Ionic effects on overstretching transition of B-DNA.

A three-dimensional model is proposed herein to study the ionic effects of a NaCl solution on the overstretching transition of long B-DNA molecules. In this model, the bending deformations of DNA backbones, cooperativity of base-stacking interactions, electrostatic interactions, and spatial effects of the DNA double-helix structure are taken into account. The energy of electrostatic interactions is given as a function of the ionic strength and of the folding angle based on the Poisson-Boltzmann equation. An expression is derived, which shows that the overstretching force is linear with the natural logarithm of the ionic strength. The analytical results of this model are in good agreement with recently reported experimental results.