Dependence of DNA persistence length on ionic strength and ion type

TL;DR

This study measures how DNA's persistence length varies with ionic strength and ion type, revealing that advanced electrostatic theories better predict the data than classical models, with ion type influencing DNA flexibility at high ionic strengths.

Contribution

It provides comprehensive experimental data on DNA persistence length across various ions and ionic strengths, and evaluates the applicability of different electrostatic theories to describe this dependence.

Findings

Debye-Hückel theories do not fit the data.

Netz-Orland and Trizac-Shen formulas fit the data well.

Ion type influences DNA flexibility at high ionic strength.

Abstract

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 presence of monovalent (Li$^+$, Na$^+$, K$^+$) and divalent (Mg$^{2+}$, Ca$^{2+}$) metallic and alkyl ammonium ions, over a large range 0.5 mM $\leq I\leq 5$ M. We show that linear Debye-H\"uckel-type theories do not describe even part of these data. By contrast, the Netz-Orland and Trizac-Shen formulas, two approximate theories including non-linear 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 $I$.