Number fluctuations of the condensing agent affects the coil-toroid coexistence for semi-stiff polyelectrolytes.

A semi-stiff charged polymer with counterions and salt was investigated using a Monte Carlo method with the Wang-Landau sampling scheme. Simulations show the coexistence of thermodynamically stable structures-prolonged and compact (toroidal-like). The transition between the two states is accompanied by concentration fluctuations for the condensing agent. Compact structures are accompanied by more multivalent counterions than prolonged ones.