Structure of Gas Phase Monohydrated Nicotine: Implications for Nicotine's Native Structure in the Acetylcholine Binding Protein.

We report a joint experimental-theoretical study of the never reported before structure and infrared spectra of gas phase monohydrated nicotine (NIC) and nornicotine (NOR) and use them to assign their protonation sites. NIC's biological activity is strongly affected by its protonation site, namely, the pyrrolidine (Pyrro-NICH, anticipated active form) and pyridine (Pyri-NICH) forms; however, these have yet to be directly experimentally determined in either the nicotinic acetylcholine receptor (nAChR, no water present) or the acetylcholine-binding protein (AChBP, a single water molecule is present) but can only be inferred to be Pyrro-NICH from the intermolecular distance to the neighboring residues (i.e., tryptophan). Our temperature-controlled double ion trap infrared spectroscopic experiments assisted by the collisional stripping method and high-level theoretical calculations yield the protonation ratio of Pyri:Pyrro = 8:2 at 240 K for the gas phase NICH···(HO) complex, which resembles the molecular cluster present in the AChBP. Therefore, a single water molecule in the gas phase enhances this ratio in NICH relative to the 3:2 for the nonhydrated gas phase NICH in a trend that contrasts with the almost exclusive presence of Pyrro-NICH in aqueous solution. In contrast, the Pyri-NORH protomer is exclusively observed, a fact that may correlate with its weaker biological activity.