溶液环境中水对甲酰胺质子迁移影响的密度泛函研究

[Article]物理化学学报(WuliHuaxueXuebao)JulyActaPhys.鄄Chim.Sin.,2006,22(7)：797耀803Received:January10,2006;Revised:February22,2006.Correspondent,E鄄mail:[e-mail];Tel:+86571鄄87952424.国家自然科学基金(20573093,20434020)资助项目鬁EditorialofficeofActaPhysico鄄ChimicaSinica溶液环境中水对甲酰胺质子迁移影响的密度泛函研究梁婉春李浩然鄢胡兴邦韩世钧(浙江大学化学系,杭州310027)摘要甲酰胺酮式和烯醇式的互变异构(FM寅FA)可被视为DNA中生物碱基点突变的一个模型.使用B3LYP密度范函的计算方法,在B3LYP/6鄄311++G鄢鄢的基组条件下运用SCRF溶液模型研究了溶液环境对甲酰胺互变异构的影响.研究表明,从水分子对甲酰胺异构的作用进行划分,分子的周围有三个不同的区域,和气态计算结果一致,在一些区域中,水对异构起到了催化的作用;而在其它区域中,水分子却能阻碍反应的进行,保护酮式,同时溶液环境的存在也使两种作用得到了加强和削弱.溶液状态下的质子转移研究将会对水分子对中心分子的异构化研究提供参考,同时也部分解释了试验中酮式比例高于烯醇式的现象.关键词：甲酰胺,异构化,水的影响,质子转移,SCIPCM中图分类号：O642DensityFunctionalStudiesonFormamide忆sWater鄄influencedProtonTransferinSolutionLIANG,Wan鄄ChunLI,Hao鄄Ran鄢HU,Xing鄄BangHAN,Shi鄄Jun(DepartmentofChemistry,ZhejiangUniversity,Hangzhou310027,P.R.China)AbstractTheinterconversionprocessbetweenketoandenolformsofformamide(FM寅FA)canberegardedasamodelfortautomerizationoflargernucleicacidbase,whichmayberesponsibleforthespontaneouspointmutationinDNA.Thepresentpaperdescribesastudyofstructuraltautomerinterconversionandtherelativestabilizinginfluencesofwaterforformamide鄄H2Oandtheenolformformamidicacid鄄H2Oinaqueoussolution.ThesolventeffectswereconsideredbyexplicitwatermoleculewithinSCRFframework.Inthevicinityofformamide(FM)andformamidicacid(FA),threedifferentregionsareconsidered,fullgeometryoptimizationsofthesecomplexeswerecarriedoutatB3LYP/6鄄311++G鄢鄢usingself鄄consistentisodensitypolarizedcontinuummodel(SCIPCM).Thecalculatedresultsindicatethatwaterintwoofthemcanprotectformamidefromtautomerizing,whileinthethirdoneworksoppositely,exactlythesameasthesituationsingasphase.However,comparingwiththetransitioningasphase,theprotectiveandassistiveeffectsinducedbytheexplicitwateraregreatlyaltered.Thecalculatedresultsshedmorelightonthewater忆sinfluenceontheprotontransferandofferanewinsightintothestructuraltautomerinterconversionofFM.Keywords：Formamide,Tautomerization,Water鄄influenced,Protontransfer,SCIPCMFormamideisthesimplestmemberoftheprimaryamidefamily,anditsstructurecontainsbothcarbonylandaminogroups.Despiteitssimpleness,formamidecontainstheessentialfeaturesofthepeptidelinkageandoftenservesasamodel[1鄄5].Especially,theprotontransfer(PT)processofformamidefromitsketoformtotheenolform(formamidetoformamidicacid,FM→FA)hasattractedtheinterestofmanytheoreticalinvesti-gators[6鄄15],becauseitcanberegardedasamodelfortautomeriza-tionoflargernucleicacidbase,inparticularconnectingwiththeguanineanduracilsystem[16鄄17].Thetautomerizationprocessofthe797ActaPhys.鄄Chim.Sin.(WuliHuaxueXuebao),2006Vol.22nucleicacidsmayberesponsibleforthespontaneouspointmu-tationinDNA[18].Experimentalresultsindicateclearlythattheketoformofformamideisthermodynamicallymorestablethantheenolforminvaporphase,inertenvironment,andpolarmedi-um[19鄄23].Asafundamentalbio鄄model,itisnotsurprisetoextendthecorrespondingtheoreticalresearchtoaqueoussolutionforfor-mamide,becausemanystructuralfeaturesthatarenecessaryforthebiologicalfunctionsofbio鄄moleculesdependontheinterac-tionswiththesurroundingwater.ClassicalMonteCarlo(MC)[24鄄26]aswellasmoleculardynamics(MD)[27鄄30]simulationswereusedtostudythesolvationofformamide.Inordertostudytheprotontransferprocessinaqueoussolution,monohydrate鄄formamidehasbeenstudiedbyquantumchemicalcalculations[6鄄12].EarlyworkperformedbySimonsandco鄄workers[6]foundthatthetau-tomerismofisolatedFM→FAhasaclassicalbarrierof217.4kJ·mol-1ingasphase,whileasinglewatermolaculelowersthebarrierto108.7kJ·mol-1.Theirfurtherworkusedtheabinitiodynamicscalculationsbasedonacanonicalvariationaltransi-tion鄄statetheoryandtherateconstantsofthereactionweregiv-en[7].Kimetal.[11]studiedthesolventeffectonthePESoftheprototropictautomerizationofFMusingself鄄consistentreactionfield(SCRF)model,water鄄assistedanddimer鄄assistedtransitionswerecompared.Recently,Fuetal.[13]havestudiedthesuperiorcatalyticeffectof(H2O)2ingasphase.AlltheaboveworkssupportthatinthePTprocess,besidesservingassolvent,watermoleculealsoplaysamediatorrolewhichgivesoracceptsprotonstopro-motethelong鄄rangePT,however,itisnoticedthatonlyonesiteS2wasstudiedpreviously.Actuallyalotoftheoreticalinvesti-gationsonthesolvationofformamidepointedoutthattherewereseveralpossiblestructuresofformamide鄄watercomplex,andwatermoleculecanbeplacedinS1,S2,andS3(Fig.1),respe-ctively[29鄄33].Wehadinvestigatedtheprotontransferprocessofmonohydratedformamidewithanexplicitwatermoleculeinthreedifferentsites,andwehadfoundthatwaterinS2hastheassistanteffectonthetautomerizationwhereaswatermoleculeinS1andS3canprotecttheFMfromtautomerizingtoFAingasphase[34].ClusterswithW2aremorestablethanthosewithW1orW3,becausetwostronghydrogenbondscanbeformedwithW2,however,onlyastrongaswellasaweakhydrogenbondscanbeformedforclusterwithW1,andforW3,thereisonlyonestronghydrogenbond(Fig.1).Thatiswhytheinteractionbe-tweenFM/FAandW2ismuchstrongerthanthatwithW1orW3.Sincetheelectroncorrelationplaysanimportantroleinde-terminingthecharacteristicsofthePESfortheprotontransferreactioningasphase,itisnecessarytocons