Stabilization of Guest Molecules inside Cation-Lidded Cucurbiturils Reveals that Hydration of Receptor Sites Can Impede Binding

Docking of alkali metal ions to water-soluble macrocyclic receptors generally reduces the affinity of guest molecules due to competitive binding. The idea that solvation water molecules could display a larger steric hindrance towards guest binding than cations has not been considered to date. We show that the docking of large cations to cucurbit[5]uril (CB5) unexpectedly increases (by a factor of 5-8) the binding of hydrophobic guests, methane and ethane. This is due to the removal of water molecules from the carbonyl portals of CB5 during cation binding, which frees up space for hydrophobe encapsulation. In contrast, smaller cations like sodium protrude deeply into the cavity of CB5 and cause the expected decrease in binding, such that the rational selection of alkali cations allows for a variation of up to a factor of 20 in binding of methane and ethane. The statistical analysis of crystallographic data shows that the cavity volume of CB5 can be enlarged by placing large alkali ions (Rb+ and Cs+ ) centro-symmetrically at the portals. The results reveal a hitherto elusive steric hindrance of solvation water molecules near receptor binding sites, which is pertinent for the design of supramolecular catalysts and the understanding of biological receptors.

Probing Reversible Guest Binding with Hyperpolarized 129Xe-NMR: Characteristics and Applications for Cucurbit[n]urils

Cucurbit[n]urils (CB[n]s) are a family of macrocyclic host molecules that find various applications in drug delivery, molecular switching, and dye displacement assays. The CB[n]s with n = 5-7 have also been studied with 129Xe-NMR. They bind the noble gas with a large range of exchange rates. Starting with insights from conventional direct detection of bound Xe, this review summarizes recent achievements with chemical exchange saturation transfer (CEST) detection of efficiently exchanging Xe in various CB[n]-based supramolecular systems. Unprecedented sensitivity has been reached by combining the CEST method with hyperpolarized Xe, the production of which is also briefly described. Applications such as displacement assays for enzyme activity detection and rotaxanes as emerging types of Xe biosensors are likewise discussed in the context of biomedical applications and pinpoint future directions for translating this field to preclinical studies.

Supramolecular chemistry of substituted cucurbit[n]urils

This review covers important advances in the field of substituted cucurbit[n]urils.

Porous supramolecular assemblies and functional properties of perhydroxylated cucurbit[6]uril and polyoxometallates

(HO)₁₂Q[6] and POMs were used as basic building blocks, and interactions yielded two novel types of supramolecular assemblies at different HCl concentrations.

4-Sulfocalix[4]arene/Cucurbit[7]uril-Based Supramolecular Assemblies through the Outer Surface Interactions of Cucurbit[n]uril

Upon mixing of aqueous solutions of the freely soluble building blocks cucurbit[7]uril (Q[7]) and 4-sulfocalix[4]arene (SC[4]A), white microcrystals instantly separate in near-quantitative yield. The driving force for this assembly is suggested to be the outer-surface interaction of the Q[n]. Dynamic light scattering, scanning electron microscopy, and NMR (diffusion-ordered NMR spectroscopy) analyses have confirmed the supramolecular aggregation of Q[7] and SC[4]A. Titration ¹H NMR spectroscopy and isothermal titration calorimetry have shown that the interaction ratio of Q[7] and SC[4]A is close to 3:1. Moreover, the Q[7]/SC[4]A-based supramolecular assembly can accommodate molecules of some volatile compounds or luminescent dyes. Thus, this work offers a simple and highly efficient means of preparing adsorbent or solid fluorescent materials.

Coordination and Supramolecular Assemblies of Fully Substituted Cyclopentanocucurbit[6]uril with Lanthanide Cations in the Presence of Tetrachlorozincate Anions, and Their Potential Applications

Coordination and supramolecular assemblies of a fully substituted cyclopentanocucurbit[6]uril (CyP₆Q[6]) with a series of lanthanide cations (Ln³⁺) have been investigated in the presence of tetrachlorozincate anion ([ZnCl₄]^2-). X-ray single-crystal diffraction analysis has revealed that the interaction of CyP₆Q[6] and a series of Ln³⁺cations unexpectedly results in the formation of at least seven different CyP₆Q[6]-based coordination complex adduct and supramolecular assemblies groups, including with (1) La³⁺, Ce³⁺cations; (2) Pr³⁺, Nd³⁺cations; (3) Eu³⁺, Gd³⁺, Tb³⁺, Dy³⁺ with P1̅ or P1 space group, in which CyP₆Q[6] molecules coordinate alternatively with Ln³⁺cations and form linear coordination polymers; (4) CyP₆Q[6] molecules interact alternatively with [Ho(H₂O)₈]³⁺ aqueous complexes and form linear supramolecular chains; CyP₆Q[6] molecules can assemble two different Ln³⁺ free porous supramolecular assemblies from CyP₆Q[6]-Ln(NO₃)₃-ZnCl₂-HCl systems, Ln = Tm, Yb, and Lu; however, no solid crystals were obtained from system containing Er³⁺cation. Thus, these differences could lead CyP₆Q[6] to be useful in not only the isolation of lighter lanthanides from their heavier lanthanides but also special selectivity for different volatile organic compounds.

Coordination supramolecular assemblies of a monohydroxycucurbit[7]uril and their potential applications in gas sorption

Coordination supramolecular assemblies of monohydroxycucurbit[7]uril ((HO)Q[7]) with alkaline earth metal ions (AE²⁺) have been formed in aqueous HCl solution in the presence of tetrachloride cadmium anions ([CdCl₄]^2-) as a structure directing agent. The driving force for the assembly could be attributed to the interaction of the positive electro-potential outer-surface of (HO)Q[7] molecules with [CdCl₄]^2- anions and ionic dipole interaction of the hydroxyl of (HO)Q[7] molecules with [CdCl₄]^2- anions. Moreover, the porous structure of the (HO)Q[7]/AE²⁺-based coordination supramolecular assemblies could result in potential applications in the selective sorption of polar volatile organic molecules, which may be useful in molecular sieves, sensors, absorption and separation.

Highly selective absorption of polychloromethanes in perhydroxylated cucurbit[6]uril-based supramolecular assemblies

Two perhydroxylated cucurbit[6]uril-based porous assemblies have been obtained by the crystallization of perhydroxylated cucurbit[6]uril {(HO)₁₂Q[6]} from aqueous HCl solutions of different concentrations, which exhibit highly selective absorption of polychloromethanes.