Charged, flexible polymers, such as single-stranded nucleic acids (ssNAs), are ubiquitous in biology and technology. Quantitative description of their solution conformation has remained elusive due to the competing effects of polymer configurational freedom and salt-screened electrostatic repulsion between monomers. We investigate this by measuring the elastic response of single ssNA molecules over a range of salt concentrations. The data are well described by a model, inspired by a mean-field approach, in which intrapolymer electrostatic repulsion creates a salt-dependent internal tension whose interplay with the external force determines the elasticity. The internal tension can be related to the polymer’s charge spacing; thus, our results show how mesoscopic polymer conformation emerges from microscopic structure
