Substantial solubility and fluorescence property changes upon inclusion of rutaecarpine in cucurbit[7]uril: Kinetics and thermodynamics of binding

The effect of acid and cucurbit[7]uril (CB7) additions on the solubility and fluorescent properties of rutaecarpine (Rut), a pharmaceutically important plant alkaloid, was studied in aqueous solution. Approximately 550-fold water-solubility enhancement was achieved upon inclusion complex formation with cucurbit[7]uril (CB7). The cationic protonated alkaloid (RutH+) was encapsulated in CB7 with a binding constant more than three orders of magnitude larger than that of the uncharged Rut. The stabilization of RutH+ by the host macrocycle led to a pKa increase of ∼3.3 units compared with that found in water. The substantial increase in fluorescence quantum yield upon RutH+-CB7 production could be exploited for the sensitive visualization of pH differences in the range of 4.5–6.5. The highly exothermic complexation was accompanied by considerable entropy diminution, partly because of the critical role of hydrogen bonding between the constituents. The rate constants of the creation and dissociation of RutH+–CB7 were 2.5 × 106 M−1 s−1 and 0.94 s−1 at 283 K, respectively.