Thermal denaturation of fluctuating finite DNA chains: the role of bending rigidity in bubble nucleation

TL;DR

This paper presents an exactly solvable coupled model of DNA denaturation that incorporates chain flexibility and bubble formation, providing insights into melting behavior, viscosity transition, and finite size effects.

Contribution

It introduces a coupled Ising and worm-like chain model for DNA denaturation, explicitly linking chain flexibility to melting transition and experimental observables.

Findings

Melting curve matches experimental data for polydA-polydT.

Model explains the thermal viscosity transition.

Finite size effects are significant for DNA strands of several thousand base pairs.

Abstract

Statistical DNA models available in the literature are often effective models where the base-pair state only (unbroken or broken) is considered. Because of a decrease by a factor of 30 of the effective bending rigidity of a sequence of broken bonds, or bubble, compared to the double stranded state, the inclusion of the molecular conformational degrees of freedom in a more general mesoscopic model is needed. In this paper we do so by presenting a 1D Ising model, which describes the internal base pair states, coupled to a discrete worm like chain model describing the chain configurations [J. Palmeri, M. Manghi, and N. Destainville, Phys. Rev. Lett. 99, 088103 (2007)]. This coupled model is exactly solved using a transfer matrix technique that presents an analogy with the path integral treatment of a quantum two-state diatomic molecule. When the chain fluctuations are integrated out, the denaturation transition temperature and width emerge naturally as an explicit function of the model parameters of a well defined Hamiltonian, revealing that the transition is driven by the difference in bending (entropy dominated) free energy between bubble and double-stranded segments. The calculated melting curve (fraction of open base pairs) is in good agreement with the experimental melting profile of polydA-polydT. The predicted variation of the mean-square-radius as a function of temperature leads to a coherent novel explanation for the experimentally observed thermal viscosity transition. Finally, the influence of the DNA strand length is studied in detail, underlining the importance of finite size effects, even for DNA made of several thousand base pairs.