Sodium-Potassium Competition as a Tool for Chemoselective Cucurbit[n]uril-Xenon Host-Guest Complexation in Imaging Applications

Cucurbit[n]urils (CB[n]) are a family of macromolecules, capable of encapsulating neutral or charged species. Due to their low toxicity, good solubility, and transport properties resulting from the structure forming the cage, they have gained popularity in imaging-oriented studies. In vitro studies have shown that one of the guests compatible with the CB[n] hosts are Xe atoms, leading to high-resolution 129Xe magnetic resonance signals in pulmonary medical imaging. This prompts the question of whether similar results could be achieved in vivo in other tissues and body fluids. Here, we demonstrate how the varying efficacy of Xe encapsulation by a CB[6] cage is governed by the Na+ : K+ molecular ratio in simulated body fluids. Moving from a high concentration of Na+ to an excess of K+ in the solution significantly increases Xe encapsulation by the macromolecules. This finding indicates chemoselectivity of cucurbit[6]uril host-guest interactions. Since Na+ and K+ are the most abundant ions in human body fluids, our results provide a theoretical foundation for future liquid-state in vivo Xe imaging in medical applications.