Anisotropy of the electrical polarizability of a model DNA fragment in aqueous salt solution

Metropolis Monte Carlo (MC) Brownian dynamics simulations are performed to determine the anisotropy of the electrical polarizability Δα of a model DNA fragment in aqueous salt solutions. A 64 base-pair fragment of the double-stranded DNA is modeled as an impenetrable cylinder and placed in a spherical MC cell. Taking into consideration the contribution of coions to the electroneutrality condition, at every simulation step we obtain a list of counterions which constitute the net charge that compensates the polyion charge. According to the fluctuation–dissipation theorem, polarizability is calculated from fluctuations of the dipole moment created in the distribution of the net charge. Electric properties of polyelectrolyte solutions are reproduced: Δα decreases with the addition of salt yielding a steady value comparable to the experiment.