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1、6928J.Phys.Chem.A2005,109,6928-6933Vacuum-UltravioletElectronicCircularDichroismofL-AlanineinAqueousSolutionInvestigatedbyTime-DependentDensityFunctionalTheoryTakayukiFukuyama,²KoichiMatsuo,²andKunihikoGekko*,²,³DepartmentofMathematicalandLifeSciences,Graduat
2、eSchoolofScience,andHiroshimaSynchrotronRadiationCenter,HiroshimaUniVersity,Higashi-Hiroshima739-8526,JapanReceiVed:April6,2005;InFinalForm:June7,2005Theelectroniccirculardichoism(ECD)ofL-alanineinthevacuum-ultravioletregionwascalculatedforvariousoptimizedstr
3、ucturesusingtime-dependentdensityfunctionaltheory(TDDFT)toassigntheCDspectrumobservedexperimentallyinaqueoussolutiondownto140nm[Matsuo,etal.Chem.Lett.2002,826].ThestructureofL-alanineinvacuowasoptimizedusingdensityfunctionaltheory(DFT)attheB3LYP/6-31G*level.I
4、tshydratedstructurewasoptimizedwithninewatermolecules(sixandthreearoundcarboxylandaminogroups,respectively)usingDFTandacontinuummodel(Onsagermodel).Thedihedralanglesofcarboxylandaminogroupsintheoptimizedhydratedstructuredifferedgreatlyfromthoseinthecrystaland
5、innonhydratedstructuresoptimizedusingacontinuummodelonly.TheECDspectrumcalculatedforthehydratedstructurehadtwosuccessivepositivepeakswithmolarellipticitiesofabout2000degcm2dmol-1ataround205and185nm,whichwereclosetothoseobservedexperimentally.Thesepositivepeak
6、swereattributabletonð*transitionsofthecarboxylgroup,withthelatterpeakalsoinfluencedbytheðð*transitionofthecarboxylgroupthatoriginatesbelow175nm.Asmallnegativepeakobservedataround252nmwasalsopredictedfromthehydratedstructure.Theseresultsdemonstratethatthehydra
7、tedwatermoleculesaroundthezwitterionsplayacrucialroleinstabilizingtheconformationofL-alanineinaqueoussolutionandthatTDDFTisusefulfortheabinitioassignmentofECDspectradowntothevacuum-ultravioletregion.I.Introductionstructure.Thetheoreticalassignmentofthesevacuu
8、m-ultravioletECDspectratakingintoaccounttheelectronictransitionisCirculardichroism(CD)isverysensitivetotheconformationindispensableforunderstandingdetailsoftheconformationofofchiralmolecu
