Dependence of DNA Persistence Length on Ionic Strength and Ion Type

Even though the persistence length L-P of double-stranded DNA plays a pivotal role in cell biology and nanotechnologies, its dependence on ionic strength I lacks a consensual description. Using a high-throughput single-molecule technique and statistical physics modeling, we measure L-P in the presence of monovalent (Li+, Na+, K+) and divalent (Mg2+, Ca2+) metallic and alkyl ammonium ions, over a large range 0.5 mM <= I <= 5 M. We show that linear Debye-Huckel-type theories do not describe even part of these data. By contrast, the Netz-Orland and Trizac-Shen formulas, two approximate theories including nonlinear electrostatic effects and the finite DNA radius, fit our data with divalent and monovalent ions, respectively, over the whole I range. Furthermore, the metallic ion type does not influence L-P(I), in contrast to alkyl ammonium monovalent ions at high I.