- Email: [email protected]
Single-molecule force spectroscopy on polysaccharides by AFM – nanomechanical fingerprint of α-(1,4)-linked polysaccharides
21 May 1999
Chemical Physics Letters 305 Ž1999. 197–201
Single-molecule force spectroscopy on polysaccharides by AFM – nanomechanical fingerprint of a- ž1,4 /-linked polysaccharides Hongbin Li a
a,b
, Matthias Rief a , Filipp Oesterhelt a , Hermann E. Gaub Xi Zhang b, Jiacong Shen b
a,)
,
Lehrstuhl fur Amalienstr. 54, D-80799 Munchen, Germany ¨ Angewandte Physik, Ludwig-Maximilians-UniÕersitat ¨ Munchen, ¨ ¨ b Key Lab for Supramolecular Structure and Spectroscopy, Jilin UniÕersity, Changchun 130 023, PR China Received 23 November 1998; in final form 26 March 1999
Abstract AFM-based single-molecule force spectroscopy was employed to measure the nanomechanical properties of 1,4-linked polysaccharides. Single-molecule force spectroscopy clearly reflected the difference in the mechanical properties of a-Ž1,4.and b-Ž1,4.-linked polysaccharides. A force-induced chair–twist boat conformational transition in a-Ž1,4.-linked polysaccharides was discovered. This chair–twist boat conformational transition is a nanomechanical fingerprint of a-Ž1,4.-linked polysaccharides. q 1999 Elsevier Science B.V. All rights reserved.
1. Introduction During its maturation, atomic force microscopy ŽAFM. has evolved into a versatile platform for experiments with individual molecules w1–3x. Beyond the topographic imaging, AFM has been used to measure and map many types of surface forces and local mechanical properties. A wealth of new information about single molecules has provided us with new insights into intermolecular forces, and the underlying molecular mechanism w4–7x. Recently, a new technique – single-molecule force spectroscopy by AFM – has been implemented and opens the possibility for directly measuring the deformation of single polymer chains w8–13x. It expands the existing
spectrum of single-molecule techniques w14–19x towards lower forces and smaller molecules. Polysaccharides are essential components of all living organisms and are the most abundant class of biological molecules. The mechanical properties of polysaccharides and their structural origin are of great interest, but these studies have lagged well behind those of proteins and nucleic acids w20x. In this Letter, single-molecule force spectroscopy on a-Ž1,4.-linked glycan Žcarboxymethyl amylose and heparin. was carried out and a nanomechanical fingerprint of a-Ž1,4.-linked glycan was reported and correlated to their molecular structure.
2. Experimental
)
Corresponding author. E-mail: [email protected]
Carboxymethyl amylose Žsodium salt, catalogue number C4947. and carboxymethyl cellulose Žsodium
0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 9 . 0 0 3 8 9 - 9
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H. Li et al.r Chemical Physics Letters 305 (1999) 197–201
salt, catalogue number C5013. were purchased from Sigma ŽDeisenhofen, Germany., heparin Žsodium salt, catalogue number 51536. was purchased from Fluka ŽBuchs, Switzerland.. In all preparation steps and experiments, PBS buffer Žphosphate buffered saline, 150 mM NaCl, pH 7.4. was used. The samples were allowed to adsorb onto a clean glass slide from a 0.5 ml drop of 1 mgrml solution Žincubated overnight.. Prior to the experiment, the sample was rinsed with PBS buffer. Experiments were conducted at ambient temperature and on a home-built AFM arrangement customized for force measurements. Si 3 N4 cantilevers from Park Scientific Instruments ŽPark, Sunnyvale, CA. were used. The spring constants of the cantilevers were calibrated by measuring their thermal excitation w21,22x. For doing the force measurements, the tip was brought into contact with the sample and kept in contact for several seconds, allowing the individual polymers to adsorb on to the tip Žfor details, see Refs. w8,9,11x.. Upon separation of the tip and the sample, the polymer was stretched. The cantilever deflection–extension curve was recorded and converted into force extension curves. The coupling of the polysaccharides to the tip in this study was based on a non-specific interaction, it has been shown in various studies that this non-specific attachment can hold forces of up to nanonewtons w8,9,11x.
3. Results and discussion Carboxymethyl amylose ŽCM-amylose. is a linear polymer of several thousand glucose residues linked by a-D-1,4-glucosidic bonds, its structure is shown in Fig. 1. Typical force extension curves of CMamylose filaments are shown in Fig. 2a. Since the polysaccharide is polydisperse and we have no control over the point at which the polymer chain is picked up by the tip, the contour lengths of the polymer being stretched between the tip and the substrate varies, as can be seen in Fig. 2a. In spite of the different contour lengths, all force curves exhibit the same deformation characteristics: firstly, the force rises monotonically with a Kuhn length of 0.54 nm and a segment elasticity of 11 000 pNrnm, then the deformation shows a plateau at ; 300 pN followed by a stiffening of the polymer Žsegment elasticity of
Fig. 1. Molecular structure of: Ža. carboxymethyl amylose; Žb. carboxymethyl cellulose; and Žc. heparin. For clarity, the sodium counter ions are omitted.
the polymer after the plateau is 28 000 pNrnm. 1. The yield strength can be as high as 1.8 nN. Force curves of different contour lengths were normalized according to their contour lengths w9,11x,
1 The force curves can be fitted with an extended Freely-JointChain model w8,15x, the Kuhn length and segment elasticity were obtained from the fit parameters.
H. Li et al.r Chemical Physics Letters 305 (1999) 197–201
Fig. 2. Force curves of CM-amylose in PBS buffer: Ža. different force curves of CM-amylose with different contour lengths and Žb. superposition of different normalized curves of Ža..
superimposed and plotted in Fig. 2b. The superposition of these curves clearly shows that all the elastic properties of CM-amylose filaments scale linearly with their lengths, and all filaments show a plateau at the same force. This result was reproduced for hundreds of CM-amylose filaments measured with different cantilevers in different experiments. This finding corroborates the view that predominantely single polymer filaments are stretched and the deformation of individual polymer chains is measured. The plateau is the dominating feature of the single-molecule force spectra of CM-amylose. This plateau is accompanied by an elongation of 0.08 nm per glucose residue and is followed by an increase of the stiffness of the polymer chain, indicating that a force-induced conformation transition occurred in this force regime. From the force spectrum, we can estimate the energy needed to induce this conformational transition to be ; 7.3 kT per glucose residue Ž4.36 kcalrmol.. In previous works w8,9,12x, we showed that it is possible to manipulate the same molecules repeat-
199
edly. By keeping the force lower than the rupture force, here we repeatedly stretched and relaxed the same CM-amylose filament and recorded the deformation curves. Shown in Fig. 3 is a series of consecutive traces on the same CM-amylose filament. No hysteresis between the stretching and relaxing traces was observed. This clearly indicates that the forceinduced conformational transition is a fully reversible process, which confirms that the transition is a purely elastic property of CM-amylose itself. The force-induced conformational transition is a unique property of CM-amylose and is thus expected to be closely related to the molecular structure of CM-amylose. For comparison, single-molecule force spectroscopy was carried out on carboxymethyl cellulose ŽCM-cellulose.. CM-cellulose has a b-Ž1,4.linked glucosidic backbone and is a structural isomer of CM-amylose Žsee Fig. 1.. In Fig. 4, the upper curve is the normalized force curve of single CMcellulose chain. Notably, no plateau is present, the force curve only shows a sharp increase in force when the extension approaches the contour length of the molecule, and the Kuhn length is ; 4 nm and the segment elasticity is ; 50 000 pNrnm which is much stiffer than CM-amylose. CM-amylose and CM-cellulose have the same chemical compositions; the structural difference is the linkage. The difference in the single-molecule force spectra of CM-amylose and CM-cellulose clearly reflects the great influence of the linkage on
Fig. 3. Subsequent deformation curves of the same CM-amylose filament.
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H. Li et al.r Chemical Physics Letters 305 (1999) 197–201
Fig. 4. Normalized force curves of CM-cellulose Župper curve. and heparin Žlower curve..
the mechanical properties: the b-glucosidic linkage of the cellulose causes each successive glucose residue to flip 1808 with respect to the preceding residue so that cellulose easily assumes a stiff, extended conformation w23x which results in the long Kuhn length and high segment elasticity; in contrast, a-glycosidic bonds in amylose can rotate easily to adopt a flexible coiled conformation w23x. The plateau presenting only in CM-amylose clearly indicates that the plateau is an inherent property of the a-Ž1,4. linkage. Hence, the force-induced conformational transition manifests itself as a nanomechanical fingerprint of an a-Ž1,4.-linked glycan. The single-molecule force spectrum of heparin further confirms the above conclusion. As an aŽ1,4.-linked glycan, heparin is a variably sulfated glycosaminoglycan that consists predominantly of alternating a Ž1,4.-linked residues of D-glucuronate2-sulfate and N-sulfo-D-glucosamine-6-sulfate w23x Žas shown in Fig. 1.. Its structure is similar to that of CM-amylose, except for the substituents. As shown in Fig. 4 Žlower curve., heparin also exhibits a plateau in the force curve but at 700 pN, compared to 300 pN for CM-amylose. The width of the plateau is exactly the same as that of CM-amylose. Moreover, the stiffness of heparin after the plateau is almost the same as that of CM-amylose. This finding
strongly indicates that the plateaus of CM-amylose and heparin originate from the same conformational transition. The difference in the height of the plateaus reflects the influence of the different substituents on the conformational transition. Recent molecular dynamic simulation ŽMD. provides us with the molecular details of this force-induced conformational transition process. The MD simulation clearly reflects the difference in the mechanical properties of a-Ž1,4.- and b-Ž1,4.-linked polysaccharides, and reveals that this force-induced conformational transition in a-Ž1,4.-linked glycan results from the chair–twist boat conformational transition of individual glucose residues in the polymer backbone w24x. The chair and twist boat are two stable conformations of the glucose ring, and the transition between them is a common stereochemical phenomenon w25–27x. However, this is the first report of a force-induced chair–twist boat conformational transition 2 . It is noteworthy that in the forceinduced chair–twist boat conformational transition, the energy difference between the chair and twist boat conformers, estimated from the force spectra of CM-amylose, agrees well with the value obtained from the free glucose monomers in solution. It is also important to point out that the force-induced chair–twist boat transition is accompanied by an elongation of the molecules and dramatically improves the polymers’ ductility. It is still a speculation that this force-induced conformational transition has relevance to its biological function. In summary, single-molecule force spectroscopy clearly reveals the nanomechanical fingerprint of an a-Ž1,4.-linked glycan: a force-induced chair–twist boat conformational transition, which cannot be measured by conventional methods. This study has demonstrated the utility of AFM for investigating the conformational changes of polymers, which may open a new field for studying their stereochemistry and provide new insight into the nanomechanical properties of biologically important polysaccharides and processes.
2 Note: during the revision of this Letter, a paper about a similar force-driven conformational transition in polysaccharides was published by Marszalek et al. w28x.
H. Li et al.r Chemical Physics Letters 305 (1999) 197–201
References w1x G. Binnig, C.F. Quate, C. Gerber, Phys. Rev. Lett. 56 Ž1986. 930. w2x M. Radmacher, M. Fritz, H.G. Hansma, P.K. Hansma, Science 265 Ž1994. 1577. w3x P. Hinterdorfer, W. Baumgartner, H.J. Gruber, K. Schilcher, H. Schindler, Proc. Natl. Acad. Sci. USA 93 Ž1996. 3477. w4x E.-L. Florin, V.T. Moy, H.E. Gaub, Science 264 Ž1994. 415. w5x V.T. Florin, E.-L. Florin, H.E. Gaub, Science 266 Ž1994. 257. w6x G.U. Lee, L.A. Chrisey, R.J. Colton, Science 266 Ž1994. 771. w7x U. Dammer, M. Hegner, D. Anselmetti, P. Wagner, M. Dreier, W. Huber, H.-J. Guentherodt, Biophys. J. 93 Ž1995. 3477. w8x M. Rief, F. Oesterhelt, B. Heymann, H.E. Gaub, Science 275 Ž1997. 1295. w9x M. Rief, M. Gautel, F. Oesterhelt, J. Fernandez, H.E. Gaub, Science 276 Ž1997. 1109. w10x A.F. Oberhauser, P.E. Marszalek, H.P. Erickson, J.M. Fernandez, Nature ŽLondon. 393 Ž1998. 1818. w11x M. Rief, M. Gautel, A. Schemmel, H.E. Gaub, Biophys. J. 75 Ž1998. 3008. w12x H.B. Li, M. Rief, F. Oesterhelt, H.E. Gaub, Adv. Mater. 10 Ž1998. 316. w13x H.B. Li, W.K. Zhang, X. Zhang, J.C. Shen, B.B. Liu, C.X. Gao, G.T. Zou, Macromol. Rapid Commun. 19 Ž1998. 609.
201
w14x S.B. Smith, L. Finzi, C. Bustamate, Science 258 Ž1992. 1122. w15x S.B. Smith, Y. Cui, C. Bustamate, Science 271 Ž1996. 795. w16x P. Cluzel, A. Lebrun, C. Heller, R. Lavery, J.L. Viovy, D. Chatenay, F. Caron, Science 271 Ž1996. 792. w17x S.R. Quate, H. Babcock, S. Chu, Nature ŽLondon. 388 Ž1997. 151. w18x K. Svoboda, C.F. Shmidt, B.J. Schnapp, S.M. Block, Nature ŽLondon. 365 Ž1993. 721. w19x J.T. Finer, R.M. Simmons, J.A. Spudich, Nature ŽLondon. 368 Ž1994. 113. w20x A.J. Furth, Lipids and Polysaccharides in Biology, Edward Arnold, London, 1970. w21x H.J. Butt, M. Jaschke, Nanotechnology 6 Ž1995. 1. w22x E.L. Florin, M. Ludwig, M. Rief, H.E. Gaub, Biosens. Bioelectron. 10 Ž1995. 895. w23x D. Voet, J.G. Voet, Biochemistry, 2nd ed., Wiley, New York, 1995. w24x B. Heymann, H. Grubmuller, Chem. Phys. Lett. 305 Ž1999. ¨ 202, in this issue. w25x J.R. Snyder, A.S. Seriann, J. Org. Chem. 51 Ž1986. 2694. w26x D.H.R. Barton, Science 539 Ž1970. 539. w27x A.D. French, J.W. Brady, Computer Modeling of Carbohydrate Molecules, Am. Chem. Soc., Washington, DC, 1990. w28x P.M. Marszalek, A.F. Oberhauser, Y.P. Pang, J.M. Fernandez, Nature ŽLondon. 396 Ž1998. 661.
Chemical Physics Letters 305 Ž1999. 197–201
Single-molecule force spectroscopy on polysaccharides by AFM – nanomechanical fingerprint of a- ž1,4 /-linked polysaccharides Hongbin Li a
a,b
, Matthias Rief a , Filipp Oesterhelt a , Hermann E. Gaub Xi Zhang b, Jiacong Shen b
a,)
,
Lehrstuhl fur Amalienstr. 54, D-80799 Munchen, Germany ¨ Angewandte Physik, Ludwig-Maximilians-UniÕersitat ¨ Munchen, ¨ ¨ b Key Lab for Supramolecular Structure and Spectroscopy, Jilin UniÕersity, Changchun 130 023, PR China Received 23 November 1998; in final form 26 March 1999
Abstract AFM-based single-molecule force spectroscopy was employed to measure the nanomechanical properties of 1,4-linked polysaccharides. Single-molecule force spectroscopy clearly reflected the difference in the mechanical properties of a-Ž1,4.and b-Ž1,4.-linked polysaccharides. A force-induced chair–twist boat conformational transition in a-Ž1,4.-linked polysaccharides was discovered. This chair–twist boat conformational transition is a nanomechanical fingerprint of a-Ž1,4.-linked polysaccharides. q 1999 Elsevier Science B.V. All rights reserved.
1. Introduction During its maturation, atomic force microscopy ŽAFM. has evolved into a versatile platform for experiments with individual molecules w1–3x. Beyond the topographic imaging, AFM has been used to measure and map many types of surface forces and local mechanical properties. A wealth of new information about single molecules has provided us with new insights into intermolecular forces, and the underlying molecular mechanism w4–7x. Recently, a new technique – single-molecule force spectroscopy by AFM – has been implemented and opens the possibility for directly measuring the deformation of single polymer chains w8–13x. It expands the existing
spectrum of single-molecule techniques w14–19x towards lower forces and smaller molecules. Polysaccharides are essential components of all living organisms and are the most abundant class of biological molecules. The mechanical properties of polysaccharides and their structural origin are of great interest, but these studies have lagged well behind those of proteins and nucleic acids w20x. In this Letter, single-molecule force spectroscopy on a-Ž1,4.-linked glycan Žcarboxymethyl amylose and heparin. was carried out and a nanomechanical fingerprint of a-Ž1,4.-linked glycan was reported and correlated to their molecular structure.
2. Experimental
)
Corresponding author. E-mail: [email protected]
Carboxymethyl amylose Žsodium salt, catalogue number C4947. and carboxymethyl cellulose Žsodium
0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 9 . 0 0 3 8 9 - 9
198
H. Li et al.r Chemical Physics Letters 305 (1999) 197–201
salt, catalogue number C5013. were purchased from Sigma ŽDeisenhofen, Germany., heparin Žsodium salt, catalogue number 51536. was purchased from Fluka ŽBuchs, Switzerland.. In all preparation steps and experiments, PBS buffer Žphosphate buffered saline, 150 mM NaCl, pH 7.4. was used. The samples were allowed to adsorb onto a clean glass slide from a 0.5 ml drop of 1 mgrml solution Žincubated overnight.. Prior to the experiment, the sample was rinsed with PBS buffer. Experiments were conducted at ambient temperature and on a home-built AFM arrangement customized for force measurements. Si 3 N4 cantilevers from Park Scientific Instruments ŽPark, Sunnyvale, CA. were used. The spring constants of the cantilevers were calibrated by measuring their thermal excitation w21,22x. For doing the force measurements, the tip was brought into contact with the sample and kept in contact for several seconds, allowing the individual polymers to adsorb on to the tip Žfor details, see Refs. w8,9,11x.. Upon separation of the tip and the sample, the polymer was stretched. The cantilever deflection–extension curve was recorded and converted into force extension curves. The coupling of the polysaccharides to the tip in this study was based on a non-specific interaction, it has been shown in various studies that this non-specific attachment can hold forces of up to nanonewtons w8,9,11x.
3. Results and discussion Carboxymethyl amylose ŽCM-amylose. is a linear polymer of several thousand glucose residues linked by a-D-1,4-glucosidic bonds, its structure is shown in Fig. 1. Typical force extension curves of CMamylose filaments are shown in Fig. 2a. Since the polysaccharide is polydisperse and we have no control over the point at which the polymer chain is picked up by the tip, the contour lengths of the polymer being stretched between the tip and the substrate varies, as can be seen in Fig. 2a. In spite of the different contour lengths, all force curves exhibit the same deformation characteristics: firstly, the force rises monotonically with a Kuhn length of 0.54 nm and a segment elasticity of 11 000 pNrnm, then the deformation shows a plateau at ; 300 pN followed by a stiffening of the polymer Žsegment elasticity of
Fig. 1. Molecular structure of: Ža. carboxymethyl amylose; Žb. carboxymethyl cellulose; and Žc. heparin. For clarity, the sodium counter ions are omitted.
the polymer after the plateau is 28 000 pNrnm. 1. The yield strength can be as high as 1.8 nN. Force curves of different contour lengths were normalized according to their contour lengths w9,11x,
1 The force curves can be fitted with an extended Freely-JointChain model w8,15x, the Kuhn length and segment elasticity were obtained from the fit parameters.
H. Li et al.r Chemical Physics Letters 305 (1999) 197–201
Fig. 2. Force curves of CM-amylose in PBS buffer: Ža. different force curves of CM-amylose with different contour lengths and Žb. superposition of different normalized curves of Ža..
superimposed and plotted in Fig. 2b. The superposition of these curves clearly shows that all the elastic properties of CM-amylose filaments scale linearly with their lengths, and all filaments show a plateau at the same force. This result was reproduced for hundreds of CM-amylose filaments measured with different cantilevers in different experiments. This finding corroborates the view that predominantely single polymer filaments are stretched and the deformation of individual polymer chains is measured. The plateau is the dominating feature of the single-molecule force spectra of CM-amylose. This plateau is accompanied by an elongation of 0.08 nm per glucose residue and is followed by an increase of the stiffness of the polymer chain, indicating that a force-induced conformation transition occurred in this force regime. From the force spectrum, we can estimate the energy needed to induce this conformational transition to be ; 7.3 kT per glucose residue Ž4.36 kcalrmol.. In previous works w8,9,12x, we showed that it is possible to manipulate the same molecules repeat-
199
edly. By keeping the force lower than the rupture force, here we repeatedly stretched and relaxed the same CM-amylose filament and recorded the deformation curves. Shown in Fig. 3 is a series of consecutive traces on the same CM-amylose filament. No hysteresis between the stretching and relaxing traces was observed. This clearly indicates that the forceinduced conformational transition is a fully reversible process, which confirms that the transition is a purely elastic property of CM-amylose itself. The force-induced conformational transition is a unique property of CM-amylose and is thus expected to be closely related to the molecular structure of CM-amylose. For comparison, single-molecule force spectroscopy was carried out on carboxymethyl cellulose ŽCM-cellulose.. CM-cellulose has a b-Ž1,4.linked glucosidic backbone and is a structural isomer of CM-amylose Žsee Fig. 1.. In Fig. 4, the upper curve is the normalized force curve of single CMcellulose chain. Notably, no plateau is present, the force curve only shows a sharp increase in force when the extension approaches the contour length of the molecule, and the Kuhn length is ; 4 nm and the segment elasticity is ; 50 000 pNrnm which is much stiffer than CM-amylose. CM-amylose and CM-cellulose have the same chemical compositions; the structural difference is the linkage. The difference in the single-molecule force spectra of CM-amylose and CM-cellulose clearly reflects the great influence of the linkage on
Fig. 3. Subsequent deformation curves of the same CM-amylose filament.
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H. Li et al.r Chemical Physics Letters 305 (1999) 197–201
Fig. 4. Normalized force curves of CM-cellulose Župper curve. and heparin Žlower curve..
the mechanical properties: the b-glucosidic linkage of the cellulose causes each successive glucose residue to flip 1808 with respect to the preceding residue so that cellulose easily assumes a stiff, extended conformation w23x which results in the long Kuhn length and high segment elasticity; in contrast, a-glycosidic bonds in amylose can rotate easily to adopt a flexible coiled conformation w23x. The plateau presenting only in CM-amylose clearly indicates that the plateau is an inherent property of the a-Ž1,4. linkage. Hence, the force-induced conformational transition manifests itself as a nanomechanical fingerprint of an a-Ž1,4.-linked glycan. The single-molecule force spectrum of heparin further confirms the above conclusion. As an aŽ1,4.-linked glycan, heparin is a variably sulfated glycosaminoglycan that consists predominantly of alternating a Ž1,4.-linked residues of D-glucuronate2-sulfate and N-sulfo-D-glucosamine-6-sulfate w23x Žas shown in Fig. 1.. Its structure is similar to that of CM-amylose, except for the substituents. As shown in Fig. 4 Žlower curve., heparin also exhibits a plateau in the force curve but at 700 pN, compared to 300 pN for CM-amylose. The width of the plateau is exactly the same as that of CM-amylose. Moreover, the stiffness of heparin after the plateau is almost the same as that of CM-amylose. This finding
strongly indicates that the plateaus of CM-amylose and heparin originate from the same conformational transition. The difference in the height of the plateaus reflects the influence of the different substituents on the conformational transition. Recent molecular dynamic simulation ŽMD. provides us with the molecular details of this force-induced conformational transition process. The MD simulation clearly reflects the difference in the mechanical properties of a-Ž1,4.- and b-Ž1,4.-linked polysaccharides, and reveals that this force-induced conformational transition in a-Ž1,4.-linked glycan results from the chair–twist boat conformational transition of individual glucose residues in the polymer backbone w24x. The chair and twist boat are two stable conformations of the glucose ring, and the transition between them is a common stereochemical phenomenon w25–27x. However, this is the first report of a force-induced chair–twist boat conformational transition 2 . It is noteworthy that in the forceinduced chair–twist boat conformational transition, the energy difference between the chair and twist boat conformers, estimated from the force spectra of CM-amylose, agrees well with the value obtained from the free glucose monomers in solution. It is also important to point out that the force-induced chair–twist boat transition is accompanied by an elongation of the molecules and dramatically improves the polymers’ ductility. It is still a speculation that this force-induced conformational transition has relevance to its biological function. In summary, single-molecule force spectroscopy clearly reveals the nanomechanical fingerprint of an a-Ž1,4.-linked glycan: a force-induced chair–twist boat conformational transition, which cannot be measured by conventional methods. This study has demonstrated the utility of AFM for investigating the conformational changes of polymers, which may open a new field for studying their stereochemistry and provide new insight into the nanomechanical properties of biologically important polysaccharides and processes.
2 Note: during the revision of this Letter, a paper about a similar force-driven conformational transition in polysaccharides was published by Marszalek et al. w28x.
H. Li et al.r Chemical Physics Letters 305 (1999) 197–201
References w1x G. Binnig, C.F. Quate, C. Gerber, Phys. Rev. Lett. 56 Ž1986. 930. w2x M. Radmacher, M. Fritz, H.G. Hansma, P.K. Hansma, Science 265 Ž1994. 1577. w3x P. Hinterdorfer, W. Baumgartner, H.J. Gruber, K. Schilcher, H. Schindler, Proc. Natl. Acad. Sci. USA 93 Ž1996. 3477. w4x E.-L. Florin, V.T. Moy, H.E. Gaub, Science 264 Ž1994. 415. w5x V.T. Florin, E.-L. Florin, H.E. Gaub, Science 266 Ž1994. 257. w6x G.U. Lee, L.A. Chrisey, R.J. Colton, Science 266 Ž1994. 771. w7x U. Dammer, M. Hegner, D. Anselmetti, P. Wagner, M. Dreier, W. Huber, H.-J. Guentherodt, Biophys. J. 93 Ž1995. 3477. w8x M. Rief, F. Oesterhelt, B. Heymann, H.E. Gaub, Science 275 Ž1997. 1295. w9x M. Rief, M. Gautel, F. Oesterhelt, J. Fernandez, H.E. Gaub, Science 276 Ž1997. 1109. w10x A.F. Oberhauser, P.E. Marszalek, H.P. Erickson, J.M. Fernandez, Nature ŽLondon. 393 Ž1998. 1818. w11x M. Rief, M. Gautel, A. Schemmel, H.E. Gaub, Biophys. J. 75 Ž1998. 3008. w12x H.B. Li, M. Rief, F. Oesterhelt, H.E. Gaub, Adv. Mater. 10 Ž1998. 316. w13x H.B. Li, W.K. Zhang, X. Zhang, J.C. Shen, B.B. Liu, C.X. Gao, G.T. Zou, Macromol. Rapid Commun. 19 Ž1998. 609.
201
w14x S.B. Smith, L. Finzi, C. Bustamate, Science 258 Ž1992. 1122. w15x S.B. Smith, Y. Cui, C. Bustamate, Science 271 Ž1996. 795. w16x P. Cluzel, A. Lebrun, C. Heller, R. Lavery, J.L. Viovy, D. Chatenay, F. Caron, Science 271 Ž1996. 792. w17x S.R. Quate, H. Babcock, S. Chu, Nature ŽLondon. 388 Ž1997. 151. w18x K. Svoboda, C.F. Shmidt, B.J. Schnapp, S.M. Block, Nature ŽLondon. 365 Ž1993. 721. w19x J.T. Finer, R.M. Simmons, J.A. Spudich, Nature ŽLondon. 368 Ž1994. 113. w20x A.J. Furth, Lipids and Polysaccharides in Biology, Edward Arnold, London, 1970. w21x H.J. Butt, M. Jaschke, Nanotechnology 6 Ž1995. 1. w22x E.L. Florin, M. Ludwig, M. Rief, H.E. Gaub, Biosens. Bioelectron. 10 Ž1995. 895. w23x D. Voet, J.G. Voet, Biochemistry, 2nd ed., Wiley, New York, 1995. w24x B. Heymann, H. Grubmuller, Chem. Phys. Lett. 305 Ž1999. ¨ 202, in this issue. w25x J.R. Snyder, A.S. Seriann, J. Org. Chem. 51 Ž1986. 2694. w26x D.H.R. Barton, Science 539 Ž1970. 539. w27x A.D. French, J.W. Brady, Computer Modeling of Carbohydrate Molecules, Am. Chem. Soc., Washington, DC, 1990. w28x P.M. Marszalek, A.F. Oberhauser, Y.P. Pang, J.M. Fernandez, Nature ŽLondon. 396 Ž1998. 661.