The Gas Phase Protonation Sites of Six Naturally Occurring Nicotinoids.

The gas phase protonation sites of six naturally occurring nicotinoids, namely nicotine (NIC), nornicotine (NOR), anabasine (ANB), anatabine (ANT), cotinine (COT), and myosmine (MYO), consisting of a common Pyridine and differing non-Pyridine rings, have been determined for the first time at the physiological temperature from cryogenic ion trap infrared spectroscopy and electronic structure calculations. The protonation site on either of these two rings is related to the nicotinoid's biological activity. At room temperature, NIC is a mixture of Pyridine and Pyrrolidine (non-Pyridine) protomers, whereas NOR, ANB, ANT, and COT are pure Pyridine protomers and finally MYO is mostly a Pyroline (non-Pyridine) protomer. The nearly planar structure of MYO-H+, induced by the presence of a conjugated π system and confirmed from calculations and the UV absorption spectra, breaks from the trends observed for NIC, NOR, and ANB, since its structure is drastically different from the structures of the other nicotinoids.

Affinity of Nicotinoids to a Model Nicotinic Acetylcholine Receptor (nAChR) Binding Pocket in the Human Brain.

The binding affinity of nicotinoids to the binding residues of the α4ß2 variant of the nicotinic acetylcholine receptor (nAChR) was identified as a strong predictor of the nicotinoid's addictive character. Using ab initio calculations for model binding pockets of increasing size composed of 3, 6, and 14 amino acids (3AA, 6AA, and 14AA) that are derived from the crystal structure, the differences in binding affinity of 6 nicotinoids, namely, nicotine (NIC), nornicotine (NOR), anabasine (ANB), anatabine (ANT), myosmine (MYO), and cotinine (COT) were correlated to their previously reported doses required for increases in intracranial self-stimulation (ICSS) thresholds, a metric for their addictive function. By employing the many-body decomposition, the differences in the binding affinities of the various nicotinoids could be attributed mainly to the proton exchange energy between the pyridine and non-pyridine rings of the nicotinoids and the interactions between them and a handful of proximal amino acids, namely Trp156, Trpß57, Tyr100, and Tyr204. Interactions between the guest nicotinoid and the amino acids of the binding pocket were found to be mainly classical in nature, except for those between the nicotinoid and Trp156. The larger pockets were found to model binding structures more accurately and predicted the addictive character of all nicotinoids, while smaller models, which are more computationally feasible, would only predict the addictive character of nicotinoids that are similar to nicotine. The present study identifies the binding affinity of the guest nicotinoid to the host binding pocket as a strong descriptor of the nicotinoid's addiction potential, and as such it can be employed as a fast-screening technique for the potential addiction of nicotine analogs.