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 a. 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.