(197d) The Influence of Topological Constraints on the Distribution of Counterions Around Model Charged Macromolecules

We use molecular dynamics simulations with explicit solvent to investigate one of the long-standings challenges in polyelectrolyte solutions, namely the interdependency between the distribution of the counterions and the molecular conformational properties. We focus on charge densities above the classical counterion condensation threshold, where the counterions form a condensed counter-ion layer about the charged polymer. We find that the that the distribution of the counterions is sensitive to the molecular structure of the charged macroion, but at the same time a fraction of the counterions remains ``bound'' to the macroion for long times , an effect that clearly alters macromolecular structure. To quantify this many-body phenomenon, we determine the counterion distribution about the polymer backbone, as well as the overall shape of the charged macromolecule-counter-ion complexes as a function of molecular topology: linear, star, ring, and trefoil knot. We find that these topological constraints regulate the saturation in the net ionic charge around the macromolecule. Finally, we discuss how conformational changes driven by the bound counterions impact molecular shape and dynamics.