Supramolecular complexation of phenylephrine by cucurbit[7]uril in aqueous solution

Cucurbiturils (CBn) are known to establish stable host-guest complexes with a variety of drug molecules. Herein, the supramolecular complexation between cucurbit[7]uril (CB7) and phenylephrine hydrochloride is reported in aqueous solution. Phenylephrine forms inclusion complex with CB7 with high binding affinity (K_affinity = 4.0 × 10⁶ M^-1), which allows for the development of a fluorescence-based sensing assay applying the dye displacement strategy. The structure of the host-guest inclusion complex is investigated by ¹H NMR spectroscopy, in which complexation-induced chemical shifts indicate the immersion of the aromatic ring inside the hydrophobic cavity of CB7. Density functional theory (DFT) calculations support the ¹H NMR results, and reveal that the complex is stabilized through intermolecular interactions between the polar groups on the phenylephrine and the carbonyl rims of CB7, as well as the hydrophobic effect. Moreover, preferential binding of phenylephrine in its protonated over the neutral form results in a complexation-induced pK_a shift.