The preparation and biological evaluation of a series of multivalent ligands for nicotinic acetylcholine receptors is proposed. The ligands will be structurally based on acetylcholine (the native ligand for the receptor), 4-acetyl-1,1-dimethylpiperazinium (ADMP, a potent quaternary ammonium ligand for nicotinic receptors), and A-84543 (a very potent and selective nicotine analog). This is an exploratory study to determine the feasibility of attaching tethers to nicotinic ligands, while retaining affinity and functional activity. Affinity will be determined using radioligand binding assays, and for the monovalent tethered ligands, functional fluorescence assays will be used to determine if the ligand is bound in an active orientation. This approach will be extended for the preparation of bivalent nicotinic ligands which will be used to probe the inter-binding site distance as well as the distance between binding sites on adjacent receptors. Finally a series of multivalent dendrimeric ligands containing 4 and 12 recognition sites/ molecules will be synthesized and evaluated for their affinity at nicotinic receptors as well as their ability to promote crystallization of these membrane proteins. Such dendrimeric multivalent ligands are potential high affinity pharmacologic probes of nicotinic receptor function that may be used for the study of receptor clustering and endocytosis. However, the ultimate goal of these studies is to produce a structurally defined scaffold for use as a template for the direction of three-dimensional (3D) crystallization of these receptors for x-ray diffraction analysis, which to date has been extraordinarily difficult. The ligands described herein will be used to prepare complexes with nicotinic receptor preparations. If successful, these studies will form the basis for a general approach to the three-dimensional (3D) crystallization of membrane proteins for x-ray structural analysis. ? ?
