D. B. McIntosh1, A. Dittmore2, O. A. Saleh2,3
(1) Physics Dept., University of California, Santa Barbara
(2) Materials Dept., University of California, Santa Barbara
(3) BMSE Program, University of California, Santa Barbara
Although long, flexible polymers form self-avoiding random walks when free in solution, most single-molecule stretching experiments elongate the polymer into a highly-aligned geometry, and prevent the long-range interactions that lead to polymer swelling. Here, we report low-force single-molecule stretching data and quantify the effects of swelling on various flexible polymers: Charged, denatured single-stranded DNA shows an immediate transition from extended chain at high forces to a swollen chain at low forces. Measurement of the salt-dependent crossover between these regimes permits estimate of how the polymer’s properties depend on electrostatics. In contrast, charge-neutral synthetic PEG molecules show a distinct ideal-to-swollen transition at a critical chain size. Single-stranded DNA composed entirely of adenine bases (poly(dA)) cooperatively base stacks; and thus, at low forces, the polymer has stiff base-stacked domains interspersed with domains of swollen coils, indicated by an elastic response that is intermediate between ideal and self-avoiding. These data permit estimates of intrinsic, microscopic properties such as the Kuhn length and excluded volume of polymers which have value in understanding their low (to zero) force structure.
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