Magnetic resonance for the study of polyelectrolytes – charge, conformation, packing, and molecular dynamics

The local nature of nuclear magnetic resonance (NMR) offers unique insight into molecular properties ranging from chemical structure, molecular arrangement, conformation, charge to molecular dynamics over many time scales. After introducing a few basic principles of modern NMR experiments the tutorial will focus on the information available from a variety of experiments. Most of the applications involve ¹H and ¹³C but examples of other nuclei will be demonstrated as well.

The chemical shift in NMR spectra enables the identification of the structures involved as well as states of dissociation. Further information is obtained from distance-dependent properties like dipolar coupling. In solids this is used to probe proximity and thus packing in multilayers and complexes.

Pulsed-field-gradient (PFG) NMR is used to probe translational motion like diffusion. From the diffusion coefficient the hydrodynamic radius as a measure of the size of molecules and thus the conformation is derived. This is used to investigate the impact of the charge on the conformation of polyelectrolytes. If an electric field is applied in situ charged molecules move as is measured by PFG NMR. From the combination of diffusion and electrophoretic NMR the effective charge of molecules and complexes is determined allowing to quantify counterion condensation and binding of ligands or formation of complexes.

Dynamic NMR ranging from exchange spectra to different kinds of NMR relaxation probes molecular dynamics over a wide range of correlation times. The transverse relaxation time T₂ is sensitive to slow motion of polymer chains.

Electron paramagnetic resonance (EPR) has significantly higher sensitivity than NMR but requires the presence of unpaired electrons that are introduced by stable spin labels. That in turn enables highly selective investigation of the local chain mobility and has been applied to polyelectrolyte complexes and multilayers.