Extended and Contorted Conformations of Alkanediammonium Ions in Symmetrical α,α′,δ,δ′-Tetramethylcucurbit[6]uril Cavity

Binding
interactions between symmetrical α,α′,δ,δ′-tetramethylcucurbit[6]­uril
(TMeQ[6]) and a series of alkyldiammonium ions in aqueous solution
and in the solid state were investigated by 1H NMR spectroscopy,
MALDI-TOF mass spectrometry, X-ray crystallography, and isothermal
titration calorimetry (ITC). Their 1H NMR spectra reveal
that the actual binding behaviors vary depending upon the alkyl chain
length. Their single-crystal X-ray diffraction analyses indicate the
guest 1,2-ethanediammonium is located outside of the TMeQ[6] portal,
while the other four alkyldiammonium guests can be accommodated in
the TMeQ[6] cavity, forming 1:1 inclusion complexes. Most importantly,
the long-chain alkyldiammoniums (1,8-octanediammonium and 1,10-decanediammonium)
take a contorted conformation when bound within the TMeQ[6] cavity.
Additionally, ITC experiments show that the complexation of the alkyldiammonium
guests with TMeQ[6] is mainly enthalpy driven, which benefits from
ion–dipole interactions.

本研究采用氢核磁共振（1H NMR）波谱法、基质辅助激光解吸电离飞行时间质谱（MALDI-TOF）、X射线晶体学以及等温滴定量热法（ITC），探究了对称α,α′,δ,δ′-四甲基六元瓜环（TMeQ[6]）与一系列烷基二铵离子在水溶液及固态下的结合相互作用。氢核磁共振谱图结果显示，实际结合行为随烷基链长度的不同而发生变化。单晶X射线衍射分析表明，客体1,2-乙二铵离子位于TMeQ[6]的端口外侧，而其余四种烷基二铵客体可嵌入TMeQ[6]的空腔，形成1:1包结配合物。尤为重要的是，长链烷基二铵离子（1,8-辛二铵与1,10-癸二铵）在结合进入TMeQ[6]空腔时会呈现扭曲构象。此外，等温滴定量热实验结果显示，烷基二铵客体与TMeQ[6]的络合过程主要为焓驱动，该过程得益于离子-偶极相互作用。