Calixarenes are bowl-shaped molecules that can be used to encapsulate small guest molecules when placed rim-to-rim to form a capsule. One guest molecule can be exchanged for another but the mechanism of exchange is unclear. Volker Böhmer and colleagues at Johannes Gutenberg University, Germany, made a series of calixarene capsules with different ether residues attached to the narrow rim. They found that the rate of exchange of guest molecules decreased as the size of the ethers increased. Böhmer explains more about the significance of his research below: 

'Self-organisation of molecules to assume a certain (active) conformation (peptides, enzymes) or to form a certain assembly (DNA) is essential for living systems. Learning from Nature, numerous and quite different artificial examples have been elaborated. Sophisticated systems of intra- and intermolecular hydrogen bonds are often the glue for such assemblies. 

Although numerous hydrogen bonded dimers and larger assemblies are known (e.g. hexameric capsules of resorcinarenes or pyrogallolarenes, to stay with calixarenes as building blocks), hydrogen bonded dimers of tetraurea calix[4]arenes present a special challenge due to their intricate and intriguing structure. The mutually interdigitating urea residues at the wide rim of the two calixarenes offer additional synthetic possibilities (e.g. by covalent intra- or intermolecular connection) and raise further questions. Thus, the detailed mechanism for the exchange of an included guest is unknown. 

Of course, our studies are still far away from such a detailed mechanism. However, we demonstrate for the first time that even subtle structural changes of the calix[4]arenes in the ether region at the narrow rim can have a drastic effect. The rate changes by more than three orders of magnitude when methyl ether groups are replaced by benzylether groups, a result which we did not expect when we started these studies. 

In the future we hope to get some further insight into the guest exchange process and its potential mechanism using different guests under various conditions.'