Experimental and theoretical study of the inclusion complexes of epinephrine with β-cyclodextrin, 18-crown-6 and cucurbit[7]uril

The binary and ternary complexes of epinephrine (EP) with β-cyclodextrin (βCD), 18-crown-6 (18C6) and cucurbit[7]uril (CB7) were probed using different experimental techniques. The results show that EP forms a stable binary complex with the three hosts. The fluorescence spectroscopy measurements showed enhancement in the emission spectrum at around 312 nm when βCD was added to the aqueous solution of EP, whereas the addition of CB7 and 18C6 caused quenching of this band accompanied by evolution of a band at around 412 nm. The ¹H NMR spectroscopy confirmed the formation of the inclusion complexes. The ¹H NMR spectroscopy indicated that the drug enters the CD nanocavity from its wider rim and deeply inserts the catechol moiety into the cavity. The association constants of binary complexes estimated by diffusion-ordered spectroscopy, DOSY, indicated a more stable complex was formed between EP and CB7. On the other hand, ESI-MS and MALDI-TOF data suggest that complexes of various stoichiometries are formed. This discloses the presence of binary and ternary complexes between EP and the three hosts. These data together with IR, Raman and PXRD for the lyophilized complexes clearly suggest that EP forms stable complexes with the three hosts in the solid and aqueous phase. These results were further clarified using molecular dynamics simulations for the five different complexes in aqueous media for 30 ns. The results obtained indicated that the hydrophobic effects and the hydrogen bonding interactions are the driving forces for the formation of the complexes and are responsible for their stabilities.