Host–guest complexations of local anaesthetics by cucurbit[7]uril in aqueous solution

The cucurbit[7]uril (CB[7]) host molecule forms very stable host–guest complexes with the local anaesthetics procaine (K_CB[7] = (3.5 ± 0.7) × 10⁴ dm³ mol⁻¹), tetracaine (K_CB[7] = (1.5 ± 0.4) × 10⁴ dm³ mol⁻¹), procainamide (K_CB[7] = (7.8 ± 1.6) × 10⁴ dm³ mol⁻¹), dibucaine (K_CB[7] = (1.8 ± 0.4) × 10⁵ dm³ mol⁻¹) and prilocaine (K_CB[7] = (2.6 ± 0.6) × 10⁴ dm³ mol⁻¹) in aqueous solution (pD = 4.75). The stability constants are 2–3 orders of magnitude greater than the values reported for binding by the comparably sized β-cyclodextrin host molecule. The inclusion by CB[7] raises the first pK_a values of the anaesthetics by 0.5–1.9 pK units, as the protonated forms are bound more strongly in acidic solution. The complexation-induced chemical shift changes in the guest proton resonances provide an indication of the site(s) of binding and the effects of protonation on the location of the binding sites.