NMR studies on puerarin and its interaction with beta-cyclodextrin

The interaction between puerarin and beta-cyclodextrin (CD) has been studied in D2O, H2O/acetone-d (6), acetone-d (6) and DMSO-d (6) solutions by H-1 NMR spectroscopy. The NMR data obtained from hydroxy protons indicate that the formation of the inclusion complex between the two molecules is not stabilized by strong hydrogen bond interactions. The sugar part of puerarin as well as the A ring are outside the beta-CD cavity while the B and C rings are located inside the cavity and the interaction is mainly stabilized by hydrophobic interactions. In DMSO at 30A degrees C and in acetone-d (6)/H2O at temperature below -5A degrees C, doubling of some signals indicated that, in these solvent systems, free rotation of the C-glycosyl bond was restricted due to the steric hindrance between the phenolic hydroxy group at C-7 and the bulky sugar moiety at C-8. In acetone, fast exchange of phenolic protons on the NMR timescale was observed, showing the effect of the solvent on the hindered rotation.