A Monte Carlo simulation study of electrostatic forces between hexagonally packed DNA double helices

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Abstract
The electrostatic contribution to the force between hexagonally packed B‐DNA double helices has been studied using different statistical mechanical descriptions and the Monte Carlo simulation method. The effects of small ion correlations and sizes are considered, and comparison with experimental results is made. It is found that for monovalent counterions, the mean field Poisson–Boltzmann theory can give a reasonable reproduction of the experimental data, even though the simulations show that its description of the electrostatic interaction can be qualitatively wrong. The chemical equilibrium of the ordered DNA phase with the surrounding bulk salt solution is found to be an important feature. Furthermore, the simulations show that the correlation between the ion clouds of different DNA polyions gives rise to a significant attractive contribution to the interaction when divalent cations are present. It is suggested that this electrostatic attraction is an important driving force for the experimentally observed condensation and collapse of DNA solutions to ordered systems, which can be induced by multivalent ions.