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

Brunet, Annaël, Tardin, Catherine, Salomé, Laurence, Rousseau, Philippe, Destainville, Nicolas, Manghi, Manoel
Macromolecules 2015 v.48 no.11 pp. 3641-3652
DNA, equations, ionic strength, ions, magnesium, salts, sodium
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.