Using Physical Chemistry To Differentiate Nicotinic from Cholinergic Agonists at the Nicotinic Acetylcholine Receptor

Abstract
The binding of three distinct agonistsacetylcholine (ACh), nicotine, and epibatidineto the nicotinic acetylcholine receptor has been probed using unnatural amino acid mutagenesis. ACh makes a cation−π interaction with Trp α149, while nicotine employs a hydrogen bond to a backbone carbonyl in the same region of the agonist binding site. The nicotine analogue epibatidine achieves its high potency by taking advantage of both the cation−π interaction and the backbone hydrogen bond. A simple structural model that considers only possible interactions with Trp α149 suggests that a novel aromatic CH···OC hydrogen bond further augments the binding of epibatidine. These studies illustrate the subtleties and complexities of the interactions between drugs and membrane receptors and establish a paradigm for obtaining detailed structural information.

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