The binding behaviours between cyclopentanocucurbit[6]uril and three amino acids

Binding behaviours between cyclopentanocucurbit[6]uril (CyP(6)Q[6]) and three amino acids have been investigated by means of X-ray crystallography, proton nuclear magnetic resonance spectroscopy and isothermal titration calorimetry. The results showed that CyP(6)Q[6] forms a 1 : 2 inclusion complex with glycine, but 1 : 1 complexes with both leucine and lysine. Whereas the carboxyl group of glycine can enter the interior of the cavity of CyP(6)Q[6], only the alkyl chains of leucine and lysine can enter this cavity. Interestingly, leucine can adopt two different self-assembly modes upon its interaction with cucurbituril, depending on the external conditions, whereas glycine and lysine do not exhibit such behaviour.