下载

0下载券

加入VIP
  • 专属下载券
  • 上传内容扩展
  • 资料优先审核
  • 免费资料无限下载

上传资料

关闭

关闭

关闭

封号提示

内容

首页 CD

CD.pdf

CD

seibo
2012-02-17 0人阅读 举报 0 0 0 暂无简介

简介:本文档为《CDpdf》,可适用于人文社科领域

CirculardichroismFromWikipedia,thefreeencyclopediaCirculardichroism(CD)isthedifferentialabsorptionofleftandrighthandedcircularlypolarizedlightACDSpectrometerisaninstrumentthatrecordsthisphenomenonasafunctionofwavelengthModerninstruments,however,cangenerallyalsorecordCDasafunctionoftemperatureorchemicalenvironment,atseveralwavelengthsThisphenomenonisexhibitedintheabsorptionbandsofanopticallyactivemoleculeCDcanbeusedtohelpdeterminethestructureofmacromolecules(includingthesecondarystructureofproteinsandthehandednessofDNA)ThephenomenonofcirculardichroismwasdiscoveredbytheFrenchphysicistAiméCottoninContentsBasicinformationCircularpolarizationInteractionofcircularlypolarizedlightwithmatterDeltaabsorbanceMolarcirculardichroismApplicationtobiologicalmoleculesExperimentallimitationsMolarellipticitySeealsoReferencesFurtherreadingExternallinksBasicinformationCircularpolarizationLinearlypolarizedlightispolarizedinacertaindirection(thatis,themagnitudeofitselectricfieldvectoroscillatesonlyinoneplane,similartoasinewave)Inofcircularlypolarizedlight,theelectricfieldvectorhasaconstantlength,butrotatesaboutitspropagationdirectionHenceitformsahelixinspacewhilepropagatingIfthisisalefthandedhelix,thelightisreferredtoasleftcircularlypolarized,andviceversaforarighthandedhelixSeetheexternallinksforademonstrativeanimationofthedifferenttypesofelectromagneticwavesInteractionofcircularlypolarizedlightwithmatterTheelectricfieldofalightbeamcausesalineardisplacementofchargewheninteractingwithamolecule,whereasthemagneticfieldofitcausesacirculationofchargeThesetwomotionscombinedresultinahelicaldisplacementwhenlightimpingesonamolecule(bothfieldvectorsinthesameplaceareofthesamedirection,butatdifferentmomentsoftime)Sincecircularlypolarizedlightitselfis"chiral",itinteractsdifferentlywithchiralmoleculesThatis,thetwotypesofcircularlypolarizedlightareabsorbedtodifferentextentsInaCDexperiment,equalamountsofleftandrightcircularlypolarizedlightofaselectedwavelengtharealternatelyradiatedintoa(chiral)sampleOneofthetwopolarizationsisabsorbedmorethantheotherone,andthiswavelengthdependentdifferenceofabsorptionismeasured,yieldingtheCDspectrumofthesampleDuetotheinteractionwiththemolecule,theelectricfieldvectorofthelighttracesoutanellipticalpathafterpassingthroughthesampleDeltaabsorbanceAtagivenwavelength,whereΔAisthedifferencebetweenabsorbanceofleftcircularlypolarized(LCP)andrightcircularlypolarized(RCP)light(thisiswhatisusuallymeasured)MolarcirculardichroismItcanalsobeexpressed,byapplyingBeer'slaw,as:whereεLandεRarethemolarextinctioncoefficientsforLCPandRCPlight,Cisthemolarconcentrationoflisthepathlengthincentimeters(cm)ThenisthemolarcirculardichroismThisintrinsicpropertyiswhatisusuallymeantbythecirculardichroismofthesubstanceApplicationtobiologicalmoleculesIngeneral,thisphenomenonwillbeexhibitedinabsorptionbandsofanyopticallyactivemoleculeAsaconsequence,circulardichroismisexhibitedbybiologicalmolecules,becauseoftheirdextrorotaryandlevorotarycomponentsEvenmoreimportantisthatasecondarystructurewillalsoimpartadistinctCDtoitsrespectivemoleculesTherefore,thealphahelixofproteinsandthedoublehelixofnucleicacidshaveCDspectralsignaturesrepresentativeoftheirstructuresCDiscloselyrelatedtotheopticalrotatorydispersion(ORD)technique,andisgenerallyconsideredtobemoreadvancedCDismeasuredinorneartheabsorptionbandsofthemoleculeofinterest,whileORDcanbemeasuredfarfromthesebandsCD'sadvantageisapparentinthedataanalysisStructuralelementsaremoreclearlydistinguishedsincetheirrecordedbandsdonotoverlapextensivelyatparticularwavelengthsastheydoinORDInprinciplethesetwospectralmeasurementscanbeinterconvertedthroughanintegraltransform(Kramers–Kronigrelation),ifalltheabsorptionsareincludedinthemeasurementsThefarUV(ultraviolet)CDspectrumofproteinscanrevealimportantcharacteristicsoftheirsecondarystructureCDspectracanbereadilyusedtoestimatethefractionofamoleculethatisinthealphahelixconformation,thebetasheetconformation,thebetaturnconformation,orsomeother(egrandomcoil)conformationThesefractionalassignmentsplaceimportantconstraintsonthepossiblesecondaryconformationsthattheproteincanbeinCDcannot,ingeneral,saywherethealphahelicesthataredetectedarelocatedwithinthemoleculeorevencompletelypredicthowmanythereareDespitethis,CDisavaluabletool,especiallyforshowingchangesinconformationItcan,forinstance,beusedtostudyhowthesecondarystructureofamoleculechangesasafunctionoftemperatureoroftheconcentrationofdenaturingagents,egGuanidiniumhydrochlorideorureaInthiswayitcanrevealimportantthermodynamicinformationaboutthemolecule(suchastheenthalpyandGibbsfreeenergyofdenaturation)thatcannototherwisebeeasilyobtainedAnyoneattemptingtostudyaproteinwillfindCDavaluabletoolforverifyingthattheproteinisinitsnativeconformationbeforeundertakingextensiveofandorexpensiveexperimentswithitAlso,thereareanumberofotherusesforCDspectroscopyinproteinchemistrynotrelatedtoalphahelixfractionestimationThenearUVCDspectrum(>nm)ofproteinsprovidesinformationonthetertiarystructureThesignalsobtainedinthenmregionareduetotheabsorption,dipoleorientationandthenatureofthesurroundingenvironmentofthephenylalanine,tyrosine,cysteine(orSSdisulfidebridges)andtryptophanaminoacidsUnlikeinfarUVCD,thenearUVCDspectrumcannotbeassignedtoanyparticularDstructureRather,nearUVCDspectraprovidestructuralinformationonthenatureoftheprostheticgroupsinproteins,eg,thehemegroupsinhemoglobinandcytochromecVisibleCDspectroscopyisaverypowerfultechniquetostudymetal–proteininteractionsandcanresolveindividualddelectronictransitionsasseparatebandsCDspectrainthevisiblelightregionareonlyproducedwhenametalionisinachiralenvironment,thus,freemetalionsinsolutionarenotdetectedThishastheadvantageofonlyobservingtheproteinboundmetal,sopHdependenceandstoichiometriesarereadilyobtainedOpticalactivityintransitionmetalioncomplexeshavebeenattributedtoconfigurational,conformationalandthevicinaleffectsKlewpatinondandViles()haveproducedasetofempiricalrulesforpredictingtheappearanceofvisibleCDspectraforCuandNisquareplanarcomplexesinvolvinghistidineandmainchaincoordinationCDgiveslessspecificstructuralinformationthanXraycrystallographyandproteinNMRspectroscopy,forexample,whichbothgiveatomicresolutiondataHowever,CDspectroscopyisaquickmethodthatdoesnotrequirelargeamountsofproteinsorextensivedataprocessingThusCDcanbeusedtosurveyalargenumberofsolventconditions,varyingtemperature,pH,salinity,andthepresenceofvariouscofactorsCDspectroscopyisusuallyusedtostudyproteinsinsolution,andthusitcomplementsmethodsthatstudythesolidstateThisisalsoalimitation,inthatmanyproteinsareembeddedinmembranesintheirnativestate,andsolutionscontainingmembranestructuresareoftenstronglyscatteringCDissometimesmeasuredinthinfilmsExperimentallimitationsCDhasalsobeenstudiedincarbohydrates,butwithlimitedsuccessduetotheexperimentaldifficultiesassociatedwithmeasurementofCDspectrainthevacuumultraviolet(VUV)regionofthespectrum(nm),wherethecorrespondingCDbandsofunsubstitutedcarbohydrateslieSubstitutedcarbohydrateswithbandsabovetheVUVregionhavebeensuccessfullymeasuredofMeasurementofCDisalsocomplicatedbythefactthattypicalaqueousbuffersystemsoftenabsorbintherangewherestructuralfeaturesexhibitdifferentialabsorptionofcircularlypolarizedlightPhosphate,sulfate,carbonate,andacetatebuffersaregenerallyincompatiblewithCDunlessmadeextremelydiluteeginthemMrangeTheTRISbuffersystemshouldbecompletelyavoidedwhenperformingfarUVCDBorateandOniumcompoundsareoftenusedtoestablishtheappropriatepHrangeforCDexperimentsSomeexperimentershavesubstitutedfluorideforchlorideionbecausefluorideabsorbslessinthefarUV,andsomehaveworkedinpurewaterAnother,almostuniversal,techniqueistominimizesolventabsorptionbyusingshorterpathlengthcellswhenworkinginthefarUV,mmpathlengthsarenotuncommoninthisworkInadditiontomeasuringinaqueoussystems,CD,particularlyfarUVCD,canbemeasuredinorganicsolventsegethanol,methanol,trifluoroethanol(TFE)Thelatterhastheadvantagetoinducestructureformationofproteins,inducingbetasheetsinsomeandalphahelicesinothers,whichtheywouldnotshowundernormalaqueousconditionsMostcommonorganicsolventssuchasacetonitrile,THF,chloroform,dichloromethanearehowever,incompatiblewithfarUVCDItmaybeofinteresttonotethattheproteinCDspectrausedinsecondarystructureestimationarerelatedtotheπtoπ*orbitalabsorptionsoftheamidebondslinkingtheaminoacidsTheseabsorptionbandsliepartlyinthesocalledvacuumultraviolet(wavelengthslessthanaboutnm)ThewavelengthregionofinterestisactuallyinaccessibleinairbecauseofthestrongabsorptionoflightbyoxygenatthesewavelengthsInpracticethesespectraaremeasurednotinvacuumbutinanoxygenfreeinstrument(filledwithpurenitrogengas)Onceoxygenhasbeeneliminated,perhapsthesecondmostimportanttechnicalfactorinworkingbelownmistodesigntherestoftheopticalsystemtohavelowlossesinthisregionCriticalinthisregardistheuseofaluminizedmirrorswhosecoatingshavebeenoptimizedforlowlossinthisregionofthespectrumTheusuallightsourceintheseinstrumentsisahighpressure,shortarcxenonlampOrdinaryxenonarclampsareunsuitableforuseinthelowUVInsteadspeciallyconstructedlampswithenvelopesmadefromhighpuritysyntheticfusedsilicamustbeusedLightfromsynchrotronsourceshasamuchhigherfluxatshortwavelengths,andhasbeenusedtorecordCDdowntonmRecentlytheCDspectrometerattheelectronstorageringfacilityISAattheUniversityofAarhusinDenmarkwasusedtorecordsolidstateCDspectradowntonmAtthequantummechanicallevel,theinformationcontentofcirculardichroismofEllipticalpolarizedlight(purple)iscomposedofunequalcontributionsofright(blue)andleft(red)circularpolarizedlightandopticalrotationareidenticalMolarellipticityAlthoughΔAisusuallymeasured,forhistoricalreasonsmostmeasurementsarereportedindegreesofellipticityMolarcirculardichroismandmolarellipticity,θ,arereadilyinterconvertedbytheequation:Thisrelationshipisderivedbydefiningtheellipticityofthepolarizationas:whereERandELarethemagnitudesoftheelectricfieldvectorsoftherightcircularlyandleftcircularlypolarizedlight,respectivelyWhenERequalsEL(whenthereisnodifferenceintheabsorbanceofrightandleftcircularpolarizedlight),θis°andthelightislinearlypolarizedWheneitherERorELisequaltozero(whenthereiscompleteabsorbanceofthecircularpolarizedlightinonedirection),θis°andthelightiscircularlypolarizedGenerally,thecirculardichroismeffectissmall,sotanθissmallandcanbeapproximatedasθinradiansSincetheintensityorirradiance,I,oflightisproportionaltothesquareoftheelectricfieldvector,theellipticitybecomes:ThenbysubstitutingforIusingBeer'sLawinnaturallogarithmform:Theellipticitycannowbewrittenas:ofSinceΔA<<,thisexpressioncanbeapproximatedbyexpandingtheexponentialsinaTaylorseriestofirstorderandthendiscardingtermsofΔAincomparisonwithunityandconvertingfromradianstodegrees:Thelineardependenceofsoluteconcentrationandpathlengthisremovedbydefiningmolarellipticityas,ThencombiningthelasttwoexpressionwithBeer'sLaw,molarellipticitybecomes:SeealsoCircularpolarizationinnatureDichroismLineardichroismMagneticcirculardichroismOpticalactivityOpticalisomerismOpticalrotationOpticalrotatorydispersionReferences^PAtkinsandJdePaula()ElementsofPhysicalChemistry,thedOxfordUniversityPress^WhitmoreL,WallaceBA()"Proteinsecondarystructureanalysesfromcirculardichroismspectroscopy:methodsandreferencedatabases"Biopolymers():–doi:bip(http:dxdoiorgFbip)PMID(http:wwwncbinlmnihgovpubmed)^GreenfieldNJ()"Usingcirculardichroismspectratoestimateproteinsecondarystructure"Natureprotocols():–doi:nprot(http:dxdoiorgFnprot)PMID(http:wwwncbinlmnihgovpubmed)FurtherreadingAlisonRodgerandBengtNordén,CircularDichroismandLinearDichroism(http:booksgooglecoukbookshl=enid=THeKGChJcC)()OxfordofThispagewaslastmodifiedonNovemberat:TextisavailableundertheCreativeCommonsAttributionShareAlikeLicenseadditionaltermsmayapplySeeTermsofUsefordetailsWikipedia®isaregisteredtrademarkoftheWikimediaFoundation,Inc,anonprofitorganizationContactusUniversityPress,Oxford,UKISBNXFasman,GD,CircularDichroismandtheConformationalAnalysisofBiomolecules()PlenumPress,NewYorkHecht,E,OpticsrdEdition()AddisonWesleyLongman,MassachusettsKlewpatinond,MandViles,JH()EmpiricalrulesforrationalisingvisiblecirculardichroismofCuandNihistidinecomplexes:ApplicationstotheprionproteinFEBSLetters,ExternallinksCircularDichroismexplained(http:wwwaplabcomcirculardichroismhtm)CircularDichroismatUMDNJ(http:wwwumdnjeducdrwjwebindexhtm#software)agoodsiteforinformationonstructureestimationsoftwareElectromagneticwaves(http:wwwenzimhu~sziacddemoedemohtm)AnimatedelectromagneticwavesTheEmanimprogramisateachingresourceforhelpingstudentsunderstandthenatureofelectromagneticwavesandtheirinteractionwithbirefringentanddichroicsamplesAnIntroductiontoCircularDichroismSpectroscopy(http:wwwphotophysicscomcirculardichroismphp)averygoodtutorialoncirculardichroismspectroscopyRetrievedfrom"http:enwikipediaorgwikiCirculardichroism"Categories:Spectroscopyarticlesneedingexpertattention|Polarization|Physicalquantities|Proteinstructureof

用户评价(0)

关闭

新课改视野下建构高中语文教学实验成果报告(32KB)

抱歉,积分不足下载失败,请稍后再试!

提示

试读已结束,如需要继续阅读或者下载,敬请购买!

评分:

/8

VIP

在线
客服

免费
邮箱

爱问共享资料服务号

扫描关注领取更多福利