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 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 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]√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.