Binding of bambusuril with bipyridinium guests in water

Bambusurils are a family of receptor molecules related to the cucurbit[n]urils and appreciated for their high affinities to inorganic anions. Here we show that a bambusuril macrocycle bearing carboxylate functional groups on its portals can form external complexes with bipyridinium cationic guests in aqueous solution. The apparent binding affinities of the complex can be modulated by the addition of potassium cations, which compete with the guests for the carboxylate portals of the macrocycle. We further illustrate that the binding mode of these host-guest systems varies depending on the choice of bipyridinium guests.