Extreme Dependence of Dynamics on Concentration in Highly Crowded Polyelectrolyte Solutions

Charge carrying species, such as polyelectrolytes, are vital to natural and synthetic processes that rely on their dynamic behavior. Here, we utilize single molecule methods to probe this dynamic behavior using polylysine as a model polyelectrolyte. Through single-particle tracking techniques, the diffusivity of individual polyelectrolyte chains and overall system viscosity are determined for concentrated polylysine solutions. These studies show experimental scaling dependences much stronger than theoretical predictions for both neutral polymers and polyelectrolytes and draw into question whether power law based scaling theories are appropriate to describe concentrated charged systems. Similar dependences are observed in concentrated solutions prepared at a variety of pH and counterion conditions. Hindered system dynamics are attributed to contributions from monomeric friction and the large effective excluded volume of polyelectrolyte chains leading to glassy dynamics. These forces restrict the movement of polymers through the sample and their effects are seen over a wide range of concentrations. The framework of the Vrentas Duda free volume theory is used to compare polyelectrolyte and neutral systems. Supported by this theory, when the mass associated with the counterions is excluded from the total polymer mass diffusive scaling across environmental conditions collapses onto a common trendline. Overall, these results are applicable to behavior in crowded biological systems, such as intracellular environments where the mobility of proteins is strongly inhibited.