ShK(StichodactylahelianthusNeurotoxin)hasbeenisolatedfromthevenomoftheCarribeanseaanemoneStoichactishelianthus.ShKinhibitsvoltage-dependentpotassiumchannels.ItblocksKv1.3(KCNA3)potentlyandalsoKv1.1(KCNA1),Kv1.4(KCNA4)andKv1.6(KCNA6)respectivelywithaKdof11pM,16pM,312pMand165pM.Interestingly,itwasalsodemonstratedthatShKpotentlyinhibitsthehKv3.2bchannelwithanIC50valueofapproximately0.6nM.
Description:
AAsequence:Arg-Ser-Cys3-Ile-Asp-Thr-Ile-Pro-Lys-Ser-Arg-Cys12-Thr-Ala-Phe-Gln-Cys17-Lys-His-Ser-Met-Lys-Tyr-Arg-Leu-Ser-Phe-Cys28-Arg-Lys-Thr-Cys32-Gly-Thr-Cys35-OH
Disulfidebonds:Cys3-Cys35,Cys12-Cys28andCys17-Cys32
Length(aa):35
Formula:C169H274N54O48S7
MolecularWeight:4054.85Da
Appearance:Whitelyophilizedsolid
Solubility:waterandsalinebuffer
CASnumber:165168-50-3
Source:Synthetic
Purityrate:>97%
Reference:
DurablepharmacologicalresponsesfromthepeptideShK-186,aspecificKv1.3channelinhibitorthatsuppressesTcellmediatorsofautoimmunedisease
TheKv1.3channelisarecognizedtargetforpharmaceuticaldevelopmenttotreatautoimmunediseasesandorganrejection.ShK-186,aspecificpeptideinhibitorofKv1.3,hasshownpromiseinanimalmodelsofmultiplesclerosisandrheumatoidarthritis.Here,wedescribethepharmacokinetic-pharmacodynamicrelationshipforShK-186inratsandmonkeys.ThepharmacokineticprofileofShK-186wasevaluatedwithavalidatedhigh-performanceliquidchromatography-tandemmassspectrometrymethodtomeasurethepeptide’sconcentrationinplasma.Theseresultswerecomparedwithsingle-photonemissioncomputedtomography/computedtomographydatacollectedwithan¹¹¹In-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraaceticacid-conjugateofShK-186toassesswhole-bloodpharmacokineticparametersaswellasthepeptide’sabsorption,distribution,andexcretion.AnalysisofthesedatasupportamodelwhereinShK-186isabsorbedslowlyfromtheinjectionsite,resultinginbloodconcentrationsabovetheKv1.3channel-blockingIC₅₀valueforupto7daysinmonkeys.PharmacodynamicstudiesonhumanperipheralbloodmononuclearcellsshowedthatbriefexposuretoShK-186resultedinsustainedsuppressionofcytokineresponsesandmaycontributetoprolongeddrugeffects.Indelayed-typehypersensitivity,chronicrelapsing-remittingexperimentalautoimmuneencephalomyelitis,andpristane-inducedarthritisratmodels,asingledoseofShK-186every2to5dayswasaseffectiveasdailyadmiNISTration.ShK-186’sslowdistributionfromtheinjectionsiteanditslongresidencetimeontheKv1.3channelcontributetotheprolongedtherapeuticeffectofShK-186inanimalmodelsofautoimmunedisease.
TarchaEJ.,etal.(2012)DurablepharmacologicalresponsesfromthepeptideShK-186,aspecificKv1.3channelinhibitorthatsuppressesTcellmediatorsofautoimmunedisease.JPharmacolExpTher.PMID: 22637724
ThebeneficialeffectofblockingKv1.3inthepsoriasiformSCIDmousemodel
TheKv1.3channelisimportantintheactivationandfunctionofeffectormemoryTcells.Recently,specificblockersoftheKv1.3channelhavebeendevelopedasapotentialtherapeuticoptionfordiverseautoimmunediseases.Inpsoriaticlesions,mostlymphocytesarememoryeffectorTcells.TheaimofthepresentstudywastodetecttheexpressionofKv1.3channelsinthesecellsinpsoriaticlesionsaswellasinhumanpsoriasiformskingraftsusingtheseverecombinedimmunodeficient(SCID)mousemodel.HistologicalandimmunohistochemicalstainingforKv1.3expressionandvariousinflammatoryMarkerswasperformedinsectionsobtainedfromsixpsoriaticpatientsand18beige-SCIDmicewithpsoriasiformhumanskingrafts.SixgraftedmiceweretreatedwithStichodactylahelianthusneurotoxin(ShK),aknownKv1.3blocker.TheresultsshowedanincreasednumberofKv1.3+cellsinthepsoriaticskinaswellasinthepsoriasiformskingraftsascomparedwithnormalskinandnormalskingrafts.InjectionsofShKshowedamarkedtherapeuticeffectinthreeofsixpsoriasiformskingrafts.AsignificantlydecreasednumberofKv1.3+cellswasobservedintheresponderscomparedwiththecontrolgrafts.Thispilotstudy,althoughperformedinasmallnumberofmice,revealsthepossIBLebeneficialeffectofKv1.3blockersinpsoriasispatients.
GilharA.,etal.(2011)ThebeneficialeffectofblockingKv1.3inthepsoriasiformSCIDmousemodel.JInvestDermatol.PMID: 20739949
Modelingthebindingofthreetoxinstothevoltage-gatedpotassiumchannel(Kv1.3)
Theconductionpropertiesofthevoltage-gatedpotassiumchannelKv1.3anditsmodesofinteractionwithseveralpolypeptidevenomsareexaminedusingBrowniandynamicssimulationsandmoleculardynamicscalculations.Employinganopen-statehomologymodelofKv1.3,wefirstdeterminecurrent-voltageandcurrent-concentrationcurvesandascertainthatsimulatedresultsaccordwithexperimentalmeasurements.Wetheninvestigate,usingamoleculardockingmethodandmoleculardynamicssimulations,thecomplexesformedbetweentheKv1.3channelandseveralKv-specificpolypeptidetoxinsthatareknowntointerferewiththeconductingmechanismsofseveralclassesofvoltage-gatedK(+)channels.ThedepthsofpotentialofmeanforceencounteredbycharyBDotoxin,α-KTx3.7(alsoknownasOSK1)andShKare,respectively,-19,-27,and-25kT.Thedissociationconstantscalculatedfromtheprofilesofpotentialofmeanforcecorrespondcloselytotheexperimentallydeterminedvalues.Wepinpointtheresiduesinthetoxinsandthechannelthatarecriticalfortheformationofthestablevenom-channelcomplexes.
ChenR.,etal.(2011)Modelingthebindingofthreetoxinstothevoltage-gatedpotassiumchannel(Kv1.3).BiophysJ.PMID: 22261053
BlockadeofT-lymphocyteKCa3.1andKv1.3channelsasnovelimmunosuppressionstrategytopreventkidneyallograftrejection
Currently,thereisanunmetclinicalneedfornovelimmunosuppressiveagentsforlong-termpreventionofkidneytransplantrejectionasalternativestothenephrotoxiccalcineurininhibitorcyclosporine(CsA).RecentstudieshaveshownthatK(+)channelshaveacrucialroleinT-lymphocyteactivity.WeinvestigatedwhethercombinedblockadeoftheT-cellK(+)channelsK(Ca)3.1andK(v)1.3,bothofwhichregulatecalciumsignalingduringlymphocyteactivation,iseffectiveinpreventionofrejectionofkidneyallograftsfromFisherratstoLewisrats.AllrecipientswereinitiallytreatedwithCsA(5mg/kgd)for7days.Inratswithintactallograftfunction,treatmentwascontinuedfor10dayswitheitherCsA(5mg/kgd),oracombinationofTRAM-34(K(Ca)3.1inhibitor;120mg/kgd)plusStichodactylahelianthustoxin(ShK,K(v)1.3inhibitor;80microg/kg3timesdaily),orvehiclealone.Kidneysectionswerestainedwithperiodicacid-Schifforhematoxylin-eosinandhistochemicallyformarkersofmacrophages(CD68),T-lymphocytes(CD43),orcytotoxicT-cells(CD8).OurresultsshowedthattreatmentwithTRAM-34andShKreducedtotalinterstitialmononuclearcellinfiltration(-42%)andthenumberofCD43+T-cells(-32%),cytotoxicCD8+T-cells(-32%),andCD68+macrophages(-26%)inallograftswhencomparedtovehicletreatmentalone.EfficacyofTRAM-34/ShKtreatmentwascomparablewiththatofCsA.Inaddition,novisibleorgandamageorotherdiscernibleadverseeffectswereobservedwiththistreatment.Thus,selectiveblockadeofT-lymphocyteK(Ca)3.1andK(v)1.3channelsmayrepresentanovelalternativetherapyforpreventionofkidneyallograftrejection.
GrgicI.,etal.(2009)BlockadeofT-lymphocyteKCa3.1andKv1.3channelsasnovelimmunosuppressionstrategytopreventkidneyallograftrejection.TransplantProc.PMID: 19715983
MolecularmechanismoftheseaanemonetoxinShKrecognizingtheKv1.3channelexploredbydockingandmoleculardynamicsimulations
ComputationalmethodsareemployedtosimulatetheinteractionoftheseaanemonetoxinShKincomplexwiththevoltage-gatedpotassiumchannelKv1.3frommice.Alloftheavailable20structuresofShKintheProteinDataBankwereconsideredforimprovingtheperformanceoftherigidproteindockingofZDOCK.ThetrADItionalandnovelbindingmodeswereobtainedamongalargenumberofpredictedcomplexesbyusingclusteringanalysis,screeningwithexpertknowledge,energyminimization,andmoleculardynamicsimulations.Thequalityandvalidityoftheresultingcomplexeswerefurtherevaluatedtoidentifyafavorablecomplexstructureby500psmoleculardynamicsimulationsandthechangeofbindingfreeenergieswithacomputationalalaninescanningtechnique.ThenovelandreasonableShK-Kv1.3complexstructurewasfoundtobedifferentfromthetraditionalmodelbyusingtheLys22residuetoblockthechannelpore.FromtheresultingstructureoftheShK-Kv1.3complex,ShKmainlyassociatesthechanneloutervestibulewithitssecondhelicalsegment.StructuralanalysisfirstrevealedthattheLys22residuesidechainoftheShKpeptidejusthangsbetweenCandDchainsoftheKv1.3channelinsteadofphysicallyblockingthechannelpore.TheobviouslossoftheShKSer20AlaandTyr23AlamutantbindingABIlitytotheKv1.3channeliscausedbytheconformationalchange.ThefivehydrogenbondsbetweenArg24inShKandH404(A)andD402(D)inKv1.3makeArg24themostcrucialforitsbindingtotheKv1.3channel.BesidesthedetailedinteractionbetweenShKandKv1.3attheatomlevel,thesignificantconformationalchangeinducedbytheinteractionbetweentheShKpeptideandtheKv1.3channel,accompaniedbythegradualdecreaseofbindingfreeenergies,stronglyimpliesthatthebindingoftheShKpeptidetowardtheKv1.3channelisadynamicprocessofconformationalrearrangementandenergystabilization.AllofthesecanacceleratethedevelopmentofShKstructure-basedimmunosuppressants.
JinL,WuY.(2007)MolecularmechanismoftheseaanemonetoxinShKrecognizingtheKv1.3channelexploredbydockingandmoleculardynamicsimulations.JChemInfModel. PMID: 17718553
K+channelblockers:noveltoolstoinhibitTcellactivationleadingtospecificimmunosuppression
DuringthelasttwodecadessincetheidentificationandcharacterizationofTcellpotassiumchannelsgreatadvanceshavebeenmadeintheunderstandingoftheroleofthesechannelsinTcellfunctions,especiallyinantigen-inducedactivation.TheirlimitedtissuedistributionandtherecentdiscoverythatdifferentTcellsubtypescarryingoutdistinctimmunefunctionsshowspecificexpressionlevelsofthesechannelshavemadeTcellpotassiumchannelsattractivetargetsforimmunomodulatorydrugs.Manytoxinsofvariousanimalspeciesandastructurallydiversearrayofsmallmoleculesinhibitingthesechannelswithvaryingaffinityandselectivitywerefoundandtheirsuccessfuluseinimmunosuppressioninvivowasalsodemonstrated.Betterunderstandingofthetopologicaldifferencesbetweenpotassiumchannelpores,detailedknowledgeoftoxinandsmall-moleculestructuresandtheidentificationofthebindingsitesofblockingcompoundsmakeitpossibletoimprovetheselectivityandaffinityoftheleadcompoundsbyintroducingmodificationsbasedonstructuralinformation.Inthisreviewthebasicpropertiesandphysiologicalrolesofthevoltage-gatedKv1.3andtheCa2+-activatedIKCa1potassiumchannelsarediscussedalongwithanoverviewofcompoundsinhibitingthesechannelsandapproachesaimingatproducingmoreefficientmodulatorsofimmunefunctionsforthetreatmentofdiseaseslikesclerosismultiplexandtypeIdiabetes.
PanyiG,etal.(2006)K+channelblockers:noveltoolstoinhibitTcellactivationleadingtospecificimmunosuppression.CurrPharmDes. PMID: 16787250
Stichodactylahelianthuspeptide,apharmacologicaltoolforstudyingKv3.2channels
Voltage-gatedpotassium(Kv)channelsregulatemanyphysiologicalfunctionsandrepresentimportanttherapeutictargetsinthetreatmentofseveralclinicaldisorders.Althoughsomeofthesechannelshavebeenwell-characterized,thestudyofothers,suchasKv3channels,hasbeenhinderedbecauseoflimitedpharmacologicaltools.ThecurrentstudywasinitiatedtoidentifypotentblockersoftheKv3.2channel.Chinesehamsterovary(CHO)-K1cellsstablyexpressinghumanKv3.2b(CHO-K1.hKv3.2b)wereestablishedandcharacterized.Stichodactylahelianthuspeptide(ShK),isolatedfromS.helianthusvenomandaknownhigh-affinityblockerofKv1.1andKv1.3channels,wasfoundtopotentlyinhibit86Rb+effluxfromCHO-K1.hKv3.2b(IC50approximately0.6nM).InelectrophysiologicalrecordingsofKv3.2bchannelsexpressedinXenopuslaevisoocytesorinplanarpatch-clampstudies,ShKinhibitedhKv3.2bchannelswithIC50valuesofapproximately0.3and6nM,respectively.DespitethepresenceofKv3.2proteininhumanpancreaticbetacells,ShKhasnoeffectontheKvcurrentofthesecells,suggestingthatitisunlikelythathomotetramericKv3.2channelscontributesignificantlytothedelayedrectifiercurrentofinsulin-secretingcells.InmousecorticalGABAergicfast-spikinginterneurons,however,applicationofShKproducedeffectsconsistentwiththeblockadeofKv3channels(i.e.,anincreaseinactionpotentialhalf-width,adecreaseintheamplitudeoftheactionpotentialafterhyperpolarization,andadecreaseinmaximalfiringfrequencyinresponsetodepolarizingcurrentinjections).Takentogether,theseresultsindicatethatShKisapotentinhibitorofKv3.2channelsandmayserveasausefulpharmacologicalprobeforstudyingthesechannelsinnativepreparations.
YanL.,etal.(2005)Stichodactylahelianthuspeptide,apharmacologicaltoolforstudyingKv3.2channels.MolPharmacol.PMID: 15709110
TargetingeffectormemoryTcellswithaselectivepeptideinhibitorofKv1.3channelsfortherapyofautoimmunediseases
Thevoltage-gatedKv1.3K(+)channelisanoveltargetforimmunomodulationofautoreactiveeffectormemoryT(T(EM))cellsthatplayamajorroleinthepathogenesisofautoimmunediseases.WedescribethecharacterizationofthenovelpeptideShK(L5)thatcontainsl-phosphotyrosinelinkedviaanine-atomhydrophiliclinkertotheNterminusoftheShKpeptidefromtheseaanemoneStichodactylahelianthus.ShK(L5)isahighlyspecificKv1.3blockerthatexhibits100-foldselectivityforKv1.3(K(d)=69pM)overKv1.1andgreaterthan250-foldselectivityoverallotherchannelstested.ShK(L5)suppressestheproliferationofhumanandratT(EM)cellsandinhibitsinterleukin-2productionatpicomolarconcentrations.NaiveandcentralmemoryhumanTcellsareinitially60-foldlesssensitivethanT(EM)cellstoShK(L5)andthenbecomeresistanttothepeptideduringactivationbyup-regulatingthecalcium-activatedK(Ca)3.1channel.ShK(L5)doesnotexhibitinvitrocytotoxicityonmammaliancelllinesandisnegativeintheAmestest.Itisstableinplasmaandwhenadministeredoncedailybysubcutaneousinjection(10mug/kg)attains“steadystate”bloodlevelsofapproximately300pM.ThisregimendoesnotcausecardiactoxicityassessedbycontinuousEKGmonitoringanddoesnotalterclinicalchemistryandhematologicalparametersafter2-weektherapy.ShK(L5)preventsandtreatsexperimentalautoimmuneencephalomyelitisandsuppressesdelayedtypehypersensitivityinrats.ShK(L5)mightproveusefulfortherapyofautoimmunedisorders.
BeetonC.,etal.(2005)TargetingeffectormemoryTcellswithaselectivepeptideinhibitorofKv1.3channelsfortherapyofautoimmunediseases.MolPharmacol. PMID: 15665253
PotassiumchannelblockadebytheseaanemonetoxinShKforthetreatmentofmultiplesclerosisandotherautoimmunediseases
Expressionofthetwolymphocytepotassiumchannels,thevoltage-gatedchannelKv1.3andthecalciumactivatedchannelIKCa1,changesduringdifferentiationofhumanTcells.WhileIKCa1isthefunctionallydominantchannelinnaiveand“early”memoryTcells,Kv1.3iscrucialfortheactivationofterminallydifferentiatedeffectormemory(TEM)Tcells.BecauseoftheinvolvementofTEMcellsinautoimmuneprocesses,Kv1.3isregardedasapromisingtargetforthetreatmentofT-cellmediatedautoimmunediseasessuchasmultiplesclerosisandthepreventionofchronictransplantrejection.ShK,a35-residuepolypeptidetoxinfromtheseaanemone,Stichodactylahelianthus,blocksKv1.3atlowpicomolarconcentrations.ShKadoptsacentralhelix-kink-helixfold,andalanine-scanningandothermutagenesisstudieshavedefineditschannel-bindingsurface.ModelshavebeendevelopedofhowthistoxineffectsK+-channelblockadeandhowitsdockingconfigurationmightdifferinShK-Dap22,whichcontainsasinglesidechainsubstitutionthatconfersspecificityforKv1.3blockade.ShK,ShK-Dap22andtheKv1.3blockingscorpiontoxinkaliotoxinhavebeenshowntopreventandtreatexperimentalautoimmuneencephalomyelitisinrats,amodelformultiplesclerosis.AfluoresceinatedanalogofShK,ShK-F6CA,hasbeendeveloped,whichallowsthedetectionofactivatedTEMcellsinhumanandanimalbloodsamplesbyflowcytometryandthevisualizationofKv1.3channeldistributioninlivingcells.ShKanditsanalogsarecurrentlyundergoingfurtherevaluationasleadsinthedevelopmentofnewbiopharmaceuticalsforthetreatmentofmultiplesclerosisandotherT-cellmediatedautoimmunedisorders.
NortonRS.,etal.(2004)PotassiumchannelblockadebytheseaanemonetoxinShKforthetreatmentofmultiplesclerosisandotherautoimmunediseases.CurrMedChem.PMID: 15578998
SolutionstructureofShKtoxin,anovelpotassiumchannelinhibitorfromaseaanemone
AnessentialbindingsurfaceforShKtoxininteractionwithratbrainpotassiumchannels.
An“Alascan”analysisofShKtoxin,a35-residuebasicpeptidepossessingthreedisulfidebonds,identifiessevensidechainswhichinfluencebindingtobraindelayedrectifierpotassiumchannels.Additionalanalogsweresynthesizedandtestedtofurtherdeciphertherolesoftheseresidues,particularlyTyr23.Theinhibitoryeffectsoftheseanalogson125I-labeleddendrotoxinbindingtoratbrainmembranesshowedthatreplacementofTyr23withAladrasticallyloweredtheaffinityofthetoxinfortheKv1.2channels.AlasubstitutionofPhe27reducedpotencymorethan15-fold.MonosubstitutedAlaanalogsforIle7,Ser20,orLys30eachdisplayed5-foldreductionsinpotency.Thus,aromaticityatposition23isimportantforeffectivedelayedrectifierbrainKchannelbinding.Incontrast,thearomaticresidueatposition27wasnotcritical,sincecyclohexylalaninesubstitutionincreasedaffinity.ThesolutionstructureofShKtoxinclustersIle7,Arg11,Ser20,Lys22,Tyr23,andPhe27incloseproximity,formingthepotassiumchannelbindingsurfaceofthetoxin.WeproposeanessentialbindingsurfaceonthetoxininwhichLys22andTyr23aremajorcontributors,throughionicandaromatic(hydrophobic)interactions,withthepotassiumchannel.
PenningtonMW.,etal.(1996)AnessentialbindingsurfaceforShKtoxininteractionwithratbrainpotassiumchannels.Biochemistry. PMID: 8987971
CharacterizationofapotassiumchanneltoxinfromtheCaribbeanSeaanemoneStichodactylahelianthus
Apeptidetoxin,ShK,thatblocksvoltage-dependentpotassiumchannelswasisolatedfromthewholebodyextractoftheCaribbeanseaanemoneStichodactylahelianthus.ItcompeteswithdendrotoxinIandalpha-dendrotoxinforbindingtosynaptosomalmembranesofratbrain,facilitiesacetylcholinereleaseatanavianneuromuscularjunctionandsuppressesK+currentsinratdorsalrootganglionneuronesinculture.ItsaminoacidsequenceisR1SCIDTIPKS10RCTAFQCKHS20MKYRLSFCRK30TCGTC35.ThereisnohomologywithotherK+channel-blockingpeptides,exceptforBgKfromtheseaanemoneBunodosomagranulifera.ShKandBgKappeartobeinadifferentstructuralclassfromothertoxinsaffectingK+channels.
Castaneda,O.,etal.(1995)CharacterizationofapotassiumchanneltoxinfromtheCaribbeanSeaanemoneStichodactylahelianthus,Toxicon. PMID: 7660365
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1.直接用固体磷酸钠配制成50mM的磷酸钠溶液,再调pH到7.4;(我们试着用这个做了下,发现挂不上柱)
2.配置磷酸钠盐缓冲液:按NaH2PO4:Na2HPO4以19:81的摩尔比配制成pH7.4的缓冲液?(附一张百度出来的配方
)
3.如果是磷酸钠盐缓冲液,可以直接将50mM的NaH2PO4的水溶液用NaOH调成pH7.4吗?
再者,2和3这两个方法配制的磷酸钠盐缓冲液有什么区别?最终效果是一样的吗?如果不一样,有什么理论的知识支撑呢?个人感觉是分析化学中酸碱理论中的缓冲液那里的知识。求帮忙解答这些疑问。
另外,我还想问一下,pH对于Ni柱对His-tagged的蛋白的分离纯化影响大吗?是怎么影响的?谢谢大家了!
由弱酸及其盐、弱碱及其盐组成的混合溶液,能在一定程度上抵消、减轻外加强酸或强碱对溶液酸碱度的影响,从而保持溶液的pH值相对稳定。这种溶液称为缓冲溶液。
是否可以理解为纯化水得PH范围为6.3-7.6?能否直接用pH计测量?谢谢!
pH(1)=pKa+lg[c(CH₃COONa)/c(CH₃COOH)]=pKa=4.74
通HCl后,溶液是c(CH₃COOH)=0.2mol/L、c(NaCl)=0.1mol/L的混合溶液,溶液pH按照弱酸溶液pH的求法求.
c(H⁺)=√[Ka*c(CH₃COOH)]=√(10^-4.74*0.2)=0.00191(mol/L)(采用了近似公式)
pH(2)=-lg{c(H⁺)}=2.72
两个pH求得,那么pH的变化量也就可得了.pH的变化量=|pH(2)-pH(1)|=|2.72-4.74|=2.02
1)PH缓冲溶液作用原理和pH值
当往某些溶液中加入一定量的酸和碱时,有阻碍溶液pH变化的作用,称为缓冲作用,这样的溶液叫做缓冲溶液.弱酸及其盐的混合溶液(如HAc与NaAc),弱碱及其盐的混合溶液(如NH3·H2O与NH4Cl)等都是缓冲溶液.
由弱酸HA及其盐NaA所组成的缓冲溶液对酸的缓冲作用,是由于溶液中存在足够量的碱A-的缘故.当向这种溶液中加入一定量的强酸时,H离子基本上被A-离子消耗:
所以溶液的pH值几乎不变;当加入一定量强碱时,溶液中存在的弱酸HA消耗OH-离子而阻碍pH的变化.
2)PH缓冲溶液的缓冲能力
在缓冲溶液中加入少量强酸或强碱,其溶液pH值变化不大,但若加入酸,碱的量多时,缓冲溶液就失去了它的缓冲作用.这说明它的缓冲能力是有一定限度的.
缓冲溶液的缓冲能力与组成缓冲溶液的组分浓度有关.0.1mol·L-1HAc和0.1mol·L-1NaAc组成的缓冲溶液,比0.01mol·L-1HAc和0.01mol·L-1NaAc的缓冲溶液缓冲能力大.关于这一点通过计算便可证实.但缓冲溶液组分的浓度不能太大,否则,不能忽视离子间的作用.
组成缓冲溶液的两组分的比值不为1∶1时,缓冲作用减小,缓冲能力降低,当c(盐)/c(酸)为1∶1时△pH最小,缓冲能力大.不论对于酸或碱都有较大的缓冲作用.缓冲溶液的pH值可用下式计算:
此时缓冲能力大.缓冲组分的比值离1∶1愈远,缓冲能力愈小,甚至不能起缓冲作用.对于任何缓冲体系,存在有效缓冲范围,这个范围大致在pKaφ(或pKbφ)两侧各一个pH单位之内.
弱酸及其盐(弱酸及其共轭碱)体系pH=pKaφ±1
弱碱及其盐(弱碱及其共轭酸)体系pOH=pKbφ±1
例如HAc的pKaφ为4.76,所以用HAc和NaAc适宜于配制pH为3.76~5.76的缓冲溶液,在这个范围内有较大的缓冲作用.配制pH=4.76的缓冲溶液时缓冲能力最大,此时(c(HAc)/c(NaAc)=1.
3)PH缓冲溶液的配制和应用
为了配制一定pH的缓冲溶液,首先选定一个弱酸,它的pKaφ尽可能接近所需配制的缓冲溶液的pH值,然后计算酸与碱的浓度比,根据此浓度比便可配制所需缓冲溶液.
以上主要以弱酸及其盐组成的缓冲溶液为例说明它的作用原理、pH计算和配制方法.对于弱碱及其盐组成的缓冲溶液可采用相同的方法.
PH缓冲溶液在物质分离和成分分析等方面应用广泛,如鉴定Mg2离子时,可用下面的反应:
白色磷酸铵镁沉淀溶于酸,故反应需在碱性溶液中进行,但碱性太强,可能生成白色Mg(OH)2沉淀,所以反应的pH值需控制在一定范围内,因此利用NH3·H2O和NH4Cl组成的缓冲溶液,保持溶液的pH值条件下,进行上述反应.
:)
我在做一细菌不同酸碱度生长状况时,发现这些奇怪现象:pH=3的培养基灭菌(TSB液体培养基)灭菌后pH上升到到9.2!而原来pH=9.0的降到8.7(基本没多少变化),请问各位大侠,这是什么原因?
一般做不同酸碱度生长实验时,该如何才能防止pH在湿热灭菌后基本不变化?
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