Ion binding to cucurbit[6] uril:: Structure and dynamics

Molecular dynamics simulations are used to study the microscopic structure and dynamics of cations bound to cucurbit[6]uril ( CB[ 6]) in water and in aqueous solutions of sodium, potassium, and calcium chloride. The molarities are 0.183 M for the salts and 0.0184 M for CB[ 6]. The cations bind only to CB[ 6] carbonyl oxygens. They are never found inside the CB[ 6] cavity. Complexes with Na+ and K+ mostly involve one cation, whereas with Ca2(+) single- and double- cation complexes are formed in similar proportions. The binding dynamics strongly depends on the type of cation. A smaller size or higher charge increases the residence time of a cation at a given carbonyl oxygen. When bound to CB[ 6], sodium and potassium cations jump mainly between nearest or second- nearest neighbors. Calcium shows no hopping dynamics. It is coordinated predominantly by one CB[ 6] oxygen. A few water molecules ( zero to four) can occupy the CB[ 6] cavity, which is limited by the CB[ 6] oxygen faces. Their residence time is hardly influenced by sodium and potassium ions. In the case of calcium the residence time of the inner water increases notably. A simple structural model for the cation activity as "lids" over the CB[ 6] portal cannot, however, be identified. The slowing of the water exchange by the ions is a consequence of the generally slower dynamics in their presence and of their stable solvation shells.