Protein-ligand complexes can also be studied using diffusion-edited experiments (also known as affinity NMR). A small molecule diffuses in solution several orders of magnitude faster than a large protein. if a small molecule binds to a protein, its diffusion coefficient will be reduced compared with that for the free ligand. Thus diffusion coeficients can be used as effective indicators of molecular interactions and to measure binding constants. Several examples have been reported: 97JACS5249, 97JOC8930, 97JACS12257, 97JB441, 97AC225, 99AC669, 98JACS10258, 99MRC269, 00JACS414).

Examples:

Interaction between the anti-inflammatory drug salicylate and the human serum albumin (HSA) ( 99SA1897)

The importance of eliminating the protein background ( 02JMR217-155)

In practice, the observed D is the averaged coefficient of free (D_free) and bound (D_bound) states weighted by the fraction of free molecule (X_free) and bound molecules (X_bound):

D = X_free*D_free + X_bound*D_bound

Study of protein-ligand interactions from heteronuclear-filtered diffusion experiments.