Entropic Elasticity of Twist-Storing Polymers

TL;DR

This paper models the statistical mechanics of torsionally constrained polymers, specifically DNA, revealing how thermal fluctuations influence twist rigidity and matching experimental data to determine microscopic properties.

Contribution

It introduces a model linking bend and twist fluctuations in polymers and validates it with experimental data on DNA, providing insights into microscopic twist stiffness.

Findings

Quantitative agreement with experiments using C=109 nm

Thermal fluctuations reduce effective twist rigidity

Chirality influences twist-stretch coupling in DNA

Abstract

We investigate the statistical mechanics of a torsionally constrained polymer. The polymer is modeled as a fluctuating rod with bend stiffness A kT and twist stiffness C kT. In such a model, thermal bend fluctuations couple geometrically to an applied torque through the relation Lk = Tw + Wr. We explore this coupling and find agreement between the predictions of our model and recent experimental results on single lambda-DNA molecules. This analysis affords an experimental determination of the microscopic twist stiffness (averaged over a helix repeat). Quantitative agreement between theory and experiment is obtained using C=109 nm. The theory further predicts a thermal reduction of the effective twist rigidity induced by bend fluctuations. Finally, we find a small reflection of molecular chirality in the experimental data and interpret it in terms of a twist-stretch coupling of the DNA duplex.