Dependence of DNA Persistence Length on Ionic Strength of Solutions with Monovalent and Divalent Salts: A Joint Theory–Experiment Study

Abstract

Using high-throughput tethered particle motion single-molecule experiments, the double-stranded DNA persistence length, Lp, is measured in solutions with Na+ and Mg2+ ions of various ionic strengths, I. Several theoretical equations for Lp(I) are fitted to the experimental data, but no decisive theory is found which fits all the Lp values for the two ion valencies. Properly extracted from the particle trajectory using simulations, Lp varies from 30 to 55 nm, and is comparable to previous experimental results. For the Na+-only case, Lp is an increasing concave function of I–1, well fitted by Manning’s electrostatic stretching approach, but not by classical Odjik–Skolnick–Fixman theories with or without counterion condensation. With added Mg2+ ions, Lp shows a marked decrease at low I, interpreted as an ion–ion correlation effect, with an almost linear law in I–1, fitted by a proposed variational approach.