• affinity pairs;
  • avidin–biotin;
  • cucurbit[n]uril;
  • host-guest systems;
  • X-ray diffraction


Host⋅guest complexes between cucurbit[7] (CB[7]) or CB[8] and diamantane diammonium ion guests 3 or 6 were studied by 1H NMR spectroscopy and X-ray crystallography. 1H NMR competition experiments revealed that CB[7]⋅6 is among the tightest monovalent non-covalent complexes ever reported in water with Ka=7.2×1017M−1 in pure D2O and 1.9×1015M−1 in D2O buffered with NaO2CCD3 (50 mM). The crystal structure of CB[7]⋅6 allowed us to identify some of the structural features responsible for the ultratight binding, including the distance between the NMe3+ groups of 6 (7.78 Å), which allows it to establish 14 optimal ion-dipole interactions with CB[7], the complementarity of the convex van der Waals surface contours of 6 with the corresponding concave surfaces of CB[7], desolvation of the C[DOUBLE BOND]O portals within the CB[7]⋅6 complex, and the co-linearity of the C7 axis of CB[7] with the N+⋅⋅⋅N+ line in 6. This work further blurs the lines of distinction between natural and synthetic receptors.