The role of charge in polyelectrolyte chain dynamics and ligand binding

The most important property of polyelectrolytes is their charge that makes them soluble in water. The conformation and the stiffness of the polyelectrolyte are determined by the repelling electrostatic force along the molecule that can be influenced by the ionic strength. This in turn influences the chain dynamics and its response to external shear. The transverse NMR relaxation time T₂ it probes slow dynamics like polymer chain segments. It nicely shows the increase in dynamics with increasing ionic strength thus shielding the electrostatics. Chain entanglements lead to restricted motion and thus two regimes of NMR relaxation are observed fast relaxation for restricted motion and longer relaxation for the more mobile free chain segments. In a concentrated solution more entanglements are formed when polyelectrolytes become more flexible shielding the electrostatic interaction as has been shown for solutions of poly(styrene sulfonate) (PSS). Under shear entanglements get lost as shown in rheological NMR in a Searle cell as it is manifested in enhanced chain dynamics and a lower fraction of restricted chain segments.

Pulsed-field-gradient (PFG) NMR measures the translation of molecules in solution, where diffusion and directed flow generated different experimental signatures and are thus distinguished. For ligands temporarily bound to macromolecules a time-weighted average is observed and knowing the diffusion coefficient for both macromolecule and the free ligand the bound fraction and thus the dissociation constant is directly determined. In combination with electrophoretic NMR the effective charge of both the polyelectrolyte and the ligand are determined as has been applied to quantify counterion condensation. While the charge of a strong polyelectrolyte like PSS is independent of pH it varies for ligands like Lysine. This offers a possibility to investigate the strength of the interaction as a function of pH and thus the charge of the ligand. While the dominating interaction is the electrostatic interaction in this case there is a non-electrostatic component. Non-binding is found only when both the macromolecule and the ligand have a net charge of the same sign and the electrostatic effect repels them.