Abstract: It is now possible to resolve local dynamics within a membrane bound protein at near physiological conditions in natural membrane fragments or in reconstituted complexes, using solid state NMR approaches [1, 2]. This information is obtained by isotopically (2H, 13C, 19F, 15N, 17O) labeling selective parts of either a ligand or the protein understudy, and observing the nucleus in non-crystalline, macromolecular complexes [3,4].
Ligands with complex structure have differential mobility at their binding sites. Substituted imidazole pyridines, for example, which inhibit the H+/K+-ATPase and have therapeutic use, are constrained in the imidazole moiety, but shows significant flexibility at the pyridine group [5] (see figure). It is this group which has a direct interaction with an aromatic (phe198) residue, with the potential for ?-electron sharing [6]. Similarly, the steroid moiety of ouabain undergoes motions which are similar to those of the target protein, the Na+/K+-ATPase, but the rhamnose undergoes a high degree of flexibility at fast rates of motions whilst interacting with Tyr198 [7]. For acetyl choline when bound in the nicotinic acetyl choline receptor (nAChR), the quaternary ammonium group undergoes fast rotation at an aromatic binding site, which is driven by thermal fluctuations which may be functionally significant [8]. Our current focus is on GPCRs, specifically the brain neurotensin receptor (NTS1) for which the structure (by single molecule cryo-EM) and ligand binding interactions are being studied [9 - 14].