The pulling force of a single DNA molecule condensed by spermidine

TL;DR

This paper presents a quantitative theory explaining the force plateau observed in single DNA molecules condensed by spermidine, showing how the plateau force varies with spermidine concentration and aligning well with experimental data.

Contribution

It introduces a theoretical model based on reentrant condensation to explain the force plateau in DNA-spermidine interactions, providing a quantitative match with experiments.

Findings

Force plateau appears at specific spermidine concentrations.

Plateau force varies parabolically with the logarithm of spermidine concentration.

The theory aligns well with experimental measurements.

Abstract

In a recent experiment, a single DNA double helix is stretched and relaxed in the presence of spermidine, a short positive polyelectrolyte, and the pulling force is measured as a function of DNA extension. In a certain range of spermidine concentration, a force plateau appears whose value shows maximum as a function of spermidine concentration. We present a quantitative theory of this plateau force based on the theory of reentrant condensation and derive almost parabolic behavior of the plateau force as a function of the logarithm of the spermidine concentration in the range of condensation. Our result is in good agreement with experimental data.