Salt Species-Dependent Electrostatic Effects on ssDNA Elasticity

The interactions of charged, flexible polymers with counterions of various valencies is a fundamental unsolved problem of polyelectrolyte physics, with specific applications to structure formation by nucleic acids, including RNA folding and DNA nanotechnology. We recently showed that single-molecule measurements of the elasticity of a model polyelectrolyte, denatured single-stranded DNA (d-ssDNA), can reveal details of the polymer's electrostatic interactions on multiple length scales. Here, we explore the effects of various salts on d-ssDNA elasticity. In agreement with our prior results in NaCl, we find that d-ssDNA elastic response in KCl, MgCl2., and CaCl2 shows a low-force Pincus regime, with a weaker response at higher forces. The data in KCl are quantitatively identical to prior NaCl data, reflecting the universality of monovalent salt electrostatics. In contrast, the behavior of d-ssDNA in divalent salt solutions shows subtantial quantitative differences, including a nonlogarithmic high-force behavior and a heightened sensitivity of elasticity to changes in divalent salt concentration. We introduce a condensed-ion model that can quantitatively account for some aspects of this sensitivity.