Monomeric Cryptophane with Record-High Xe Affinity Gives Insights into Aggregation-Dependent Sensing.

Area of Science:

• Supramolecular Chemistry
• Nuclear Magnetic Resonance (NMR) Spectroscopy
• Medical Imaging

Background:

• Cryptophane host molecules are promising for ultrasensitive ¹²⁹Xe NMR/MRI contrast agents.
• Understanding cryptophane-Xe sensing mechanisms, including aggregation effects, is crucial for developing advanced imaging tools.

Purpose of the Study:

• To design and characterize a novel water-soluble cryptophane (AFCA) with a pendant adamantyl group for studying Xe sensing behavior.
• To investigate the influence of cryptophane aggregation and receptor binding (β-CD) on ¹²⁹Xe NMR signals.

Main Methods:

• Synthesis and characterization of adamantyl-functionalized cryptophane-A (AFCA).
• Determination of Xe-AFCA association constant using fluorescence quenching.
• ¹²⁹Xe NMR spectroscopy and dynamic light scattering (DLS) to assess Xe binding, aggregation state, and chemical shift changes.

Main Results:

• AFCA exhibited the highest reported Xe affinity (K_A = 114,000 ± 5000 M^-1) in buffer.
• ¹²⁹Xe NMR signals were only observed for monomeric AFCA, not aggregated forms.
• No ¹²⁹Xe chemical shift change occurred upon β-CD binding, indicating aggregation state change is key for sensing.

Conclusions:

• ¹²⁹Xe NMR chemical shift in cryptophane sensing is critically dependent on the host molecule's aggregation state.
• High-affinity Xe sensors require careful design to maintain a monomeric state for signal detection.
• Findings provide insights into cryptophane-based Xe sensing limitations and guide the development of improved monomeric sensors.