Twist-bend coupling and the statistical mechanics of the twistable worm-like chain model of DNA: perturbation theory and beyond

TL;DR

This paper investigates how twist-bend coupling affects DNA's statistical mechanics, presenting perturbative and non-perturbative analyses that align well with simulations but reveal discrepancies with experimental data, indicating additional mechanics at play.

Contribution

It introduces a perturbative approach to quantify the impact of twist-bend coupling G on DNA mechanics and extends results to non-perturbative regimes, validated by simulations.

Findings

Effective torsional stiffness is influenced by twist-bend coupling G.

Thermal fluctuations screen the bare G, modifying long-wavelength elastic properties.

Model predictions align with simulations but deviate from experimental measurements.

Abstract

The simplest model of DNA mechanics describes the double helix as a continuous rod with twist and bend elasticity. Recent work has discussed the relevance of a little-studied coupling $G$ between twisting and bending, known to arise from the groove asymmetry of the DNA double helix. Here, the effect of $G$ on the statistical mechanics of long DNA molecules subject to applied forces and torques is investigated. We present a perturbative calculation of the effective torsional stiffness $C_\text{eff}$ for small twist-bend coupling. We find that the "bare" $G$ is "screened" by thermal fluctuations, in the sense that the low-force, long-molecule effective free energy is that of a model with $G=0$, but with long-wavelength bending and twisting rigidities that are shifted by $G$-dependent amounts. Using results for torsional and bending rigidities for freely-fluctuating DNA, we show how our perturbative results can be extended to a non-perturbative regime. These results are in excellent agreement with numerical calculations for Monte Carlo "triad" and molecular dynamics "oxDNA" models, characterized by different degrees of coarse-graining, validating the perturbative and non-perturbative analyses. While our theory is in generally-good quantitative agreement with experiment, the predicted torsional stiffness does systematically deviate from experimental data, suggesting that there are as-yet-uncharacterized aspects of DNA twisting-stretching mechanics relevant to low-force, long-molecule mechanical response, which are not captured by widely-used coarse-grained models.