Hydration-induced protomer switching in p-aminobenzoic acid studied by cold double ion trap infrared spectroscopy
Hydration-induced protomer switching in p-aminobenzoic acid studied by cold double ion trap infrared spectroscopy
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通过冷双离子阱红外光谱研究对氨基苯甲酸中水合诱导的启动子转换
DOI：
10.1039/d2cp04497h
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发表时间：
2023
影响因子：
3.3
通讯作者：
Fujii Masaaki
中科院分区：
文献类型：
--
作者：
Akasaka Kyota;Hirata Keisuke;Haddad Fuad;Dopfer Otto;Ishiuchi Shun-ichi;Fujii Masaaki
对氨基苯甲酸 (PABA) 是研究溶剂诱导的质子位点转换的基准分子。 PABA 的羧基和氨基质子化分别生成 PABAH+ 的 O- 和 N- 原体。离子淌度质谱（IMS）和红外光解离（IRPD）研究表明，气相中最稳定的O-原体在与气相簇中的六个水分子水合后转化为溶液中最稳定的N-原体。然而，阈值大小仍然不明确，因为 IMS 实验中的到达时间分布表现出多个峰值。另一方面，IRPD 光谱无法检测较小水合簇的 N-原聚体，因为减少动力学捕获所需的退火导致背景较宽。在此，我们使用双离子阱光谱仪中的红外光谱仪报告了 O → N 原聚体切换的阈值大小，没有歧义，范围为 1300 至 1800 cm−1。通过电喷雾制备纯O-原体，并在反应离子阱中形成特定尺寸的水合簇。所得簇被转移到第二个低温离子阱中，O-和N-原聚体的分布由中红外光谱确定，无需展宽。促进O→N原聚体转换的阈值确实是五个水分子。它比之前报道的值要小，因此它的五水结构不支持之前提出的Grotthuss机制。通过碰撞辅助剥离红外光谱评估 O → N 质子转移的程度，N-原体数量随着水分子数量的增加而增加。该结果与水溶液中 N-原体的优势群体一致。
Para-Aminobenzoic acid (PABA) is a benchmark molecule to study solvent-induced proton site switching. Protonation of the carboxy and amino groups of PABA generates O- and N-protomers of PABAH+, respectively. Ion mobility mass spectrometry (IMS) and infrared photodissociation (IRPD) studies have claimed that the O-protomer most stable in the gas phase is converted to the N-protomer most stable in solution upon hydration with six water molecules in the gas-phase cluster. However, the threshold size has remained ambiguous because the arrival time distributions in the IMS experiments exhibit multiple peaks. On the other hand, IRPD spectroscopy could not detect the N-protomer for smaller hydrated clusters because of broad background due to annealing required to reduce kinetic trapping. Herein, we report the threshold size for O → N protomer switching without ambiguity using IR spectroscopy in a double ion trap spectrometer from 1300 to 1800 cm−1. The pure O-protomer is prepared by electrospray, and size-specific hydrated clusters are formed in a reaction ion trap. The resulting clusters are transferred into a second cryogenic ion trap and the distribution of O- and N-protomers is determined by mid-IR spectroscopy without broadening. The threshold to promote O → N protomer switching is indeed five water molecules. It is smaller than the value reported previously, and as a result, its pentahydrated structure does not support the Grotthuss mechanism proposed previously. The extent of O → N proton transfer is evaluated by collision-assisted stripping IR spectroscopy, and the N-protomer population increases with the number of water molecules. This result is consistent with the dominant population of the N-protomer in aqueous solution.