Absolute and relative binding affinity of cucurbit[7] uril towards a series of cationic guests

We determined the relative binding constants (K-rel) for guests 1-19 towards cucurbit[7]uril by H-1 NMR competition experiments in 100 mM Na3PO4-buffered D2O. In these experiments, we use guest 11 as the reference guest because of its strong binding towards CB[7] and its advantageous spectroscopic properties (e.g. slow exchange on NMR timescale and distinct resonances for key protons). To convert the determined K-rel values to absolute binding constants, we performed a direct UV-vis titration of 1 with CB[7] to determine K-a for CB[7]root 1. The trends in the determined values of K-rel and K-a are discussed with respect to the importance of the concentration of metal ions in the buffer, the influence of hydroxyl groups located at the portals or inside the CB[7] cavity, geometry of the guest (e.g. regioisomers), the number of guest C atoms and secondary electrostatic interactions.