Non-ideal Elasticity Of Single Stranded DNA At Low Forces

Abstract

Single molecule force-extension experiments on dsDNA and ssDNA are usually compared to the worm-like chain (WLC) or freely-jointed chain (FJC) models. The WLC and FJC are ideal polymer models: they account for local stiffness through the persistence or Kuhn length, but ignore long-range interactions between monomers, accounted for by, e.g., the excluded-volume parameter in the Flory scaling theory, that determine the self-avoiding random-walk structure of polymers not under tension. To attempt to bridge the gap between ideal force/extension models and the classic scaling picture of real polymers, we explore the low-force elasticity of chemically-denatured single-stranded DNA (ssDNA) using magnetic tweezers. We find a low-force regime where extension grows as a non-linear power-law with force, in contradiction with ideal predictions, but in agreement with scaling predictions made by P. Pincus (Macromolecules 9, 386 (1976)). By analyzing this power-law regime, we extract the dependence of the Kuhn length of ssDNA on monovalent salt concentration, and find that it is linearly proportional to the Debye screening length. This result is in contrast to the quadratic dependence predicted in the well-known OSF theory, but agrees with other theories and simulations. Finally, we identify a high-salt point ([NaCl] ∼ 3 M) where ideal polymer behavior returns, and the data is well-fit by the WLC model at all forces. We identify this as a theta point of the polymer, and show that, beyond 3M salt, the ssDNA aggregates in a manner consistent with a polymer in poor solution.