Salt effects on the apparent stability of the cucurbit[7]uril-methyl viologen inclusion complex

The effects of the medium ionic composition on the apparent equilibrium association constant (K) for the formation of a 1:1 inclusion complex between the guest methyl viologen (MV2+) and the host cucurbit[7]uril (CB7) were studied in aqueous solutions. The K values were found to decrease with increasing ionic strength, with more pronounced effects for solutions containing divalent Ca2+ ions than for solutions containing monovalent Na+ ions. The competing ion-dipole interactions between Ca2+ or Na+ and MV2+ ions appear to be responsible for the remarkable modulation of the K values observed in this work.