Unique nuclear magnetic resonance behaviour of γ-cyclodextrin in organic solvents

The nuclear magnetic resonance (NMR) behaviour of dry α-, β-, and γ-cyclodextrin (CyD) in non-aqueous solutions with the solvents pyridine-d5, N,N-dimethyl formamide-d7 (DMF-d7), and dimethyl sulfoxide-d6 (DMSO-d6) were examined in order to study the interactions among hydroxyl groups on the rims of CyD. All signals were assigned using H-H correlation spectroscopy (COSY) and 1H-detected multiple quantum coherence spectroscopy (HMQC) spectra. In pyridine-d5, the hydroxyl groups of γ-CyD and water were observed as appreciably broad signals. Over 95 °C, a white precipitate appeared immediately, and after 4 days, it disappeared completely. All signals of α- and β-CyDs were sharp and exhibited a high field shift as the temperature increased. These spectral changes were reversible, but in the case of γ-CyD in dry pyridine-d5, the time temperature-fall for reaching the equilibrium state took longer than that of temperature rise. The dependency of NMR spectroscopy of γ-CyD on the solvent, the concentration of water, and temperature suggested that hydroxyl groups on γ-CyD are sufficiently flexible to interact via intermolecular hydrogen bond formation and to form insoluble aggregates.