BROWNIAN DYNAMICS OF THE POLARIZATION OF RODLIKE POLYELECTROLYTES

The effect of external electric field pulses on the ion atmosphere around linear polyelectrolytes is characterized by Brownian dynamics simulations. The polymer is fixed at the center of a cube and is surrounded by mobile counterions and byions; electrostatic interactions beyond the cube are evaluated using periodic boundary conditions and the Ewald summation technique; hydrodynamic interactions are not included in the simulation. The polymers are constructed in analogy; to DNA double helices with a constant charge spacing of 0.17 nm. Electric fields applied parallel to the long axis of the polymer induce a dipole moment parallel to this axis with time constants around 10 ns (at ion concentrations around 1 mM). The dipole of a polymer with 40 charged residues approaches saturation at field strengths in the range around 50 kV/cm. Parallel to the dipole rise, there is a dissociation of counterions from the polymer reflected by an increase of the root mean square distance [S-c(2))(1/2)of the counterions from the center of the cube. The [S-c(2)](1/2) value increases linearly with the field strength E up to E approximate to 100 kV/cm; this effect and its dependence on E are in close analogy to the ''dissociation field effect''. The dipole moment at a given field strength and ion concentration does not increase with more than the square of the chain length under the conditions of our present simulations. A relatively small decrease of the dipole moment is observed when the ion concentration is increased.