Salt effects on condensed protamine-DNA assemblies: anion binding and weakening of attraction

Using osmotic stress coupled with X-ray scattering, we have directly examined the salt sensitivity of the intermolecular forces between helices in condensed protamine-DNA arrays. Thermodynamic forces are measured from the dependence of DNA helical interaxial spacings on external salt concentration or the osmotic pressure applied by neutral polymer solutions in equilibrium with the condensed phase. Force curves of salmon protamine-DNA condensates are highly dependent on salt species and concentration, indicating salt binding to protamine-DNA complexes. This dependence of the forces on salt species follows the Hofmeister series for anions. Chaotropic anions bind more tightly to protamine-DNA arrays than kosmotropic anions, thus more greatly disrupting the attractive thermodynamic forces. Variations with cation type are small compared with those observed for anions. Further, osmotic stress is used to estimate the number of ions bound in the condensed phase through a Gibbs-Duhem relationship. We estimate that at equilibrium, ∼1 Br(-) is bound per protamine molecule at 200 mM NaBr concentration. Remarkably, this one bound anion results in a change of ∼12% in the surface-to-surface distance between DNA helices. Potential biological implications of this attractive force salt sensitivity are discussed.