5 September 2007 Direct measurement of the intermolecular forces confining a single entangled DNA molecule
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Concentrated solutions of long polymer molecules exhibit reduced molecular diffusivity and striking non-Newtonian fluid properties that arise due to intermolecular entanglements. The most successful theories for describing these properties are based on the notion that on short time scales each polymer is confined to move within a tube-shaped region following its contour. Such a confining field is proposed to arise due to collective intermolecular interactions, yet this has remained a rather vague concept since the confining forces have never been directly measured. Here, we directly measure these forces by using optical tweezers to manipulate single entangled DNA molecules. We found that the forces opposing displacement of a molecule parallel to its local contour were negligible compared with those opposing transverse displacement. A time-dependent harmonic potential opposed transverse displacement. Work per unit length of order 1 kT was required to displace the molecule by a distance roughly equal to the theoretically predicted tube radius in the calculated thermal equilibration time. The required work also decreased gradually with the rate of displacement, in accord with predictions of a recent simulation study that found that the tube radius expands with time. Following the displacement, we measured the relaxation of force acting on the molecule and observed three distinct exponential decay times of ~0.4, 5, and 34 s, which are consistent with theoretically proposed molecular relaxation mechanisms. These measurements quantify the notion of a tube-like molecular confining field assumed in reptation theories.
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Rae M. Robertson, Rae M. Robertson, Douglas E. Smith, Douglas E. Smith, } "Direct measurement of the intermolecular forces confining a single entangled DNA molecule", Proc. SPIE 6644, Optical Trapping and Optical Micromanipulation IV, 66440C (5 September 2007); doi: 10.1117/12.739632; https://doi.org/10.1117/12.739632

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