Melting transition of covalently closed DNA with supercoil-induced cruciforms.

The melting curve for covalently closed supercoiled DNA has been studied by assuming the existence of cruciforms as significant structural perturbations in the pre-melting region. The statistical mechanical treatment used incorporates these cruciform structures through an appropriate sequence generating function. The variation of the effective hydrogen bond energy with temperature is taken into account by an empirical procedure. The results obtained are in close agreement with the corresponding experimental data in TEA solution where the effect of heterogeneity of the base pairs is minimized.