Department of Chemistry, University of Washington, Seattle, Washington 98195, United States.
Laboratory for Chemistry and Life Science, Institute of Innovative Research, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8503, Japan.
J Am Chem Soc. 2022 Sep 21;144(37):16698-16702. doi: 10.1021/jacs.2c04064. Epub 2022 Aug 31.
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.
我们报告了一项联合实验理论研究，研究对象是之前从未报道过的气相单水合尼古丁（NIC）和去水尼古丁（NOR）的结构和红外光谱，并利用它们来确定其质子化位置。NIC 的生物活性强烈受到其质子化位置的影响，即吡咯烷（Pyrro-NICH，预期的活性形式）和吡啶（Pyri-NICH）形式；然而，这些尚未在烟碱型乙酰胆碱受体（nAChR，不存在水）或乙酰胆碱结合蛋白（AChBP，存在单个水分子）中直接实验确定，只能从与相邻残基的分子间距离（即色氨酸）推断为 Pyrro-NICH。我们的温度控制双离子阱红外光谱实验，辅以碰撞剥离方法和高级理论计算，得出气相 NICH···（HO）配合物中 Pyri:Pyrro 的质子化比例为 8:2，在 240 K 下，这类似于 AChBP 中存在的分子簇。因此，气相中单水分子相对于非水合气相 NICH 的 3:2 增加了 NICH 中的这种比例，这与水溶液中几乎仅存在 Pyrro-NICH 的趋势形成对比。相比之下，仅观察到 Pyri-NORH 单体，这一事实可能与其较弱的生物活性相关。
