Sodium-Potassium Competition Controls Xenon Uptake in Cucurbiturils.

The remarkable specificity of 129Xe magnetic resonance has motivated efforts to expand its application to medical diagnostics and to develop Xe-cage guest-host complexes compatible with in vivo conditions. Cucurbit[n]urils (CB[n]) are among the most promising host molecules for direct biocompatible Xe encapsulation. Building on earlier observations that alkali cations influence the binding of Xe to CB[n], this study provides a comprehensive experimental and computational analysis of how the ratio of Na+ to K+ in the solution governs Xe uptake by CB[6] and CB[5]. Using one- and two-dimensional 129Xe NMR together with quantum-mechanical modeling, we elucidate the molecular mechanism by which alkali cations modulate cage Xe uptake. Our calculations reveal that Na+ can adopt a near-central position inside CB[6], directly competing with Xe for the hydrophobic core, thereby suppressing encapsulation. In contrast, K+ remains confined to peripheral portal sites, leaving the central cavity accessible, and enabling Xe to displace the cation and stabilize inside the cage. Elevated temperatures and cage-cage competition further amplify these effects. By combining experimental and theoretical results, we thus provide the first detailed mechanistic explanation of Na+/K+ competition in Xe-CB[n] systems, and establish clear design principles to optimize Xe hosts for future in vivo applications.