Cucurbit[n]urils (CB[n]s), which are macrocycles that are formed from glycouril units, are often employed as host molecules as they form stable complexes with cationic guests. Lyle Isaacs and his team at the University of Maryland, USA, wanted to investigate the binding properties of CB[n] dimers, and synthesized molecules containing two CB[6] units connected by phenyl or biphenyl units. They then studied the formation of ternary complexes between the dimers and mono- and diammonium guests.

Having confirmed that the recognition properties of the dimers are similar to those of the nonmodified CBs, the researchers then investigated the binding of compounds containing multiple alternating hexanediammonium and viologen units, and found that the systems self-assemble to form molecular “ladders” with the CB dimers as “rungs” (pictured). Mass spectrometry and NMR experiments were used to show that a three-rung ladder has dimensions of 2.6×4.0 nm and a molecular weight of 7389, which is in the range of small proteins.

The authors hope that higher-order multivalent CB[n] scaffolds can be synthesized in order to expand the range of binding possibilities of these compounds.

• Supramolecular Ladders from Dimeric Cucurbit[6]uril,
  James B. Wittenberg, Peter Y. Zavalij, Lyle Isaacs,
  Angew. Chem. Int. Ed. 2013.
  DOI: 10.1002/anie.201300404