DNA condensation and redissolution:: interaction between overcharged DNA molecules

The effective DNA-DNA interaction force is calculated by computer simulations with explicit tetravalent counterions and monovalent salt. For overcharged DNA molecules, the interaction force shows a double-minimum structure: The positions and depths of these minima are regulated by the counterion density in the bulk. Using two-dimensional lattice sums and free energy perturbation theories, the coexisting phases for DNA bundles are calculated. A DNA condensation and redissolution transition and a stable mesocrystal with an intermediate lattice constant for high counterion concentration are obtained.