Structural features and protonation site of epibatidine in the gas phase: an investigation through infrared multiphoton dissociation spectroscopy and computational chemistry

Abstract

The gas phase structures of epibatidine, one of the most potent agonists of nicotinic acetylcholine receptors (nAChRs), are determined by means of infrared multiphoton dissociation (IRMPD) spectroscopy and quantum chemistry calculations. Comparison of the experimental and theoretical spectra provides evidence that about 15% of epibatidine is protonated on the Nsp(2) nitrogen in the gas phase. In contrast, the computational study of deschloroepibatidine shows that in the gas phase, the molecule is present only protonated on the Nsp(2) nitrogen. The main minima of the Nsp(2) protonated forms of the two molecules are strongly stabilized by intramolecular CH···Nsp(3) hydrogen bonds. The fundamental insights obtained in the present study on these two important nAChRs agonists show how subtle chemical modifications can have a deep impact on important physicochemical properties.