The conformation and the stiffness of polyelectrolyte are determined by the repelling electrostatic force along the molecule that can be influenced by the ionic strength. The effective charge is reduced by the condensation of counterions if the charge density along the chain is so high that the thermal energy of the counterions is insufficient to escape it.
Pulsed-field-gradient (PFG) NMR measures the translation of molecules in solution distinguishing diffusion and directed flow. Both the self-diffusion coefficient and the electrophoretic mobility are measured on the same time and length scales. From the combination of both the effective charge of macromolecules is calculated and compared to the nominal charge permitting quantification of the counterion condensation. Charged small molecules may bind as ligands to macromolecules. Knowing the diffusion coefficient for both macromolecule and the free ligand compared to the effective diffusion coefficient in the interacting state the bound fraction and thus the dissociation constant is directly determined. Electrophoretic NMR allows determining the effective charge of both the macromolecule and the ligand and gives insight on the electrostatic interaction as a major contribution to binding. 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.