Cucurbit[7]uril⋅guest pair with an attomolar dissociation constant

Area of Science:

• Supramolecular Chemistry
• Host-Guest Chemistry
• Crystallography

Background:

• Cucurbiturils (CBs) are macrocyclic hosts known for their ability to bind guests.
• Diamantane diammonium ions are guests with specific structural features.
• Understanding host-guest interactions is crucial for designing new molecular systems.

Purpose of the Study:

• To investigate host-guest complex formation between cucurbit[7] (CB[7]) or CB[8] and diamantane diammonium ion guests.
• To characterize the binding affinity and structural basis of these complexes.
• To compare the binding strength with existing non-covalent complexes.

Main Methods:

• Proton Nuclear Magnetic Resonance (1H NMR) spectroscopy for binding studies.
• X-ray crystallography for structural elucidation.
• Competition experiments to determine binding constants (Ka).

Main Results:

• CB[7]⋅6 complex demonstrated exceptionally high binding affinity in water (Ka = 7.2×10^17 M^-1 in D2O).
• Structural analysis revealed key interactions: optimal ion-dipole interactions, surface complementarity, portal desolvation, and axis co-linearity.
• The binding strength surpasses many previously reported monovalent non-covalent complexes.

Conclusions:

• The CB[7]⋅6 complex represents one of the tightest synthetic host-guest complexes in water.
• Specific structural features of both the host (CB[7]) and guest (6) contribute to the ultratight binding.
• This research advances the understanding of synthetic receptors and blurs the line between natural and synthetic binding systems.