Conformational Instability of Rodlike Polyelectrolytes due to Counterion Fluctuations

TL;DR

This paper investigates how counterion fluctuations can destabilize the rigid conformations of highly charged polyelectrolytes, leading to possible collapse or complex structures, with implications for DNA condensation.

Contribution

It introduces a theoretical framework showing counterion fluctuations induce conformational instabilities and re-entrant behavior in polyelectrolytes, including a generalized electrostatic persistence length.

Findings

Counterion fluctuations can cause instability in stiff polyelectrolytes.

Re-entrant conformational transitions depend on salt concentration and polymer length.

Theoretical insights may explain DNA re-entrant condensation phenomena.

Abstract

The effective elasticity of highly charged stiff polyelectrolytes is studied in the presence of counterions, with and without added salt. The rigid polymer conformations may become unstable due to an effective attraction induced by counterion density fluctuations. Instabilities at the longest, or intermediate length scales may signal collapse to globule, or necklace states, respectively. In the presence of added-salt, a generalized electrostatic persistence length is obtained, which has a nontrivial dependence on the Debye screening length. It is also found that the onset of conformational instability is a re-entrant phenomenon as a function of polyelectrolyte length for the unscreened case, and the Debye length or salt concentration for the screened case. This may be relevant in understanding the experimentally observed re-entrant condensation of DNA.