Mechanism of Threshold Elongation of DNA Macromolecule

The mechanism of threshold elongation (overstretching) of DNA macromolecules under the action of external force is studied within the framework of phenomenological approach. When considering the task it is taken into account that double-stranded DNA is a polymorphic macromolecule with a set of metastable states. Accordingly to proposed mechanism, DNA threshold elongation can take place as a cooperative structural transition of macromolecule to metastable form, which is stabilized by the external force. For the description of DNA overstretching, the model included external (stretching) and internal (conformation) displacement components is constructed. As assumed, both components are coupled on the pathway of double helix transformations. It is shown, that under force action DNA deformation proceeds in two stages. First one is the restructuring of the double helix, which leads to formation of conformational bistability in DNA macromolecule. On the second stage, the conformational transition and the deformation induced by it consistently cover the macromolecule as a threshold process. The propagation of overstretched deformation in bistable macromolecule occurs as the motion of transition boundaries (domain walls), which are topologically stable structural excitations. Using the proposed approach, the contributions to DNA overstretching process are determined. This is elastic stretching of the double helix, deformation induced by conformational changes in macromolecule before overstretching transition, and threshold elongation as such. The greatest contribution to overstretching process in DNA (more then 80%) is introduced by threshold elongation. As shown, the effect of threshold deformation in DNA double helix takes place due to the critical correspondence of acting force with elastic properties of macromolecule, and achieves by restructuring of the double helix. The comparison of calculated characteristics of overstretching process for heteronomous DNA with experimental data shows good agreement. Analysis of the results obtained, together with the available literature data allow to conclude, that overstretching transition in the double helix is dynamical process and can spread in DNA chain. In the same time, in DNA with A·T-rich content due to large dissipation the overstretching process leads to force induced melting transition, and should have nearly static character.