Statistical Mechanics of Torque Induced Denaturation of DNA

A unifying theory of the denaturation transition of DNA, driven by temperature $T$ or induced by an external mechanical torque $\Gamma$ is presented. Our model couples the hydrogen-bond opening and the untwisting of the helicoidal molecular structure. We describe denaturation as a first order transition from $B$-DNA to $d$-DNA phases. The coexistence region is naturally parametrized by the degree of supercoiling $\sigma$. The denaturation free energy, the temperature dependence of the twist angle, the phase diagram in the $T,\Gamma$ plane, and isotherms in the $\sigma ,\Gamma$ plane are calculated and show a good agreement with experimental data.