Meditbio, University of Algarve, Campus de Gambelas, 8005-139, Faro, Portugal.
Department of Chemistry, University of Miami, Coral Gables, FL, 33124, USA.
J Am Soc Mass Spectrom. 2017 Nov;28(11):2508-2514. doi: 10.1007/s13361-017-1758-0. Epub 2017 Jul 29.
Electrospray ionization mass spectrometry (ESI-MS) is a powerful tool to study host-guest supramolecular interactions. ESI-MS can be used for detailed gas-phase reactivity studies, to clarify the structure, or simply to verify the formation of complexes. Depending on the structure of the host and of the guest, negative and/or positive ESI are used. Here we report the unexpected formation of host-guest complexes between cucurbit[n]urils (n = 7, 8, CB[n]) and amine, styryl pyridine, and styryl pyridine dimer cations, under negative ESI. Non-complexed CB[n] form double charged halide (Br, Cl, F) adducts. Under negative ESI, halide ions interact with CB[n] outer surface hydrogen atoms. One to one host-guest complexes (1:1) of CB[n] with positive charged guests were also observed as single and double charged ions under negative ESI. The positive charge of guests is neutralized by ion-pairing with halide anions. Depending on the number of positive charges guests retain in the gas phase, one or two additional halide ions are required for neutralization. Complexes 1:2 of CB[8] with styryl pyridines retain two halide ions in the gas phase, one per guest. Styryl pyridine dimers form 1:1 complexes possessing a single extra halide ion and therefore a single positive charge. Negative ESI is sensitive to small structural differences between complexes, distinguishing between 1:2 complexes of styryl pyridine-CB[8] and corresponding 1:1 complexes with the dimer. Negative ESI gives simpler spectra than positive ESI and allows the determination of guest charge state of CB[n] complexes in the gas phase. Graphical Abstract ᅟ.
电喷雾电离质谱（ESI-MS）是研究主体-客体超分子相互作用的强大工具。ESI-MS 可用于详细的气相反应研究，以阐明结构，或简单地验证配合物的形成。根据主体和客体的结构，可以使用负电喷雾和/或正电喷雾。在这里，我们报告了在负电喷雾下，葫芦[n]脲（n = 7、8、CB[n]）与胺、苯乙烯吡啶和苯乙烯吡啶二聚阳离子之间形成主体-客体配合物的意外情况。未配位的 CB[n]形成双电荷卤化物（Br、Cl、F）加合物。在负电喷雾下，卤化物离子与 CB[n]外表面的氢原子相互作用。在负电喷雾下，还观察到与带正电荷客体的 CB[n]（1:1）形成的单电荷和双电荷离子的主体-客体 1:1 配合物。客体的正电荷通过与卤化物阴离子的离子对中和而中和。根据客体在气相中保留的正电荷数，需要一个或两个额外的卤化物离子来中和。带有苯乙烯吡啶的 CB[8]的 1:2 配合物在气相中保留两个卤化物离子，每个客体一个。苯乙烯吡啶二聚体形成 1:1 配合物，带有一个额外的卤化物离子，因此带有一个单一的正电荷。负电喷雾对配合物之间的微小结构差异很敏感，能够区分苯乙烯吡啶-CB[8]的 1:2 配合物和相应的带有二聚体的 1:1 配合物。负电喷雾比正电喷雾产生更简单的谱图，并允许确定气相中 CB[n]配合物的客体电荷状态。
