Calix[4]arenes in their 1,3-alternate conformation are particularly convenient scaffolds for the construction of extended tubes. These molecules can act as polytopic cation receptors and have the fascinating property of allowing cation transport |("shuttling") along the tube by passing through the "π-basic tube" formed by the macrocyclic rings of the calixarene units. Although calix tubes can also influence anion transport, this does not appear to involve anion shuttling.
