Complexation of Aliphatic Ammonium Ions with a Water-Soluble Cucurbit[6]uril Derivative in Pure Water: Isothermal Calorimetric, NMR, and X-ray Crystallographic Study

Abstract

Complexation of a water-soluble cucurbituril (CB) derivative, cyclohexanocucurbit[6]uril (CB*[6]), the cavity dimensions of which are essentially the same as those of CB[6] with various organic mono- and diammonium ions has been studied by isothermal titration calorimetry and H-1 NMR spectroscopy. The binding affinity of CB*[6] with the guest molecules in water is 3-5 and 2-3 orders of magnitude higher than those of CB[6] in 50% formic acid and in 0.05 M NaCl solution, respectively, which is mainly due to the larger enthalpic gains upon complex formation in the absence of interfering ions, such as protons and Na+. In particular, the binding constant (K) of spermine to CB*[6] was measured to be 3.4 x 10(12) M-1, which is the highest binding constant ever reported for CB[6] or its derivatives. We also obtained the X-ray crystal structures of alpha,omega-alkanediammonium ions (C(n)DA(2+), n=4-8) and spermine complexes with CB[6], in which the aliphatic chains of the guest molecules take an extended or partially bent conformation inside the CB[6] cavity, depending on the chain length. The hexamethylene chain of C(6)DA(2+) takes a twisted conformation, which not only allows strong ion-dipole interactions between the ammonium groups and the carbonyl groups at the portals, but also increases hydrophobic interactions between the alkyl part of the guest and the inner wall of the host, which results in the largest enthalpic gain among alpha,omega-alkanediammonium ions. The thermodynamic parameters associated with the complexation are discussed in relation to the binding modes and conformations of the aliphatic chain of the guest molecules inside the host, which were investigated by H-1 NMR spectroscopy and X-ray crystallography.