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1、NANOLETTERSMagneticTweezersMeasurementof2009Vol.9,No.4SingleMoleculeTorque1720-1725AlfredoCeledon,,^,3IlanaM.Nodelman,BridgetWildt,
2、RohitDewan,§PeterSearson,
3、DenisWirtz,§,^GregoryD.Bowman,,^andSeanX.Sun*,,^,#DepartmentofMechanicalEngineering,DepartmentofBiophysics,D
4、epartmentofChemicalandBiomolecularEngineering,DepartmentofMaterialScienceandEngineering,HowardHughesMedicalInstituteGraduateTrainingProgramandInstituteforNanoBioTechnology,andWhitakerBiomedicalEngineeringInstitute,JohnsHopkinsUniVersity,BaltimoreMaryland21218,andDep
5、artamentodeIngenierı´aMeca´nicayMetalu´rgica,PontificiaUniVersidadCato´licadeChile,Santiago,ChileReceivedFebruary26,2009;RevisedManuscriptReceivedMarch11,2009ABSTRACTTorsionalstressinlinearbiopolymerssuchasDNAandchromatinhasimportantconsequencesfornanoscalebiological
6、processes.Wehavedevelopedanewmethodtodirectlymeasuretorqueonsinglemolecules.Usingacylindricalmagnet,wemanipulateanovelprobeconsistingofananorodwitha0.1µmferromagneticsegmentcoupledtoamagneticbead.Weachievecontrolledintroductionofturnsintothemoleculeandprecisemeasure
7、mentoftorqueandmoleculeextensionasafunctionofthenumberofturnsatlowpullingforce.WeshowtorquemeasurementofsingleDNAmoleculesanddemonstrateforthefirsttimemeasurementsofsinglechromatinfibers.DNAandotherlinearbiopolymersaccumulatetorsionalstressThedependenceoftorqueontwist
8、forDNAhasbeenundertheactionofarotaryforceortorque.Thispropertyhasmeasuredwithopticalangulartrappingmethods,9-11andhasimportantbiologicalimplications,forexample,torsionalstressbeenanalyzedfromviscousdragforces.12However,likeaffectstheactionofenzymesthatactonDNA.1-3Fu
9、ndamentalotherbiopolymers,thestructureofDNAissensitivetobiologicalprocessessuchasDNAreplicationandtranscrip-pullingforce,andthesetechniquesemploypullingforcestioninvolvebiologicalnanomachinesthatexerttorqueongreaterthan1pN,sufficientformeltingduplexDNAwhendouble-stra
10、nded(ds)DNAandchromatin.4,5Thus,inordernegativetwistsareintroduced.13DNAandchromatininvivotorevealthemechanismsandenergeticsoftheseprocess
