At higher forces the dextran filaments underwent a

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At higher forces the dextran filaments underwent a distinct conformational change. The polymer stiffened and the segment elasticity was dominated by the bending of bond angles. The conformational change was found to be reversible and was corroborated by molecular dynamics calculations. R ecently a series of single molecule experi - ments provided detailed insight into intermo - lecular and intramolecular forces, providing relevant information on molecular mecha - nisms ( 1–4 ). In previous experiments we and others chemically linked molecular pairs such as biotin and avidin ( 3, 5 ), or conjugated DNA strands ( 6 ), between the tip of an atom - ic force microscope (AFM) cantilever and support structures. Molecule - specific bond forces between binding pairs were measured upon separation and compared with known thermodynamic parameters ( 4 ). Here we used this approach to probe elastic properties of single polymer strands. The experimental geometry is depicted in Fig. 1A. Dextrans (average molecular weight 500,000) linked to a gold surface through epoxy - alkanethiols were activated with one carboxymethyl group per glucose unit on average ( 7 ) and reacted with streptavidin such that several molecules were chemically bound to each dextran fil - ament (Sensor Chip SA5, Pharmacia Bio - sensor AB, Uppsala, Sweden). The mean distance between the grafting points of two different polymer strands was about 200 Å, and the hydrated “polymer brush” extended 1000 to 2000 Å into the solution ( 7 ). Be - cause in physiological buffer dextran be - haves like an ideal polymer, the coil overlap is expected to be low. In our experiments streptavidin served as a molecular handle for the manipulation of the polymer to be investigated. An AFM cantilever with bi - otin bound to the AFM tip, following the protocol given in ( 3 ), was used to pull on individual dextran filaments through the biotin - streptavidin bond ( 8 ). To minimize the number of multiple bonds, which typi - cally occur when the tip penetrates the polymer brush, we let the tip approach and retract step by step without it indenting into the sample until a binding event was registered. In this “fly fishing mode” the undesirable multiple bonds can be efficient - ly avoided ( 9 ). Alternatively, one can “manually” disentangle an individual fila - ment from the polymer brush by slowly pulling back the tip while monitoring all multiple bonds and tangles rupturing until just one last filament is stretched (see the first trace of Fig. 4, discussed further below). This filament can then be repeatedly ma - nipulated as long as the force is kept below the force limit of the molecular handles. Several measured elongation curves of dextran strands of various lengths are shown in Fig. 1B ( 10 ). At the given extension rate of 0.5 m m/s the biotin - streptavidin bond is known to hold up to a force F of 250 6 25 pN ( 4 ). The measured deformation curves were modeled by entropy springs with segment elasticity ( 11 ). Although the contour lengths L contour of the polymers varied from 0.4 to 1.6 m m, the measured Kuhn length I K 5 6 6 0.5
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