ABSTRACT
Studying the translational (diffusive) motion of molecules in solution provides immense amounts of information about the environment that the motion is occurring in as well as information about binding equilibria and any restrictions or boundaries present. Nuclear magnetic resonance (NMR) is a powerful tool for studying
4.1 Introduction .................................................................................................. 125 4.2 Measuring Diffusion Using NMR ................................................................ 129
4.2.1 Relaxation, Gradients and Echoes .................................................... 129 4.2.2 Note on the Effect of Gradients ........................................................ 136
4.3 Diffusive Averaging Effects ......................................................................... 136 4.3.1 Association and Polydispersity ......................................................... 136 4.3.2 Obstruction ....................................................................................... 139 4.3.3 Binding Equilibria ............................................................................ 142
4.4 Diffusion and Ion/Molecule Radius in Solution ........................................... 144 4.4.1 Stokes-Einstein-Sutherland Equation ............................................. 144 4.4.2 Hydration and Solvation ................................................................... 145
4.4.2.1 Hydration of Non-Electrolytes ........................................... 146 4.4.2.2 Hydration of Ions ............................................................... 146 4.4.2.3 Hydrated Radius by Experiment ........................................ 154
4.5 NMR Measurement Interference .................................................................. 155 4.5.1 Eddy Currents ................................................................................... 156 4.5.2 Field Inhomogeneities ....................................................................... 158 4.5.3 Solvent Suppression .......................................................................... 160 4.5.4 Convection ........................................................................................ 162
4.6 Concluding Remarks .................................................................................... 163 References .............................................................................................................. 163
these motions, and this chapter will cover some of the basic aspects of NMR diffusion measurements and their application to studying free diffusion, obstructed systems, and systems where aggregation and binding events are present, and will also introduce some of the experimental complications and their resolutions. We start by describing what is meant by ‘diffusion’.
