Oligopeptide-CB complexation with switchable binding pathways†
Host–guest complexes exhibiting a 1 : 1 binding stoichiometry need not consist of a single host and guest. A series of oligopeptides, which were previously reported to have abnormally high binding enthalpies were investigated to deduce whether they exist as a 2 : 2 quaternary or a 1 : 1 binary complex with cucurbituril (CB). Through a systematic study of the sequence-specific binding pathways of peptide-CB association, a phenylalanine-leucine dipeptide was found to be capable of switching from a 1 : 1 stoichiometric complex to a 2 : 1 complex. By studying the differences in size-based diffusion properties of these two binding modes, the presence of a 1 : 1 pairwise inclusion complex was verified for the regime where CB is in excess. Findings in this study can be utilised to ‘customise’ the precise CB-oligopeptide self-assembly pathway, acting as a useful toolbox in the design of supramolecular systems.