Deciphering ephedrine inclusion complexes with β-cyclodextrin, 18-crown-6 and cucurbit[7]uril using spectral and molecular modeling methods

Abstract

Inclusion complexes of ephedrine (EPh) with β-cyclodextrin (βCD), 18-crown-6 (18C6) and cucurbit[7]uril (CB7) were investigated using experimental and theoretical methods. The addition of βCD to the aqueous solution of EPh enhances the fluorescence emission spectrum at 312 nm. Whereas, the addition of CB7 and 18C6 cause enhancement of this band accompanied by growth of a band at around 412 nm. Electrospray ionization mass spectrometry, ESI-MS, results suggested that complexes of various stoichiometries are formed. The 1:1 complexes are the most dominant ones however, 1:2 (host: guest) stoichiometry and 2:1 (host: guest) stoichiometry for EPh with βCD and CB7 are also observed. The 1HNMR has indicated that EPh enters the CD hydrophobic cavity from the secondary rim and the aromatic ring is deeply inserted into the cavity with the ammonium ion left outside exposed to the aqueous solution. The molecular dynamics, MD, calculations show that EPh forms a stable complex with βCD and CB7 but not with 18C6. Furthermore, the ternary complexes of EPh–βCD with 18C6 and CB7 were found unstable according to the MD simulation studies, and ESI-MS experiments. The solid-state complexes prepared by freeze-drying and characterized by Fourier transform infrared spectroscopy and powder X-ray diffraction confirmed that only binary complexes are formed.