ARTICLE IN PRESS

ContentslistsavailableatScienceDirect

Journal of Chromatography A

jou rn al h om ep a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h r o m a

Combinatorial effects of the configuration of the cationic and the anionic chiral subunits of four zwitterionic chiral stationary phases leading to reversal of elution order of cyclic ˇ ³ -amino acid

enantiomers as ampholytic model compounds

Nóra Grecsó

, Enik ˝o Forró

, Ferenc Fülöp

, Antal Péter

, István Ilisz

, Wolfgang Lindner

aDepartmentofInorganicandAnalyticalChemistry,UniversityofSzeged,H-6720Szeged,Dómtér7,Hungary

bInstituteofPharmaceuticalChemistry,UniversityofSzeged,H-6720Szeged,Eötvösu.6,Hungary

cDepartmentofAnalyticalChemistry,UniversityofVienna,Währingerstrasse38,1090Vienna,Austria

a r t i c l e i n f o

Articlehistory:

Received6April2016

Receivedinrevisedform4May2016 Accepted11May2016

Availableonlinexxx

Keywords:

Enantiomerseparation

Zwitterionicchiralstationaryphases N␣-methyl-protectedcyclicˇ³-aminoacids Temperatureeffect

a b s t r a c t

Inasystematicwayenantioseparationsofnon-methylatedandthecorrespondingN-monomethylated ampholyticcyclicß³-aminoacidswerecarriedoutonfourzwitterionicchiralstationaryphases(CSPs;

ZWIX(+)^TM,ZWIX(−)^TM,ZWIX(+A),ZWIX(-A)).CSPswerebasedonthecombinationsofquinineand quinidineasthecationicandof(R,R)-and(S,S)-aminocyclohexanesulfonicacidastheanionicsites.In polar-ionicmobilephasesystems,theeffectsofthecompositionofthebulksolvents,theadditives,the concentrationoftheco-andcounter-ions,thetemperature,andthestructuresoftheampholyticanalytes wereinvestigated.Thechangesinstandardenthalpy,(H^◦),entropy,(S^◦),andfreeenergy,(G^◦), werecalculatedfromthelinearvan’tHoffplotsderivedfromtheln˛vs1/Tcurvesinthestudiedtempera- turerange(5–40^◦C).UnusualtemperaturebehaviorwasobservedontheZWIX(−)^TMcolumn:decreased retentiontimeswereaccompaniedbyincreasedseparationfactorswithincreasingtemperature,andsep- arationwasentropically-driven.FortheotherthreeCSPs,enthalpically-drivenenantioseparationswere observed.Viatheconsequentdeterminationoftheelutionorderoftheresolvedenantiomers,theeffects oftheabsoluteconfigurationofthechiralanionicandcationicsubunitsofthezwitterionicCSPscould beelucidated.N-methylationoftheaminoacidsledunexpectedlytoareversaloftheelutionsequence, whichcanbeinterpretedbyasubtleshiftofthehierarchicalorderofthestericallymostimportantdriving interactionsitesfromthecationictotheanionicunits,andviceversa.

©2016ElsevierB.V.Allrightsreserved.

1. Introduction

Conceptually,thispaperraisestwoprincipalquestions:(i)What willbetheeffectsofasystematicvariationofthepositivelyandneg- ativelychargedchiralinteractionsiteswithinzwitterionicselectors (SOs)andtherelatedchiralstationaryphases(CSPs)?;and(ii)What effectwilltheN-monomethylationoftheaminogroupofcyclic ˇ³-amino acids (AAs) haveregarding the retention andelution sequenceoftheresolvedenantiomers?

Asgenerally accepted,thespatial structure of a chiral com- pounddescribedbytheabsoluteconfigurationofthechiralcenters

∗Correspondingauthors.

E-mailaddresses:ilisz@chem.u-szeged.hu(I.Ilisz), wolfgang.lindner@univie.ac.at(W.Lindner).

andtheconformationalflexibilityoftheentireSOactingaschiral hostdeterminestheextentofmolecularinteractionsandmolec- ularrecognitionoftheanalytes(selectands,SAs).Inasimplified way,thisconceptrelatestothedifferenceofGibbsfree binding energy(G^◦)ofthe[SO-(R)-SA]andthe[SO-(S)-SA]associates thatinchromatographicterms,leadstoastereoselectivityfactor,

␣.Incombinationwiththeefficiencyofa“chiralcolumn,”itrelates totheresolutionvalueoftheenantiomers.AchiralSOmoietycanin principlebedividedintostructuralsubunits,whichmayparticipate moreorlessstronglybutsimultaneouslyintheoverallintermolec- ularinteractionprocesswiththechiralSAs.Inthecaseofionizable subunits,cationicandanionicsitesintheSOcaneachhaveindepen- dent(R)and(S)configurations.ForzwitterionicSOs,itisassumed thatthesetwositesinteractviaasimultaneouslyoccurringdou- bleionpairingeventwiththechiralzwitterionic(ampholytic)SAs, wherebythespatialenvironmentofalltheionizedsiteswillhavea http://dx.doi.org/10.1016/j.chroma.2016.05.041

0021-9673/©2016ElsevierB.V.Allrightsreserved.

ARTICLE IN PRESS

G Model

CHROMA-357568; No.ofPages10

2 N.Grecsóetal./J.Chromatogr.Axxx(2016)xxx–xxx

Fig.1. StructureofthefourcombinatoricallycomposedzwitterionicCSPsbasedonQNorQDandon(R,R)-and(S,S)-ACHSA.

directiveeffect,andthusdeterminetheresultingelutionsequence oftheresolvedenantiomers.

Onthebasisofpreviousworks[1,2],weusedfourZWIXtype CSPs,whichdifferonlyinthechiralityofthesubunits(depicted inFig.1),fortheresolutionofthecyclicˇ³-AAderivativesandto probetheinitiallyraisedquestions.

Duetotheirpharmacologicaleffects,carbocyclicˇ-AAshave gained great interest among synthetic and medicinal chemists in the past two decades, and they have become a “hot” topic inorganicand bioorganicchemistry. A considerablenumber of syntheses of carbocyclic ˇ-AA derivatives in both racemic and enantiopureformhavebeenreportedinrecentyears,andmost ofthemhavebeenreviewed[1,3].Controllingtheenantiomeric purity(e.g.underpeptidesynthesis)requiresreliable andaccu- rate analytical methods to be at hand. At an analytical level, high-performanceliquidchromatography(HPLC)isundoubtedly themostimportantmethodofchoicetoday.HPLCenantiosepara- tionsofß-AAshavebeenperformedbybothindirectand direct methods and inthe pastdecade new typesof chiral derivatiz- ingagentsandCSPshavebeenappliedforthispurposeandwere reviewedinnumerouspapers[4–11].Severalpapersdealwiththe enantioseparationofN-acylandN-aryl-protectedAAderivatives;

however,onlyfewreportscanbefoundfortheenantioresolution ofN-methylatedvariants. Enantioseparationof N-methyl-˛-AAs by ligand-exchange HPLC was reported by Brückner [12]. N- methylleucine,N-methylisoleucine,N-methyl-phenylalanine and N-methylglutamateandaspartatewasresolvedbytheapplication ofchiralderivatizingagentsbyHessetal.[13]andTsesarskaiaetal.

[14].Enantiomersof N-methylleucine andits2,4-dinitrophenyl- and 3,5-dinitrobenzoyl-derivatives were separated on Cinchona alkaloid-based CSPs [15,16]. Piette et al. reported enantiosep- aration of 3,5-dinitrobenzoyl-N-methylleucine by non-aqueous capillaryelectrophoresisusing1,3-phenylene-bis(carbamoylated quinine)asaselector[17].

Chromatographicresolutioncanbetunedbyvariationofmobile phasecomposition,mobilephaseadditives,andespeciallyinchiral chromatography,temperature.Inmostcases,theachiralandchiral (stereoselective)interactionsaresensitivetotemperature[18–21].

Therefore,duringchiralseparations,thecolumntemperatureoften hasbeenoptimizedandkeptwellcontrolled.

Thetemperaturedependenceofchromatographicretentioncan beexpressed by thevan’t Hoff equation.The differencein the changeinstandardenthalpy(H^◦)andentropy(S^◦)forenan- tiomerscanbeexpressedas[20]:

ln˛=−(H^o)

RT +(S^o) R

whereRistheuniversalgasconstant,TistemperatureinKelvin, and␣istheselectivityfactor.Ifalinearvan’tHoffplotisobtained, aplotofRln˛versus1/Thasaslopeof−(H^◦)andanintercept of(S^◦).Thevan’tHoffplotsinterpretedthiswayyieldappar- entchangesinstandardenthalpyandentropyvalues,inwhichthe respectivecontributionsofthechiralandachiralinteractionsare combined.Amoresophisticatedapproachshouldbebasedonthe differentiationofstereoselectiveandnon-selectivesites[21].

In this work, we present the enantioseparation of non- methylatedandN˛-monomethylatedcyclicß³-AAs(Fig.2)onthe quinine(QN)orquinidine(QD)andaminocyclohexanesulfonicacid [(R,R)-and(S,S)-ACHSA]basedandchemicallyfusedzwitterionic CSPs.FocuswasgiventotheeffectofN-methylationincompar- isontothenon-methylatedcongeners, aswellasin themobile phase composition, temperature,and the structure of thevari- antsofchiralselectorsonchromatographicperformance,including enantioselectivityand elutionorder. Theeffectsof thedifferent selectorsandtheN˛-methylationofthecyclicß³-AAsonthesepa- rationperformanceswerestudiedindetailwiththeaimtoevaluate thedrivinginfluencesoftheconfigurationofthechiralionizable subunitsofthecombinatoricallycomposedzwitterionicselector units.

Fig.2. Chemicalstructuresofampholyticcyclicˇ³-aminoacids.

2. Experimental

2.1. Chemicalsandreagents

TheN-methylatedcarbocyclic ˇ-AAswere preparedthrough thegeneralmethodologydescribedbyArvidssonetal.[22],from thecorrespondingFmoc-AAswithparaformaldehyde,viaoxazi- nanoneintermediates,followedbytriethylsilanereduction,under microwaveheating.

HPLC-grade MeOH, MeCN and THF, as wellas diethylamine (DEA)andaceticacid(AcOH)modifierswereobtainedfromVWR

International(Radnor,PA,USA).Purifiedwaterwasobtainedfrom theUltrapure WaterSystem,PuranityTUUV/UF(VWRInterna- tional).

2.2. Apparatusandchromatography

HPLCseparations wereperformed on:A WatersBreeze sys- tem consisting of a 1525 binary pump, a 487 dual-channel absorbance detector, a 717plus autosampler, Empower 2 data manager software(WatersChromatography,Milford,MA,USA), andaLaudaAlphaRA8thermostat(LaudaDr.R.WobserGmbh,

ARTICLE IN PRESS

G Model

CHROMA-357568; No.ofPages10

4 N.Grecsóetal./J.Chromatogr.Axxx(2016)xxx–xxx

Fig.3.Effectsofthecompositionsofthebulksolventsonthechromatographic parametersfor1and3onCSP-2.

Chromatographicconditions:column,CSP-2;mobilephase,MeOH/MeCN(75/25, 50/50or25/75v/v)containing25mMDEAand50mMAcOH;flowrate,0.6mlmin⁻¹; detection,215and230nm;temperature25^◦C;symbols,fork1,␣andRSvaluesfor 1,䊏,for3,䊉.

Lauda-Königshofen,Germany);orona1100SeriesHPLCsystem fromAgilentTechnologies(Waldbronn,Germany),consistingofa solventdegasser,apump,anautosampler,acolumnthermostat, amultiwavelengthUV–visdetectorandacorona-chargedaerosol detectorfromESABiosciences,Inc.(Chelmsford,MA,USA).Data acquisitionandanalysisonthelattersystemwerecarriedoutwith ChemStationchromatographicdatasoftwarefromAgilentTech- nologies.

ThecommerciallyavailableChiralpakZWIX(+)^TM(CSP-1)and ZWIX(−)^TM (CSP-2)(150×3.0mmI.D.,3-␮mparticlesize)were provided by Chiral Technologies Europe (Illkirch, France). The preparation ofZWIX(+A) (CSP-3)(150×3.0mm I.D.,5-␮mpar- ticlesize)isdescribed in [23,24], andthat ofZWIX(-A)(CSP-4) (150×3.0mmI.D.,3-␮mparticlesize)in[25].

StocksolutionsofAAs(1mgml⁻¹)werepreparedbydissolving theminthemobilephase.Fordeterminationofthedead-times(t₀) ofthecolumns,amethanolicsolutionofacetonewasinjected.The flowratewas0.6mlmin⁻¹andthecolumntemperaturewas25^◦C ifnototherwisestated.

3. Resultsanddiscussion

3.1. Effectofnatureandconcentrationofbulksolvent componentsandmobilephaseadditives

ThefourzwitterionicCinchonaalkaloidQN-orQD-and(R,R)-or (S,S)-ACHSA-basedCSPs(Fig.1)performwellinnon-aqueouspolar organicsolventscontainingMeOHasaproticbulksolventcompo- nent(whichcansuppressH-bondinginteractions)andMeCNasan aproticbulksolventcomponent(whichisadvantageousforionic interactions,butdisadvantageousfor-interactions)[1].Addi- tionally,acidandbaseadditivesareoftenappliedtoensureionic propertiesofthemobilephase[1,24–27].

Forthechromatographicexperimentswiththecyclicß³-and N˛-monomethylatedcyclicß³-AAs,variationofvolumeratiosof MeOH/MeCN (75/25,50/50 and 25/75v/v)as bulk solventcon- taining50mMAcOHand 25mM DEAinmostcases resultedin theseparationof theenantiomers.As depictedinFig.3, inthe caseofCSP-2for1and3,withincreasingMeCN/MeOHratio,the retentioncontinuouslyincreased.Theobservedchromatographic behaviorcanbeexplainedonthebasisofthecharacteristicsofthe solventsandAAs.Nonaqueous,polarorganicsolventsarefavored

Table1

Chromatographicdata,separationfactor(k),selectivityfactor(␣),resolution(RS) andelutionsequenceoffree-andN-monomethylatedcyclicß³-aminoacidsonCSP- 1.

Compound Eluent k1 ␣ RS Elutionsequence

1 a 3.13 1.00 0.00 –

g 3.46 1.02 <0.3 A<B

2 a 2.55 1.71 7.07 B<A

3 a 2.82 1.00 0.00 –

g 2.61 1.10 0.68 A<B

4 a 1.60 1.54 3.75 B<A

5 a 3.96 1.21 1.31 A<B

6 a 2.13 1.50 3.59 B<A

7 a 6.22 1.12 0.99 B<A

8 a 2.46 2.00 9.24 B<A

9 a 3.39 1.05 0.49 A<B

f 0.70 1.11 0.69 A<B

10 a 1.47 1.38 3.37 B<A

Chromatographic conditions: column, CSP-1; mobile phase, a, MeOH/MeCN (50/50v/v)containing25mMDEAand50mMAcOH,f,H2O/MeCN(25/75v/v)con- taining25mMDEAand50mMAcOH,g,MeOH/THF(80/20v/v)containing25mM DEAand50mMAcOH;flowrate,0.6mlmin⁻¹;detection,215and230nm.

Table2

Chromatographicdata,separationfactor(k),selectivityfactor(␣),resolution(RS) andelutionsequenceoffree-andN-monomethylatedcyclicß³-aminoacidsonCSP- 2.

Compound Eluent k1 ␣ RS Elutionsequence

1 a 6.59 1.07 0.52 B<A

g 4.66 1.17 0.86 B<A

2 a 3.34 1.70 6.45 A<B

3 a 3.36 1.04 0.30 B<A

g 3.48 1.20 1.21 B<A

4 a 1.78 1.63 3.41 A<B

5 a 4.33 1.35 2.17 B<A

6 a 2.55 1.35 2.30 A<B

7 a 8.42 1.41 2.83 A<B

8 a 2.55 2.23 8.74 A<B

9 a 3.69 1.16 0.95 B<A

i 1.66 1.25 1.25 B<A

10 a 1.23 1.64 3.29 A<B

Chromatographic conditions: column, CSP-2; mobile phase, a, MeOH/MeCN (50/50v/v)containing25mMDEAand50mMAcOH,g,MeOH/THF(80/20v/v)con- taining25mMDEAand50mMAcOH,i,MeOH/THF(95/5v/v)containing25mMDEA and50mMAcOH;flowrate0.6mlmin⁻¹;detection,215and230nm.

forCinchonaalkaloid-basedCSPs[1,23,24].Theincreasedreten- tionobserved for theapplied AAsat higherMeCN contentcan probablybeexplainedintermsofthedecreasedsolvationofthe ionizablecompounds,i.e.solvationofpolarcompoundsinamobile phasewithahigherMeCNcontentislesseffective,withtheconse- quencethattheelectrostaticinteractionsbetweentheSOandthe AAbecomestronger,resultinginhigherretention.Ataconstant acid-to-baseratiowiththechangeofbulksolventcomposition,the acid–baseequilibriumandprotonactivitymayalsochange,leading tofurthereffectsonthechromatographicbehavior.

Inthecaseof1and3,theselectivityslightlyincreasedinMeCN- richmobilephases.Thisislikelyduetoenhancedelectrostaticand H-bondinginteractions.Forresolution,bothaslightincreaseand decreasecouldbeobserved.

ApplyingMeOH/MeCNasbulksolventinsomecasesprovided onlypartialorlackofseparation(Tables1–4).Inordertoobtain betterseparations,THFwasappliedinsteadofMeCN.Incertain cases,theapplicationofTHFresultedinanimprovementinenan- tioselectivityandresolution;however,thisimprovementstrongly dependedontheSOsandAAsinvolvedintheexperiment.InFig.4, for1,3,and9onCSP-2andCSP-3,theMeOH/THFratioexerteda markedeffectontheretention.WithMeOH/THFmobilephasesys- temscontaining50mMAcOHand25mMDEA,amoderatedecrease in k1 with increasingMeOH content wasobserved. The higher

Fig.4. EffectsoftheTHFcontentinMeOH/THFmobilephaseonthek1and␣valuesforAAs1,3and9onCSP-2.

Chromatographicconditions:column,CSP-2;mobilephase,MeOH/THF(98/2,95/5,90/10and80/20v/v)containing25mMDEAand50mMAcOH;flowrate,0.6mlmin⁻¹; detection,215and230nm;temperature25^◦C;symbols,fork1and␣valuesfor1䊏,for3,䊉,andfor9,.

Table3

Chromatographicdata,theseparationfactor(k),theselectivityfactor(␣),theres- olution(RS)andelutionsequenceoffree-andN-monomethylatedcyclicß³-amino acidsonCSP-3.

Compound Eluent k1 ␣ RS Elutionsequence

1 a 1.89 1.69 2.89 B<A

g 1.44 1.66 2.87 B<A

2 a 1.41 1.00 0.00 –

g 0.57 1.00 0.00 –

3 a 1.70 1.40 1.82 B<A

g 1.27 1.39 1.40 B<A

4 a 1.33 1.00 0.00 –

g 0.98 1.00 0.00 –

5 a 1.76 1.25 1.21 B<A

6 a 1.31 1.21 1.23 A<B

7 a 2.24 1.11 0.58 B<A

8 a 1.42 1.00 0.00 –

g 1.20 1.00 0.00 –

9 a 2.00 1.10 0.46 B<A

g 1.55 1.00 0.00 –

10 a 1.01 1.32 1.69 A<B

Chromatographic conditions: column, CSP-3; mobile phase, a, MeOH/MeCN (50/50v/v)containing25mMDEAand50mMAcOH,g,MeOH/THF(80/20v/v) containing25mMDEAand50mMAcOH;flowrate,0.6mlmin⁻¹;detection,215, 230nmandcoronadetector.

Table4

Chromatographicdata,theseparationfactor(k),theselectivityfactor(␣),theres- olution(RS)andelutionsequenceoffree-andN-monomethylatedcyclicß³-amino acidsonCSP-4.

Compound k1 ␣ RS Elutionsequence

1 2.80 1.80 3.73 A<B

2 1.50 1.46 3.31 B<A

3 2.47 1.51 2.21 A<B

4 1.56 1.07 0.65 B<A

5 2.38 1.24 0.90 A<B

6 1.85 1.11 0.75 B<A

7 3.00 1.44 2.53 A<B

8 1.50 1.32 2.09 A<B

9 3.36 1.14 0.68 A<B

10 1.35 1.19 1.46 B<A

Chromatographic conditions: column, CSP-4; mobile phase, a, MeOH/MeCN (50/50v/v)containing25mMDEAand50mMAcOH;flowrate,0.6mlmin⁻¹;detec- tion,215,230nmandcoronadetector.

contentofproticMeOHprogressivelyweakenstheionicinterac- tionsbetweentheAAandtheSO;theenhancedsolvationofpolar AAsresultsinadecreaseofretention.Theselectivity(andR_S,data

notshown)increasedslightlywhentheTHFcontentwasincreased from2to20%(v/v).

InastudyofzwitterionicCSPsundernon-aqueousandaqueous conditions,theadditionofwatertotheMeOH-containingeluent systemuptoawatercontentof20%(v/v)decreasedtheretention ofthepolarAAs[24].Similarly,Zhangetal.foundthatthepresence of2.0%(v/v)waterinthemobilephaseshortenedretention,but enhancedresolutionofsomeAAenantiomerpairsonzwitterionic CSPs[26].Inourcase,onCSP-1andCSP-2for1,3,and9,theaddition of2%(v/v)H₂OtotheMeOHorMeOH/THF(95/5v/v)mobilephase containing50mMAcOHand25mMDEAwasalsoaccompaniedby adecreaseofabout20–25%ink1,andaslightdecrease(5–10%) for␣andR_S(datanotshown).Thus,thepresenceofsuchalow concentrationofwaterinthemobilephasemightbeadvantageous becauseofthedecreased retentionobtained,withoutsignificant lossinselectivityandresolution.

3.2. Influenceofthecounter-ionconcentration

Forbothanion-andcation-exchangetypeSOsunderpolar-ionic conditions,apredominantion-exchange-drivenretentionmecha- nismhasbeenconfirmed[27],andintheion-pairingprocess,the counter-ionspresentinthemobilephaseactascompetitors.Thus, retentioncanbeadjustedbyvariationoftheconcentrationsofthe counter-ions.Incaseslikethis,theretentioncanbedescribedby thesimpledisplacementmodel[28];theplotoflogkvs.logcresults inalinearrelationship,wheretheslopeofthestraightlineispro- portionaltotheeffectivechargeinvolvedintheion-exchange.The effectsofcounter-ionconcentrationfor1,2,7,8,9,and10(selected AAsexhibitedcis-ortrans-configurationswithprimaryaminoorN- monomethylatedstructure)wereinvestigatedintheMeOH/MeCN (50/50v/v)mobilephasesystemcontainingAcOHandDEA.The concentrationsofAcOHandDEAwerevariedfrom12.5mMupto 100mMandfrom6.25mMupto50mM,respectively,whilethe acidtobaseratiowasmaintainedat2:1.Underthestudiedcondi- tions,linearrelationshipswerefoundbetweenlogk₁andlogc_AcOH, withslopesvaryingaround0.20–0.35(Fig.5).Theobservedslopes practicallywereinvariantwiththestructuresofAAs.According tothesimple displacementmodel,theslopesoftheseplotsare determinedbytheratiooftheeffectivechargesofthesoluteand thecounter-ions.Thedataobtainedwiththefour,inacombina- torialfashion,modulatedZWIXcolumnsrevealedthatingeneral, anincreasingcounter-ionconcentrationresultedinreducedreten- tionfactors,asexpectedforion-exchangers.Onacation-exchange

ARTICLE IN PRESS

G Model

CHROMA-357568; No.ofPages10

6 N.Grecsóetal./J.Chromatogr.Axxx(2016)xxx–xxx

Fig.5.Effectsofcounter-ionconcentrationonthek1valuesfor1,2,7,8,9and10onthefourCinchonaalkaloid-basedCSPs.

Chromatographicconditions:column,CSP-1−CSP-4;mobilephase,MeOH/MeCN(50/50v/v)containingDEA/AcOHinconcentration6.25/12.5,12.5/25.0,25.0/50,and 50/100mM/mM;flowrate,0.6mlmin⁻¹;detection,215and230nm;temperature25^◦C;symbols,for1䊏,for2,䊉,for7,,for8,for9,,andfor10,*.

CSP,assuminga“singleionic”ion-exchangemechanism,slopesof logkvs.logcplotsaround0.9havebeenreportedforthesepara- tionofdifferentchiralamines[23].However,theslopesofthelog kvs.logcplotsinthisstudywere0.20-0.35.Thesevaluesindicate amarkeddifferencebetweenthezwitterionicanda“singleionic”

ion-exchangemechanism.In thecase ofzwitterionicCSPs,with increasedcounter-ion concentration, retention can be reduced.

However,almostanorderofmagnitudeincreaseintheconcen- trationof thecounter-ions(from 12.5mMto100mM)resulted onlyin40–50%reductionintheretentionfactor.Underthestud- iedconditionsonallthefourCSPs,practicallyidenticalslopeswere obtainedforeachenantiomer,i.e.theindividualenantioselectiv- itycharacteristicsremainedalmostconstantwhenthecounter-ion concentrationwasvaried(datanotshown).Thisisinlinewiththe generalobservationconcerningthebehaviorof enantioselective ion-exchangers[2].

3.3. Comparisonofseparationperformancesofthefour combinatoricallycomposedzwitterionicCSPs

For the purpose of comparison, all of the CSP separations investigatedwerecarriedoutatconstantmobilephasecomposi- tion(MeOH/MeCN(50/50v/v)containing25mMDEAand50mM AcOH).ThedatainTables1–4revealthatretentiononthestud- iedCSPsforthefirsteluting enantiomerfollowed thesequence

CSP–2>CSP–1>CSP–4>CSP-3(exceptfor10)withk₁ varyingin therangefrom1.01–8.42.

Comparingthetwopairs ofcolumnsbased onQN,and pos- sessingtheACHSAsubunitswithoppositeconfiguration[CSP-1vs.

CSP-3],orbasedonQD,andpossessingtheACHSAsubunitswith oppositeconfiguration[CSP-2vs.CSP-4],itbecameevidentthat higherk₁ valueswereobtainedonCSP-1andCSP-2thanonthe pseudoenantiomericCSP-3andCSP-4(exceptfor10)phases.The samewasvalidforvaluesof␣(exceptionswere1,3,5,and9on CSP-1/CSP-3and1and3onCSP-2/CSP-4).

ThebehavioroftwopairsofCSPsbasedonQDorQNmoietyand possessingACHSAunitwiththesameconfiguration[CSP-2vs.CSP- 3andCSP-1vs.CSP-4]isalsoworthelaborating.Itwasobserved thatinthecasewhentheACHSAunitpossessesa1”R,2”Rconfig- uration,theQD-basedCSP(CSP-2)exhibitedhigherk1valuesthan theQN-basedCSP(CSP-3),whilewhentheACHSAunitpossesses a1”S,2”Sconfiguration,QN-basedCSP-1exhibitedhigherk₁val- uesthantheQD-basedCSP-4.Thehigherk1valuesgenerallywere accompaniedwithhigherselectivities(exceptionswereSAs1and 3onCSP-2/CSP-3andSAs1,3,5,7,and9onCSP-1/CSP-4).The chromatographicallyvisibleeffectofthestructureoftheQN/QD and(R,R)/(S,S)-ACHSAsubunitsoftheSOonk,␣,andR_Sturnsout toberathercomplex.However,intermsoftheelutionorderofthe resolvedcyclicß³-AAenantiomers,trendscanclearlybeseento dependonQD/QNand(R,R)/(S,S)-ACHSAsubunits(discussedlater).

Table5

Thermodynamicparameters,(H^◦),(S^◦),Tx(S^◦),(G^◦),correlationcoefficients(R²)andQtemperatureoffree-andN-monomethylatedcyclicß³-aminoacidson CinchonaalkaloidandACHSAbasedCSPs.

Compound Column Correlationcoefficients(R²) −(H^◦)(kJmol⁻¹) −(S^◦)(Jmol⁻¹K⁻¹) −Tx(S^◦)298K(kJmol⁻¹) −(G^◦)298K(kJmol⁻¹) Q

1 CSP-2 0.9911 −0.7 −2.8 −0.8 0.1 0.8

CSP-4 0.9951 5.5 13.5 4.0 1.5 1.4

CSP-3 0.9951 6.0 15.8 4.7 1.3 1.3

2 CSP-2 0.9942 2.9 5.4 1.6 1.3 0.5

CSP-4 0.9908 5.4 14.9 4.4 1.0 1.2

CSP-1 0.9914 3.0 5.7 1.7 1.3 1.8

7 CSP-2 0.9923 1.3 1.5 0.5 0.8 0.2

CSP-4 0.9909 2.4 5.0 1.5 0.9 1.6

CSP-1 0.9941 0.7 1.6 0.5 0.2 1.6

CSP-3 0.9990^* 0.4 0.3 0.1 0.3 3.9

0.9975^** 2.1 6.2 1.8 0.3 1.1

8 CSP-2 0.9922 5.2 10.7 3.2 2.0 0.6

CSP-4 0.9973 2.5 6.0 1.8 0.7 1.4

CSP-1 0.9934 4.3 8.6 2.6 1.7 1.7

Chromatographicconditions:columns,CSP-1–CSP-4;mobilephase,a,MeOH/MeCN(50/50v/v)containing25mMDEAand50mMAcOH;flowrate,0.6mlmin⁻¹;detection, 215,230nmandcoronadetector;R²,correlationcoefficientofvan’tHoffplot,ln␣vs1/Tcurves;Q=(H^◦)/298×(S^◦)

*Temperaturerange5–20^◦C.

**Temperaturerange20–40^◦C.

ItseemsthattheQD-basedCSP-2andCSP-4ensureamorepro- nouncedstericallytriggeredenvironment fortheintermolecular interactionsbetweenSOandAA.ThefourCinchonaalkaloid-based zwitterionicCSPsdisplayacomplementarycharacter.Thisbehav- iorrevealsagaintheimportanceofthestericandspatialposition oftheionicinteractionsiteswithintheSOmoieties.

3.4. Effectsofthestructureoftheanalytes(non-methylated versusN-monomethylatedSAsandcis/trans-isomers)on chromatographicbehaviorandelutionsequence

ItisfullyconsistentforallAAsandforallthefourCSPsthatk valuesweresmallerforN-methylatedcyclicß³-AAscomparedto thenon-methylatedones.However,␣andR_Svalueschangedin differentways.WithreversedelutionorderonCSP-1andCSP-2, despitethelowerkvalues,theenantioselectivityandtheR_Swere higherforN-methylatedAAs; onCSP-3and CSP-4the␣andRS

valueschangedparallelwithkvalues,i.e.theywerelowerforN- methylatedAAs(exceptionswere9and10).Itwasalsoobserved thatthepresenceofadoublebondinthemolecule(1vs.3or2vs.

4)generallyresultedinlowerk,␣,andR_Svalues.Themorepolar moleculeswithadoublebondprobablyweresomewhatmoresol- vatedinthemethanolcontainingmobilephaseresultinginsmaller k,␣,andR_Svalues.Acomparisonofcyclohexylskeleton-containing AAswithcis-vs.trans-configurations(5vs.7and6vs.8)revealed thatAAswithtrans-configuration(7and8)inmostcasesexhib- itedhigherk,␣,andR_Svalues(exceptionswere5vs.7onCSP-1 andCSP-3and6vs.8onCSP-3).7onallfourCSPsexhibitedthe largestkvalues.Probablythestericarrangementintrans-isomerof carboxyl-andamino-(orN-methylamino)-groupsfavorstheinter- actionswiththecationic-andanionic-sitesoftheSO.Subsequently, itbecameevidentthatwithswitchingthecolumnsfromCSP-1to CSP-2orCSP-3toCSP-4orfromCSP-1toCSP-3orfromCSP-2to CSP-4,theelutionsequencecouldbereversed(exceptionwas7).

Thisisinlinewiththepreviouslyobtainedresults,aswitchfrom theQNbasedselectorCSP-1tothepseudoenantiomericQDbased CSP-2ledtoareversaloftheelutionorder.(1,3,5,and9hadan elutionorderA<BonCSP-1andanelutionorderB<AonCSP-2.)

However, a reversal of elution order became evident when changingfromtheQNbasedCSP-1tothealsoQNbasedCSP-3.

Thisfactcorroboratesthatinessencetheabsoluteconfigurations ofallthechiralsubunits(theQNandtheACHSAsubunit)determine ina concertedfashion theoverall spatiallyorientedinteraction sitesoftheparticularZWIXselectorwiththeampholyticAAs.As already outlinedabove, a simultaneouslyoccurringdouble ion-

pairingeventisenvisionedandastrongelectrostaticinteraction betweentheconfigurationallydefinedchiralACHSAunitoftheSO andtheprimaryaminogroupoftheAAseemtoberesponsiblefor thedirectingeffect,whichleadstotheobservedenantioselectivity.

Inotherwords,switchingfroman(R,R)-toa(S,S)-ACHSAsubunit leadstoareversaloftheelutionsequenceof theprobedenan- tiomers.ExactlythesametrendcanbeseenfortheQD-basedCSPs (CSP-2andCSP-4).Asaconsequenceofthesesystematicstudies, itbecomes evidentthatalsotheCSP-3 andCSP-4behavepseu- doenantiomericallytoeachotherforthegivencyclicˇ³-AAs.

In comparison to the non-methylated cyclic ˇ³-AAs we observed a reversal of the elution sequence for the N- monomethylatedanalogues(seeTables1–4).Thiseffectwasvery surprisingasitindicatesthatinthecaseofCSP-1andCSP-2the basicQNsitebecomesnowmoredominantthantheACHSAsite.

ThesameargumentappliesforthecomparisonofCSP-3andCSP-4.

Inotherwords,thereexistsasubtlebuthierarchicallyswitched balancebetweenthesterically drivingsubunits ofthezwitteri- onicCSPs(SOs)in termsoftheoverallobservedintermolecular recognitionphenomenaforagivensetofampholyticanalytes.This determinesconsequentlytheenantioselectivityandelutionorder oftheAAs.

Switching from the cis-configurated 5 and 6 to the trans- configurated 7 and 8 (Fig.2 and Tables 1–4), we observed an exception oftheabovediscussedtrend.In essence,theactively involved and sterically demanding intermolecular interaction motifsoftheAAandtheSOsitesaredifferent.These,however, determinetheelutionorder.

Selectedchromatogramswithindicationofelutionsequences aredepictedinFig.6.

3.5. Temperaturedependenceandthermodynamicparameters

Inordertoinvestigatetheeffectsoftemperatureonchromato- graphicbehavior, a variable-temperaturestudy wascarriedout on all fourCinchona alkaloid-based CSPsover thetemperature range5–40^◦C.Experimentaldatacollectedfor1,2,7,and8onthe fourcolumnswiththemobilephaseMeOH/MeCN(50/50v/v)con- taining25mMTEAand50mMAcOHarelistedinSupplementary material(TableS1).

Thetabulateddataindicatethattheretentiondecreasedinall cases withincreasing temperature.Transfer oftheSAfrom the mobilephasetothestationaryphaseisgenerallyanexothermic processand consequentlyk decreaseswithincreasingtempera- ture.Thechangesobservedintheselectivityandresolutionwith

ARTICLE IN PRESS

G Model

CHROMA-357568; No.ofPages10

8 N.Grecsóetal./J.Chromatogr.Axxx(2016)xxx–xxx

Fig.6.Selectedchromatogramsfor1–10.

Chromatographicconditions:column,for1,3and7CSP-4,for2,4,6,8and10CSP-1andfor5and9CSP-2;mobilephase,for1and7MeOH/MeCN(50/50v/v)containing 6.25mMDEAand12.5mMAcOH,for2-6and8MeOH/MeCN(50/50v/v)containing25mMDEAand50mMAcOH,for9MeOH/THF(95/5v/v)containing25mMDEAand 50mMAcOHandfor10MeOH/MeCN(50/50v/v)containing12.5mMDEAand25mMAcOH;flowrate,0.6mlmin⁻¹;detection,215,230nmandcoronadetector;temperature 25^◦C,exceptfor2and8itwas5^◦C

temperaturewerenotconsistent.Inmostcases,␣andRSdecreased withincreasingtemperature.However,onCSP-2for1,␣andR_S slightlyincreasedwithincreasingtemperatureundertheapplied conditions.Increasingtemperaturemayimprovethepeaksymme- tryandefficiency,andthereforeresolutionmayalsoimprove.Such unusualbehaviorwasrecentlyobservedforenantioseparationof unusualAAsonCinchonaalkaloid-basedCSPs[29–32]andforan entirelydifferentchiralchromatographicsystembyChankvetadze etal.[33].

Sincetheeffectoftemperatureontheenantiomerseparationis complex,anextensivestudyrelatingtothethermodynamicswas carriedout.Accuratechromatographicdatawerecollectedtocon- structvan’tHoffplotsandthethermodynamicparameterswere calculatedfromtheslopesandinterceptsofplotsofln␣vs.1/T (Table5).

Thedifferencesinthechangesinstandard enthalpy(H^◦) provideinformationontherelativeeaseoftransferofAAsfromthe mobiletothestationaryphase.Anegative(H^◦)valueindicates anexothermictransferofAAsfrommobiletostationaryphase.The (H^◦)valuesrangedfrom−6.0to+0.7kJmol⁻¹.

Thedifferencesinentropy(S^◦)isameasureofthelossofthe degreeoffreedomortheenergybalanceofdesolvation/solvation processwhenAA-SOcomplexisformingduringtheadsorptionpro- cess.Thetrendinthechangein(S^◦)issimilartothatin(H^◦).

Undertheconditionswhere(H^◦)hasnegativevalues,(S^◦) wasalsonegative[andthepositive(H^◦)wasaccompaniedby thepositive(S^◦)].Anegative(S^◦)indicatesanincreasein order and/or loss in the degrees of freedom of the adsorbed

enantiomers. The (S^◦) values ranged from −15.8 to +2.8Jmol⁻¹K⁻¹. The interactions of 1 and 2 withCSP-4 and 1 or8withCSP-3orCSP-2,respectivelywerecharacterizedbythe lowest(H^◦)and(S^◦)valuesreflectingathermodynamically unfavorableprocess.

Whentheselectivityincreasedwithincreasingtemperature, (H^◦)and(S^◦)werepositive(e.g.,1onCSP-2).Inthiscase, thechangeintheadsorptionenthalpywithincreasingtempera- turehadapositiveeffectontheenantioselectivity.ForthisAAin thistemperaturerange,enantioresolutionisentropically-driven, andtheselectivityincreaseswithincreasingtemperature.Ther- modynamically,thisunusualbehavior maybeattributedtothe positive(S^◦)values,indicatingtheimportanceoftheentropy contributiontothechiralseparation.Ontheotherhand,thepos- itive(S^◦)compensatedthepositive(H^◦)andresultedina negative(G^◦).

Therelative contributionoftheenthalpicandentropicterms to the free energy of adsorption can be visualized by the enthalpy/entropyratiosQ=(H^◦)/298x(S^◦)(Table5).Com- parison of theQ values for the SAs ondifferent CSPsrevealed that the enantioselective discrimination was in most cases enthalpically-driven(Q>1)butforallinvestigatedSAsonCSP-2 theentropiccontributiontothefreeenergysurpassestheenthalpic one(Q<1).For7onCSP-3,theln␣vs.1/Tcurvecanbedivided intotwolinearrangesbetween5–20^◦Cand20–40^◦C,respectively.

Whilethesignofthesloperemainedthesameinbothregions, theentropiccontributiontothefreeenergyintheregion20–40^◦C

ismoreexpressed(Q=1.1)denotingamoresignificantrearrange- mentinthesolventsphere.

Comparisonofthe(G^◦)valuesforthefourcolumnsdemon- stratedthatinmostcaseslower(G^◦)wereobtainedonCSP-2or CSP-4.ItseemsthattheQD-basedCSPsunderwentmoreefficient bindingwiththeSAsstudied.

4. Conclusions

Theenantiomers of non-methylatedand N-monomethylated cyclic ß³-AAs were separated onfour zwitterionicCSPs, which containedthestronglyacidicchiral(R,R)-and(S,S)-ACHSAsub- units fused with the weakly basic chiral QN- or QD subunits.

Theseparationscouldbeaccomplishedinpolar-ionicmodeand thechromatographicretentionbehaviorandresolutionprovedto dependonthenatureandconcentrationofthebulksolventandthe acidandbasemodifiers,thetemperature,andtheN-methylation ofthecyclicß³-AAs.Thevaluesofthethermodynamicparameters, suchas (H^◦), (S^◦)and (G^◦), dependedon thestruc- tures of theanalytes and onthechiral selectorsused.Most of theseparationswereenthalpically-driven,butentropically-driven separationswerealsoobserved.OfthestudiedCSPs,theQDbased CSP-2 and CSP-4 appeared more suitable for the direct enan- tioseparation of non-methylated and N-monomethylated cyclic ß³-AAs.

Particularlystrikingweretheresultsobtainedfortheinvestiga- tionofthemolecularrecognitionphenomenaofthesystematically variedzwitterionicCSPs.Inasystematicfashion,thesefourCSPs werescreenedfortheresolutionoffiveˇ³-AAsandfiverelated N-monomethylatedˇ³-AAs.Throughtheconsequentelucidation oftheelutionordersoftheenantiomersofthisparticularsetof probesitbecameevidentthat:

(i)ThepairsofCSP-1andCSP-2,andofCSP-3andCSP-4behave pseudo-enantiomericallytoeachother,thustheelutionorderof thestructurallysimilaranalytes1,3,5,and9werereversed.

(ii)TheQNbasedCSP-1andCSP-3,differinginthechiralityof theACHSAsubunits,ledalsotoareversaloftheelutionorderof thescreenedprobes,whichindicatesthatbothchiralsubunitsare simultaneouslyinvolvedintheintermolecularrecognitionprocess.

However,fornon-methylatedˇ³-AAstheabsoluteconfigurationof theanionicACHSAsubunitsiteprovidedthedominantdirecting effect.

(iii)FortheN-methylatedcongenersofthenon-methylatedˇ³- AAs,anunexpectedreversalofelutionorderwasnoticed,which indicatesthatinthesecasesthecationicCinchonaalkaloidinter- action site becomes the dominant and elution order-directing interactionsite.

EffortstoelucidatethesediverseandsurprisingandsubtleSO- AAinteractionphenomenaonamolecularlevelbyspectroscopic, X-ray,andcomputermodelingstudiesarecurrentlyunderway.

Fromananalyticalpointofviewitisadvantageoustoelutethe minorenantiomerinfrontofthemajorenantiomer,soitmakes indeedsensetohaveasetofsimilarCSPsavailablewhichoffer thereversalofelutionorder.InthecaseofChiralpakZWIX(+)^TM andZWIX(−)^TM thisconceptisalreadyestablished.Insummary, fromtheexploredexamplesofenantiomerseparationsandelu- tionordersitbecomesevidentthat zwitterionicchiral selectors composed of oppositely charged chiral subunits which inter- act simultaneously viaelectrostatic forces withe.g. ampholytic analyteshaveagreatpotential.Fusingchiralsubunitsinacombi- natorialwayofferthepotentialtooptimizetheoverallstructure of the SO to discriminate most efficiently the enantiomers of givenSAs.ItbecamealsoevidentthatsuchcomplexSOsmaynot alwayswork perfectlyand a loss of enantioselectivity canalso occur.

Conflictsofinterest

Theauthorsdeclarenoconflictsofinterest.

Acknowledgments

ThisworkwassupportedbyHungarianNationalScienceFoun- dation grant OTKA K108847. We gratefully thank Pilar Franco (ChiralTechnologiesEurope,Illkirch,France)fortheprovisionof theCinchonaalkaloid-basedcolumnsoftheCSP-1andCSP-2type.

CSP-4wasprovidedbyDr.MichalKohout(DepartmentofOrganic Chemistry,InstituteofChemicalTechnology,Prague,CzechRepub- lic).WealsothankDr.KevinSchug(UniversityofArlington,Texas, USA)forproofreadingthearticle.

AppendixA. Supplementarydata

Supplementarydataassociatedwiththisarticlecanbefound,in theonlineversion,athttp://dx.doi.org/10.1016/j.chroma.2016.05.

041.

References

[1]C.V.Hoffmann,R.Pell,M.Lämmerhofer,W.Lindner,Effectson

enantioselectivityofzwitterionicchiralstationaryphasesforseparationsof chiralacidsbases,andaminoacidsbyHPLC,Anal.Chem.80(2008) 8780–8789.

[2]I.Ilisz,A.Péter,W.Lindner,State-of-the-artenantioseparationsofnaturaland unnaturalaminoacidsbyhigh-performanceliquidchromatography,Trends Anal.Chem.(2016),http://dx.doi.org/10.1016/j.trac.2016.01.016.

[3]L.Kiss,F.Fülöp,Synthesisofcarbocyclicandheterocyclicü-aminocarboxylic acids,Chem.Rev.114(2014)1116–1169.

[4]I.Ilisz,R.Berkecz,A.Péter,Applicationofchiralderivatizingagentsinthe high-performanceliquidchromatographicseparationofaminoacid enantiomers:areview,J.Pharm.Biomed.Anal.47(2008)1–15.

[5]I.Ilisz,Z.Pataj,A.Aranyi,A.Péter,High-performanceliquidchromatography ofbiologicallyimportantsmallepimericpeptidesandtheirl,d-aminoacid content,MiniRev.Med.Chem.10(2010)287–298.

[6]M.Lämmerhofer,Chiralrecognitionbyenantioselectiveliquid chromatography:mechanismsandmodernchiralstationaryphases,J.

Chromatogr.A1217(2010)814–856.

[7]I.Ilisz,Z.Pataj,A.Aranyi,A.Péter,Macrocyclicantibioticselectorsindirect HPLCenantioseparations,Sep.Purif.Rev.41(2012)207–249.

[8]I.Ilisz,A.Aranyi,Z.Pataj,A.Péter,Recentadvancesintheenantioseparation ofaminoacidsandrelatedcompounds:areview,J.Pharm.Biomed.Anal.69 (2012)28–41.

[9]I.Ilisz,A.Aranyi,Z.Pataj,A.Péter,Enantiomericseparationof

nonproteinogenicaminoacidsbyhigh-performanceliquidchromatography, J.Chromatogr.A1269(2012)94–121.

[10]I.Ilisz,A.Aranyi,Z.Pataj,A.Péter,Enantioseparationsbyhigh-performance liquidchromatographyusingmacrocyclicglycopeptide-basedchiral stationaryphases–anoverview,in:G.Scriba(Ed.),ChiralSeparations, MethodsandProtocols,HumanaPress,NewYork,2013,pp.137–163.

[11]M.H.Hyun,Developmentandapplicationofcrownether-basedHPLCchiral stationaryphases,Bull.KoreanChem.Soc.26(2005)1153–1163.

[12]H.Brückner,EnantiomericresolutionofN-methyl-␣-aminoacidsand

␣-alkyl-␣-aminoacidsbyligand-exchangechromatography, Chromatographia24(1987)725–738.

[13]S.Hess,K.R.Gustafson,D.J.Milanowski,E.Alvira,M.A.Lipton,L.K.Panell, Chiralitydeterminationofunusualaminoacidsusingprecolumn derivatizationandliquidchromatography-electrosprayionizationmass spectrometry,J.Chromatogr.A1035(2004)211–219.

[14]M.Tsesarskaia,E.Galindo,G.Szókán,G.Fisher,HPLCdeterminationofacidic d-aminoacidsandtheirN-methylderivativesinbiologicaltissues,Biomed.

Chromatogr.23(2009)581–587.

[15]R.Pell,S.Sic,W.Lindner,MechanisticinvestigationofCinchonaalkaloid-based zwitterionicchiralstationaryphases,J.Chromatogr.A1269(2012)287–296.

[16]A.Mandl,L.Nicoletti,M.Lämmerhofer,W.Lindner,Quinineversus carbamoylatedquinine-basedchiralaninonexchangers.Acomparison regardingenantioselectivityforN-protectedaminoacidsandotherchiral acids,J.Chromatogr.A858(1999)1–11.

[17]V.Piette,W.Lindner,J.Crommen,EnantiomerseparationofN-protected aminoacidsbynon-aqueouscapillaryelectrophoresiswithdimericformsof quinineandquinidinederivativesservingaschiralselector,J.Chromatogr.A 948(2002)295–302.

[18]S.Allenmark,V.Schurig,Chromatographyonchiralstationaryphases,J.

Mater.Sci.7(1977)1955–1963.