Nuclear Magnetic Resonance Spectroscopic and Thermodynamic Characterization of Hydroxypropyl-cyclodextrin Inclusion Complexes with Bioactive Molecules from Plant Mother Tinctures

This study investigates the formation of inclusion complexes using the cyclodextrins, specifically HP-β-CD and HP-γ-CD, with bioactive molecules that mask the unpleasant tastes found in herbal extracts, such as horehound, eucalyptus, plantain, and thyme. The selected bioactive molecules (BAMs), including thymol, carvacrol, and eucalyptol, were subjected to inclusion complexation with host hydroxypropyl-cyclodextrins (HP-CDs) and studied by ¹H nuclear magnetic resonance (NMR) spectroscopy. The stoichiometry of the complexes was determined to be 1:1 (HP-CD:BAM). The analysis of ¹H chemical shift changes in BAM NMR spectra allowed the calculation of binding constants, indicating better stability with HP-γ-CD due to its larger cavity size and enhanced flexibility, which favors stronger and more stable inclusion with bioactive molecules. Two-dimensional rotating-frame Overhauser effect spectroscopy experiments provided insight into the binding mode. A simple thermodynamic model highlighted the hydrogen bonding contribution to complexation. Furthermore, the inclusion complex of the carvacrol and eucalyptol mixture showed that carvacrol consistently maintains the most stable complex, even in the presence of both active compounds combined with HP-CD.