Two- and Three-Dimensional Smectic Ordering of Single-Handed Helical Polymers(5)

Two- and Three-Dimensional Smectic Ordering of Single-Handed Helical Polymers

Figure2.Right-andleft-handedhelicalpoly-L-1.(A-C)SECchromatogramsoftheas-preparedpoly-L-1(A)andtheisolatedacetone-insolublepoly-L-1(-)(B)andacetone-solublepoly-L-1(+)(C)usingUV(redlines)andCD(bluelines)detectorsinTHFcontaining0.1wt%tetra-n-butylammoniumbromide.(D)CDandabsorptionspectraofpoly-L-1(a),poly-L-1(-)(b),andpoly-L-1(+)(c)(0.2mg/mL)inchloroformat25°C.Thenumber-averagemolecularweight(Mn)anditsdistribution(Mw/Mn)ofeachpolymerasdeterminedbySECcoupledwithamulti-anglelight-scattering(MALS)detector(SEC-MALS)measurementsarealsoshowninA-C.

differentsolubilityinacetone,andthereby,theycanbeseparated.Thisconclusionissupportedbythefactthatevenamixtureofpoly-L-1(-)andpoly-L-1(+)withcomparableMw’scouldbealsoseparatedintoeachhelixusingacetone.Inthesameway,theas-preparedpoly-D-1canbefractionatedusingacetoneintoright-andleft-handedhelicalpoly-D-1’swithdifferentMw’sandanarrowMWD(FigureS1).

Polyisocyanidesbearingabulkysubstituenthavebeenconsideredtohavea4unitsperturn(4/1)helicalconformationeveninsolution,althoughtheirexacthelicalstructureshavenotyetbeenelucidated6c,9probablybecauseofdifficultyinobtainingauniaxiallyorientedpolymerfilmsuitableforXRDmeasurements.Thepoly-L-1(-)andpoly-L-1(+)arerigid-rodhelicalpolymersandexhibitalyotropicsmecticLCphase(seebelow),whichenablesustodeterminetheirstructuresbyXRDandAFM.Figure3Ashowsawide-angleX-raydiffraction(WAXD)patternofanorientedpoly-L-1(-)filmpreparedfromaconcentratedLCbenzenesolutioninanelectricfield;foraWAXDpatternofpoly-L-1(+),seeFigureS2.Theelectricfield-inducedalignmentofpoly-L-1moleculesevidencedalargedipolemomentofpoly-L-1alongitshelicalaxis,thatisalltheintramolecularlyhydrogen-bondedN-HandCdOgroupsareorientedinonedirectionsoastoaccumulatethelargedipolemomentasobservedinthetypicalR-helicalpolypeptides.13TheWAXDpatternsofthefilmsofpoly-L-1(-)andpoly-L-1(+)showdiffuse,butapparentmeridionalandequatorialreflections,suggestingthattheypossessasimilarhelicalstructure;a15unitsper4turns(15/4)helixwithahexagonallattice(TableS1)whichsatisfiesthedensityrequirements.14Optimizedmolecularstructuresforthe15/4helicesoftheleft-handedhelicalpoly-L-1(-)model(158-mer)andright-handedhelicalpoly-L-1(+)model(58-mer),basedontheabsolutemolecularweights(Mn)5.65×104and2.06×104,respectively)

(13)Wada,A.AdV.Biophys.1976,9,1-63.

calculatedbytheSECmeasurementscoupledwithamulti-anglelightscattering(MALS)detector,areillustratedinBandCofFigure3,respectively.Thedetailedstructures(11-mer)takenfromFigure3BarealsoshowninDandEofFigure3(seealsoSupportingInformation).Thepolymermodelsappeartohavefoursetsofhydrogen-bondedhelicalarrayslinkingnand(n+4)pendantswiththequarterhelicalpitchandchainlengthof1.31and13.7nm,and1.31and5.1nm,forpoly-L-1(-)andpoly-L-1(+),respectively.IRspectrasuggestedtheformationofsuchintramolecularhydrogenbondsbetweenthependantamideresiduesofpoly-L-1(-)andpoly-L-1(+)(FigureS3).TheequivalentinterpendanthydrogenbondswereobservedinthecrystallinestructureofL-1,inwhichtheamidelinkagewasobliquetothephenylring(ca.30°)(FigureS4).HelicalpolyisocyanidesstabilizedbyintramolecularhydrogenbondshavebeenreportedbyNolteandco-workers.6c,9

PartsFandGofFigure3showhigh-resolutionAFMimagesofpoly-L-1(-)andpoly-L-1(+),respectively,castfromabenzenesolution(0.015mg/mL)onhighlyorientedpyrolyticgraphite(HOPG)followedbybenzenevaporexposureatca.20°Cfor12h.8,15,16Thepoly-L-1self-assemblesintowell-defined2Dhelixbundleswithacontrolledmolecularlength,17mostofwhichareclearlyresolvedintoindividualleft-(Figure3F)andright-handed(Figure3G)helicespackedparalleltoeachother.TheAFMimagesaswellasthoseoflargerareasconfirmedthehelicalpitch,helicalsense,helix-senseexcess,

(14)Theobserveddensitiesofthepoly-L-1(-)andpoly-L-1(+)filmswere

1.0739and1.0624g/cm3,respectively,measuredbythestandardflotationmethodinanaqueousNaClsolutioncontainingasmallamountofasurfactantatambienttemperature(20-25°C).The15/4helicesofpoly-L-1(-)andpoly-L-1(+)inthehexagonallatticesrequirethedensitiesof1.102and1.089g/cm3,respectively,whichareingoodagreementwiththeobservedvalues.Whenthenumberofrepeatingunitsperfiberperiodisassumedtobeotherthan15,thecalculateddensitieswereconsiderablydifferentfromtheobserveddensities.Formoredetails,seeSupporting

Information.

J.AM.CHEM.SOC.

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VOL.130,NO.1,2008231

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