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

We determined the relative binding constants (Krel) for guests 1–19 towards cucurbit[7]uril by 1H 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 Krel values to absolute binding constants, we performed a direct UV–vis titration of 1 with CB[7] to determine Ka for CB[7]√1. The trends in the determined values of Krel and Ka 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.