Stretching DNA: Role of electrostatic interaction

TL;DR

This paper models how electrostatic interactions influence DNA stretching, revealing different scaling behaviors at small and large forces, and successfully compares theoretical predictions with experimental data across salt concentrations.

Contribution

It introduces a self-consistent variational approach to analyze electrostatic effects on DNA extension, providing a unified framework for small and large force regimes.

Findings

Extension scales as f r_D at small forces

Extension approaches contour length as f^{-1/2} at large forces

Theory matches experimental data across salt concentrations

Abstract

The effect of electrostatic interactions on the stretching of DNA is investigated using a simple worm like chain model. In the limit of small force there are large conformational fluctuations which are treated using a self-consistent variational approach. For small values of the external force f, we find theoreticlly and by a simple blob picture that the extension scales as fr_D where r_D is the Debye screening length. In the limit of large force the electrostatic effects can be accounted for within the semiflexible chain model of DNA by assuming that only small excursions from rod-like conformations are possible. In this regime the extension approaches the contour length as f^{-1/2} where f is the magnitude of the external force. The theory is used to analyze experiments that have measured the extension of double-stranded DNA subject to tension at various salt concentrations. The theory reproduces nearly quantitatively the elastic response of DNA at small and large values of f and for all concentration of the monovalent counterions. The limitations of the theory are also pointed out.