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1、56BiophysicalJournalVolume96January200956–66Diffusion,Exclusion,andSpecificBindinginaLargeChannel:AStudyofOmpFSelectivityInversion†‡††‡AntonioAlcaraz,EkaterinaM.Nestorovich,M.Lido´nLo´pez,ElenaGarcı´a-Gime´nez,SergeyM.Bezrukov,†andVicenteM.Aguilella*†‡DepartamentodeFı
2、´sica,UniversitatJaumeI,12080Castello´n,Spain;andLaboratoryofPhysicalandStructuralBiology,EuniceKennedyShriverNationalInstituteofChildHealthandHumanDevelopment,NationalInstitutesofHealth,Bethesda,Maryland20892ABSTRACTWefindthatmoderatecationicselectivityofthegeneralba
3、cterialporinOmpFinsodiumandpotassiumchloridesolutionsisinversedtoanionicselectivityinconcentratedsolutionsofbarium,calcium,nickel,andmagnesiumchlorides.Toun-derstandtheoriginofthisphenomenon,weconsiderseveralfactors,whichincludethebindingofdivalentcations,electrostat
4、icandstericexclusionofdifferentlychargedanddifferentlysizedions,size-dependenthydrodynamichindrance,electrokineticef-fects,andsignificant‘‘anionic’’diffusionpotentialforbulksolutionsofchloridesofdivalentcations.Thoughallthesefactorscon-tributetothemeasuredselectivityo
5、fthislargechannel,theobservedselectivityinversionismostlyduetothefollowingtwo.First,bindingdivalentcationscompensates,orevenslightlyovercompensates,forthenegativechargeoftheOmpFprotein,whichisknowntobethemaincauseofcationicselectivityinsodiumandpotassiumchloridesolut
6、ions.Second,thehigheranionic(versuscationic)transportrateexpectedforbulksolutionsofchloridesaltsofdivalentcationsistheleadingcauseofthemeasuredan-ionicselectivityofthechannel.Interestingly,athighconcentrationsthebindingofcationsdoesnotshowanypronouncedspec-ificitywith
7、inthedivalentseriesbecausethereversalpotentialsmeasuredintheseriescorrelatewellwiththecorrespondingbulkdiffusionpotentials.Thusourstudyshowsthat,incontrasttothehighlyselectivechannelsofneurophysiologythatemploymostlytheexclusionmechanism,quitedifferentfactorsaccountf
8、ortheselectivityoflargechannels.Theelucidationofthesefactorsisessentialforunderstandinglargechannelselectivityanditsregulationinviv
