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Comparison between static and dynamic mode measurement of the forces acting on a chainlike molecule. (a) Typical experimental setup in the static mode where a chainlike molecule attached to a tip is stretched. Measured quantities are the bending of the cantilever (i.e., the force) vs the extension of the molecule. (b) In dynamic force spectroscopy the cantilever oscillates with an amplitude around its equilibrium position while it is retracted from the surface covered with chainlike molecules. If a molecule binds to the tip the resonance frequency of the cantilever changes in dependence of the cantilever position. As explained in the text this signal is used to determine a force vs extension curve.
An application of the introduced dynamic force spectroscopy technique. (a) Amplitude and (b) frequency shift curves measured during approach to and retraction from the surface covered with dextran molecules. During the retraction one dextran molecule bound to the tip as revealed by the change in the frequency and amplitude signal. At a position of about 135 nm (see arrows) the maximum binding force was exceeded and the cantilever oscillated freely again. Only the data before this jump is used for the subsequent analysis. (c) The force acting on the dextran molecule (symbols) can be reconstructed as a function of the actual tip position . The experimental result is well described by a single-click model using only the number of molecules as fitting parameter (solid line). (d) The energy dissipated per oscillation cycle can be calculated from Eq. (2) for approach and retraction. The zero of the -axes is arbitrarily set to the left of the graphs.
Force vs extension curves of three different dextran strands measured with the proposed dynamic approach. The symbols represent experimental data which is well reproduced by the theoretical force curves where only the number of dextran monomers were used as a fit parameter (, 266, and 381, respectively).
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