Stimuli Responsive Systems Constructed Using Cucurbitnuril-Type Molecular Containers

Conspectus This Account focuses on stimuli responsive systems that function in aqueous solution using examples drawn from the work of the Isaacs group using cucurbitnuril (CBn) molecular containers as key recognition elements. Our entry into the area of stimuli responsive systems began with the preparation of glycoluril derived molecular clips that efficiently distinguish between self and nonself by H-bonds and π-π interactions even within complex mixtures and therefore undergo self-sorting. We concluded that the selectivity of a wide variety of H-bonded supramolecular assemblies was higher than previously appreciated and that self-sorting is not exceptional behavior. This lead us to examine self-sorting within the context of CBn host-guest chemistry in water. We discovered that CBn homologues (CB7 and CB8) display remarkably high binding affinity (Ka up to 10(17) M(-1)) and selectivity (ΔΔG) toward their guests, which renders CBns prime components for the construction of stimuli responsive host-guest systems. The CB7·adamantaneammonium ion complex, which is particularly privileged (Ka = 4.2 × 10(12) M(-1)), was introduced by us as a stimulus to trigger constitutional changes in multicomponent self-sorting systems. For example, we describe how the free energy associated with the formation of host-guest complexes of CBn-type receptors can drive conformational changes of included guests like triazene-arylene foldamers and cationic calix4arenes, as well as induced conformational changes (e.g., ammonium guest size dependent homotropic allostery, metal ion triggered folding, and heterochiral dimerization) of the hosts themselves. Many guests display large pKa shifts within their CBn-guest complexes, which we used to promote pH controlled guest swapping and thermal trans-to-cis isomerization of azobenzene derivatives. We also used the high affinity and selectivity of CB7 toward its guests to outcompete an enzyme (bovine carbonic anhydrase) for a two-faced inhibitor, which allowed stimuli responsive regulation of enzymatic activity. These results prompted us to examine the use of CBn-type receptors in both in vitro and in vivo biological systems. We demonstrated that adamantaneammonium ion can be used to intracellularly sequester CB7 from gold nanoparticles passivated with hexanediammonium ion·CB7 complexes and thereby trigger cytotoxicity. CB7 derivatives bearing a biotin targeting group enhance the cytotoxicity of encapsulated oxaliplatin toward L1210FR cells. Finally, acyclic CBn-type receptors function as solubilizing excipients for insoluble drugs for drug delivery purposes and as a broad spectrum reversal agent for the neuromuscular blocking agents rocuronium, vecuronium, and cis-atracurium in rats. The work highlights the great potential for integration of CBn-type receptors with biological systems.