The ensemble approach to distance geometry: application to the nicotinic pharmacophore.

We develop an extension of conventional distance geometry techniques that treats two or more molecules as a single "ensemble". This extension can be used to find a common pharmacophore, i.e., the spatial arrangement of essential groups, from a small set of biologically active molecules. The approach can generate, in one step, coordinates for the set of molecules in their "active" conformations such that their essential groups are superimposed. As an example, we show how the nicotinic pharmacophore can be deduced from a set of four nicotinic agonists: nicotine, cytisine, ferruginine methiodide, and muscarone. Three essential groups in each agonist are chosen: the cationic center (A), an electronegative atom (B), and an atom (C) that forms a dipole with B. There is only one pharmacophore possible for the superposition of these essential groups: a triangle with sides 4.8 A (A-B), 4.0 A (A-C), and 1.2 A (B-C). The pharmacophore triangle, which is consistent with previous models in the literature, can also be achieved by the agonist trans-3,3'-bis[(trimethylammonio)methyl]azobenzene and the antagonists strychnine, trimethaphan, and dihydro-beta-erythroidine. An examination of the common volumes of agonists suggests a specific disposition of molecular volume relative to the pharmacophore triangle. We discuss the relative strengths and drawbacks of the ensemble approach vs. other conformational search methods.