Structure investigations of agonists of the natural neurotransmitter acetylcholine, V. Structure-activity correlations for cholinergic stimulants derived from crystal structures of their halides.

General features of crystal structures of halide salts of cholinergic stimulants can be interpreted in terms of substrate-receptor interactions. The monoatomic counterions in the crystal structures are discussed as models for the binding site of the receptor with respect to the ammonium group of the cholinergic neurotransmitters. In the crystal structures the anions occupy the tetrahedral faces of the quaternary trimethylammonio methyl or related groups in a specific geometry. Crystallographic and pharmacological evidence indicates that these groups should preferentially interact with the receptor via a specific face type (B-type face). The directionality of the interaction is derived from the vectors joining N+ with the anions occupying B-type faces. So called "activity triangles", formed by the nitrogen of the ammonium group, a second polar centre of the neurotransmitter cation and a counterion occupying a B-face of the ammonium group, provide a structural criterion for the differentiation between muscarinic and nicotinic activity. It is shown that structure-activity relationships of cholinergic stimulants do not depend on the conformational details of the neurotransmitter cations, but primarily on the relative positions of the polar centres of the cations with respect to the anionic binding site of the receptor.