Stick-release pattern in stretching single condensed polyelectrolyte toroids

TL;DR

This study uses simulations to analyze how salt-condensed DNA toroids unfold under tension, revealing a sawtooth force pattern due to loop-by-loop unraveling, providing insights into DNA elasticity and structure.

Contribution

It demonstrates the microscopic unfolding mechanism of DNA toroids under force, linking structural dynamics to observed force-extension patterns.

Findings

Force-extension curve shows sawtooth pattern.

Unfolding occurs loop-by-loop from the toroid.

Toroid size oscillates during unfolding.

Abstract

Using Langevin dynamics simulations, we study elastic response of single semiflexible polyelectrolytes to an external force pulling on the chain ends, to mimic the stretching of DNA molecules by optical tweezers. The linear chains are condensed by multivalent counterions into toroids. The force-extension curve shows a series of sawtooth-like structure, known as the stick-release patterns in experiments. We demonstrate that these patterns are a consequence of the loop-by-loop unfolding of the toroidal structure. Moreover, the dynamics, how the internal structure of chain varies under tension, is examined. At the first stage of the stretching, the toroidal condensate decreases its size until the loss of the first loop in the toroid and then, oscillates around this size for the rest of the unfolding process. The normal vector of the toroid is pulled toward the pulling-force direction and swings back to its early direction repeatedly when the toroidal chain looses a loop. The results provide new and valuable information concerning the elasticity and the microscopic structure and dynamic pathway of salt-condensed DNA molecules being stretched.