Prototypical Allosterism in a Simple Ditopic Ligand: Gas-Phase Topologies of Cucurbit[n]uril· n -Alkylammonium Complexes Controlled by Binding in the Second Site.

We have employed mass spectrometry, ion mobility, and computational techniques to characterize complexes of n -alkylammonium ions with cucurbit[5]uril (CB[5]) and cucurbit[6]uril (CB[6]) ligands in the gas phase. Nonrotaxane structures are energetically preferred and experimentally observed for all CB[5] complexes. Pseudorotaxane structures are computationally favored and experimentally observed for [CB[6]· n -alkylammonium] + complexes, but the addition of a second cation (proton, alkali metal ion, another alkylammonium ion, or guanidinium) on the opposite rim of CB[6] causes sufficiently unfavorable steric interactions that n -pentylammonium and longer chains no longer remain threaded through the CB[6] cavity; nonrotaxane topologies are then favored. This provides a very simple example of negative allosteric interactions and molecular structure switching in these complexes.