Allosterically Controlled Capture of Mutually Repulsive Guests With Positive Cooperativity and Ultrahigh Affinity

Area of Science:

• Supramolecular Chemistry
• Host-Guest Chemistry
• Chemical Sensing

Background:

• Biological systems exhibit allosteric regulation, where binding events influence distant sites.
• Encapsulating mutually repulsive anions within synthetic hosts presents significant challenges due to electrostatic repulsion.

Purpose of the Study:

• To design and synthesize a hexacationic cage capable of binding two mutually repulsive anions.
• To investigate the binding mechanisms, affinities, and conformational changes upon guest encapsulation.
• To explore potential applications in anion extraction and catalysis.

Main Methods:

• Synthesis of a hexacationic cage featuring three urea units.
• Spectroscopic studies (NMR) to determine binding constants and conformational changes.
• Computational modeling to understand host-guest interactions.

Main Results:

• The cage exhibits exceptionally high binding affinities (K₁×K₂ ≈ 10²⁰ M⁻²) for chloride (Cl⁻) and fluoride (F⁻) anions.
• Anion binding induces a conformational change from a collapsed to an unfolded state, facilitating the binding of the second anion.
• The second anion binds significantly stronger (three orders of magnitude) than the first, demonstrating an allosteric effect.
• The cage successfully extracts F⁻ from CaF₂ and Cl⁻/Br⁻ from organic halides.

Conclusions:

• A synthetic strategy for mimicking biological allosteric regulation via anion encapsulation has been developed.
• The cage's strong halide binding affinity has potential applications in fluorine recovery, catalysis, and corrosion inhibition.