maltosyl)-b-cyclodextrin (2) in aqueous buffer solution at pH 7.20. Theconcentration of 2 increases in the range 0±1.01.0� 10ÿ3 mol dm ÿ3 from a to k. Excitation wavelength,520 nm


INCLUSION COMPLEXATION OF CYCLODEXTRINS WITH FLUORESCENTDYES 13

systemshouldbeemployed:17

I0

�I� 1

faK�CD� �1fa

�1�

where I0 is the total fluorescencein the absenceofquencher (CD), K is the Stern–Volmer quenchingconstantof the accessiblefraction and fa is the fractionof theinitial fluorescencewhich is accessibleto quencher(CD).

On the basisof Eqn. ((1)), a typical plot is showninFig. 3(b) for the quenchingof RhB by 2, where I0/DIvaluesare plotted against1/[CD] to give an excellentlinear relationship(R = 0.9978),verifying the probableaccessible–inaccessiblemechanismproposedabove.Weare further interestedin which speciesis accessibletocyclodextrinsinceit is useful to understandthe bindingconstantsobtained.Changet al.18 pointed out that, inaddition to the zwitterionic and cationic forms, a thirdspeciesof RhB in dilute solutionis thecolorlesslactone,whichdoesnotcontributeto thefluorescenceemissioninthe visible region.Therefore,it may be concludedthatcyclodextrins1–3 prefer to complex with the lactonicform of RhB.Basedon this hypothesis,we mayinterpretthe phenomenathat the fluorescenceof RhB decreaseswith theadditionof cyclodextrinhostsin dilute solution,sincesomeof thezwitterionsaretransformedto lactonewith the lactone–cyclodextrincomplex formation. Thisinclusion behavior is very similar to the inclusioncomplexationof phenolphthaleinwith b-cyclodextrin,where when the ionized form of phenolphthaleinisenclosedin the cyclodextrincavity, it is forced into itscolorlesslactonestructure.19 Furthermore,Hinckley etal.18b reportedthatin thelactone zwitterionequilibrium,81.5% of RhB dissolvedin water and 70.6% of RhBdissolvedin ethanol is in the zwitterionic form in theconcentrationrange(6–8)� 10ÿ6 mol dmÿ3. In fact, thefraction(fa) of theinitial fluorescencewhich is accessibleto cyclodextrin calculated from the intercept of themodifiedStern–Volmerplots is in therange30–40%fortheb-cyclodextrinhosts1–3 in thepresentstudy.

Fluorescence spectral titrations

Assuming1:1 stoichiometry,the inclusioncomplexationof a guest(Dye) with a host (CD) is expressedby theequation

CD� Dye�Ks

CD � Dye �2�

The stability constant(KS) can be obtainedfrom theanalysis of the sequential changes of fluorescenceintensity(DI) of guestdyesat variousconcentrationsofhost,usinga non-linearleast-squaresmethodaccordingto thecurve-fittingequation20

�I �f��CD�0� �Dye�0� 1=Ks�

��2��CD�0 � �Dye�0� 1Ks�2ÿ 4�2�CD�0�Dye�0

qg=2�3�

where[Dye]0 and[CD]0 refer to the total concentrationsof the guestdyes and host cyclodextrinsand a is theproportionality coefficient, which may be taken as asensitivity factor for the fluorescencechange.For eachfluorescentdyeexamined,theplot of DI asa functionof[CD]0 gavean excellentfit, verifying the validity of the1:1 complex stoichiometry assumedabove. Figure 4illustratestypical curve-fitting plots for the titrations ofAR with cyclodextrins 1–3. There are no seriousdifferencesbetweentheexperimentalandcalculateddatafor the CD–Dye system, indicating 1:1 complexationonly throughout the concentrationrange of host b-cyclodextrins(0–2mmol dmÿ3). The complexstabilityconstants(KS) obtainedarelisted in Table1, alongwiththe freeenergychangeof complexformation(ÿDG°).

Binding constants and molecular recognition

In previous studies,21,22 we examined the inclusion

Figure 3. (a) Stern±Volmer and (b) modi®ed Stern±Volmer plots for RhB,whose ¯uorescence was quenched by the addition of mono(6-O-maltosyl)-b-cyclodextrin (2)



complexation of a variety of chemically modifiedcyclodextrins with diverse guest molecules, such asaminoacids,aliphatic alcoholsandnaphthalenesulfonicacid, and found that severalnon-covalentweak forces,including van der Waals,hydrophobic,hydrogen-bond-ing and dipole–dipole interactions,cooperativelycon-tributeto theinclusioncomplexationof cyclodextrins.Inthe presentcase,we found that the size/shapematchingbetweenhost and guestdominatesthe stability of thecomplexesformed,which indicatesthat van der Waalsand hydrophobicinteractionsmainly contribute to theformation of supramolecularcomplexes,as thesetwoforcesareclosely relatedto the distanceandcontactingsurfaceareabetweenhostandguest

As canbeseenfrom Table2, althoughANS andTNScontainbothphenylandnaphthylresidues,their bindingconstantswith a certainhost are dramaticallydifferent,

e.g.,thebindingconstantsof hosts1 and2 with TNS aregreaterby afactorof 35–36thanthatwith ANS.Wehavedemonstratedthat the naphthalenering prefers topenetrate into the cavity of b-cyclodextrin in thelongitudinal direction.23 Examinationwith the Corey–Pauling–Koltun(CPK) molecularmodel indicated thatthe naphthaleneresidueof TNS may be embeddedintotheb-cyclodextrincavity fully andthenaffordsa largerbinding constant with b-cyclodextrin hosts 1–3. Incontrast, the naphthalene residue of ANS cannotpenetrateinto the cavity of b-cyclodextrinowing to thesterichindranceof theanilino group.In this context,thebinding of ANS to b-cyclodextrinprobablyoccur withthe anilino group and then affords the lowest bindingconstantswhosemagnitudeis consistentwith thethoseofaniline(KS = 61)andN-methylaniniline(KS = 83)with b-cyclodextrin.

As a linearxanthenederivative,AR maybeembeddeddeeplyinto b-cyclodextrinbut affordsmoderatebindingconstantsof (1.8–2.6)� 103 with hosts1–3, which aresmallerthanthoseof TNS with thecorrespondinghosts.It is well known that the cyclodextrincavity is lined byhydrogenatomsandglycosidicoxygenbridges,andthenon-bondingelectron pairs of the glycosidic oxygenbridges are directed toward the inside of the cavityproducinga high electrondensitythereandlendingto itsome Lewis base characteristics.In this context, thecyclodextrin cavity should favor complexation withcationic guestssuchasAR. However,it shouldalsobenoted that the positive charge of AR may reverselyreducethehydrophobicityof thewholemolecule.In thepresentcase,it seemsthesecondfactorplaysthecrucialrole. As a result, the cyclodextrin hostsusedprefer toencapsulateneutralTNS to chargedAR. Unexpectedly,although RhB possessesa large sterically hinderinggroup on the xantheneresidue, it affords the largestbinding constantsamong the four fluorescentdyes. Aprobableexplanationis that the lactonic form of RhBparticipatesin the complexationwith b-cyclodextrin,aswe proposed previously. Phenolphthalein,which isbelieved to transform into the lactone in the b-cyclodextrin cavity, can form an extra stablecomplexwith b-cyclodextrinandgivesahighstability constantof2.3� 104 evenin pH 10.5aqueoussolution.19

We may note from Table2 that the functionalgroupintroducedon to hosts2 and 3 significantly affects theoriginal molecularbinding ability of b-cyclodextrin(1).Theadditionof maltoseto therim of thecylcodextrincupnot only increasesthesolubility of theb-cyclodextrininwater, but also marginally attenuatesthe binding ofguestswithin the cyclodextrin cavity. As can be seenfrom Table 2, the binding constantsof the fluorescentdyeswith host2 areabouttwo-thirdsof thecorrespond-ing values with native b-cyclodextrin (1). This resultprobablyindicatesthat the introductionof a hydrophilicsubstituentmayreducetheoriginalhydrophobicityof thecavity andthe binding ability towardhydrophobicguest

Table 2. Complex stability constants (KS) and Gibbs freeenergy changes (ÿDG°) for 1:1 inclusion complexation ofvarious guest dyes with b-cyclodextrins 1±3 in aqueousbuffer solution (pH 7.20) at 25°Ca

Host Guest KS Log KS

ÿDG°(kJmolÿ1)

1 ANS 102 2.01 11.5TNS 3700 3.57 20.4AR 2630 3.42 19.5RhB 5100 3.71 21.2Aniline 61 1.79 10.2N-Methylaniline 83 1.92 11.0

2 ANS 69 1.84 10.5TNS 2410 3.38 19.3AR 1790 3.25 18.6RhB 3300 3.52 20.1

3 ANS 260 2.41 13.8TNS 3030 3.48 19.9AR 2400 3.38 19.3RhB 3840 3.58 20.5

a When repeatedmeasurementswere performed,the KS value wasreproducible within an error of �5%, which correspondsto anestimatederror of 0.12kJmolÿ1 in the free energyof complexation(DG°).

Figure 4. Curve-®tting analyses of ¯uorescence spectraltitrations of AR with b-cyclodextrin hosts 1±3 in aqueousbuffer solution at pH 7.20. The differential ¯uorescenceintensity DI (open circles) was ®tted to the theoretical value(closed circles) calculated for stoichiometric 1:1 complexation


INCLUSION COMPLEXATION OF CYCLODEXTRINS WITH FLUORESCENTDYES 15

molecules. The 2-hydroxypropyl group in host 3 ishydrophobiccomparedwith the hydroxy group on thesecondaryside,andmay extendthe hydrophobiccavityof nativeb-cyclodextrin(1). However,host3 still affordsaweakerbindingability thannativeb-cyclodextrin.As iswell known, guestmoleculesusually accessthe cyclo-dextrin cavity from the more opening side in thecomplexationprocedure,24 andthereforetheintroductionof a subsituent on to the secondary side of thecyclodextrin may causesteric hindranceof the guestaccessingto the cavity, which probably leads to theweakerbinding ability of 3 with large-sizedguests.Onthe other hand, as we reported previously, 22b,23b ahydrophobic substituent in cyclodextrin derivativesprefers to be self-included into its cavity to form anintramolecularcomplexandis excludedfrom the cavityonadditionof externalguests.Hence,theintroductionofa self-including substituent in cyclodextrin is notfavorablefor enhancingthecyclodextrin’scomplexationability. As a result of the above factors, the bindingability of thethreehostsfor TNS,AR or RhBdecreasesintheorder1> 3> 2.

FromTable2, it canbeseenthat thebindingconstantof host 3 with ANS is greaterthan that of host 1 by afactor of 2.5,which is consistentwith the resultsfor theinclusion complexationof ANS with polyamine-mod-ified b-cyclodextrin(KS = 266for diethylenetriamino-b-cyclodextrinand280 for triethylenetetraamino-b-cyclo-dextrin.23b A reasonableexplanationis that the anilinoresiduein ANSparticipatesin theinclusioncomplexationwith b-cyclodextrin,althoughits sizeis smallerthanthevolumeof theb-cyclodextrincavity (262 A3).25,26 If so,the self-including propyl group in 3 may adjust theeffective spaceof its cavity and 3 affords more stablecomplexationwith ANS thandoesnativeb-cyclodextrin.The present results indicate that the hydrophobicsubsituentcan servenot only as a competitive group,but alsoasanadjusterof thecavity size.

Acknowledgements

This work was supportedby the National OutstandingYouth Fund (Grant No. 29625203),Natural ScienceFoundation of China (Grant No. 29992590-8 and29972029), Trans-CenturyQualified PersonnelFund(Sun-Light Plan) of Tianjin Municipality and TianjinNaturalScienceFund(GrantNo. 993601311),which isgratefully acknowledged.

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