Using high-throughput tethered particle motion single-molecule experiments, the double-stranded DNA persistence length is measured in solutions with Na+ and Mg2+ ions of various ionic strengths.
Several theoretical equations for the persistence length are fitted to the experimental data, but no decisive theory is found which fits all the values for the two ion valencies. Properly extracted from the particle trajectory using simulations, the persistence length varies from 30 to 55 nm, and is comparable to previous experimental results.
For the Na+ only case,it is well fitted by the Manning's electrostatic stretching approach, but not by classical Odjik-Skolnick-Fixman theories with or without counterion condensation.
With added Mg2+ ions, the persistence length shows a marked decrease at low ionic strength, interpreted as an ion-ion correlation effect, with an almost linear law in the ionic strength inverse, fitted by a proposed variational approach.
Reference : Annael? Brunet, Catherine Tardin, Laurence Salome, Philippe Rousseau, Nicolas Destainville, and Manoel Manghi, Dependence of DNA Persistence Length on Ionic Strength of Solutions with Monovalent and Divalent Salts : A Joint Theory? Experiment Study, Macromolecules, 48, 3641 (2015).