The effects of lithium, rubidium, cesium and magnesium ions on the close packing of persistence-length DNA fragments.

By application of scaled particle theory to persistence-length DNA fragments in sedimentation-equilibrium at speeds high enough to maintain close packing, the range of interhelical electrostatic repulsion was evaluated with LiCl, RbCl, CsCl, and MgCl2 as supporting electrolytes. Analysis of the data in terms of the Zimm cluster function confirmed that the net interaction between helices is purely repulsive in all cases. At constant ionic strength the electrostatic radius of the rod-like DNA decreases as the counterion changes from Li+ to Rb+ to Cs+. In contrast to univalent counterions, electrostatic radius increases with Mg2+ concentration, except at very low (mM) MgCl2 concentrations. All solutions undergo a reversible transition to a turbid, optically anisotropic phase at a slightly salt-dependent, critical DNA concentration, as observed previously for NaDNA.