Physicochemical and Structural Characterization of Quercetin-β-Cyclodextrin Complexes

Quercetin is a bioactive flavonoid widely used as a health supplement. Being sparingly soluble and chemically unstable in aqueous intestinal fluids, quercetin is poorly absorbed orally. This study aimed to investigate the effects of three β-cyclodextrins, namely, unsubstituted β-cyclodextrin (β-CD), hydroxypropyl-β-cyclodextrin (HP-β-CD), and sulfobutyl ether β-cyclodextrin (SBE-β-CD) on the chemical stability and water solubility of quercetin, and to elucidate the complexation mechanisms of these β-CDs with quercetin. Quercetin-β-CD complexes in solution were characterized by stability assessment, phase solubility measurements, and 1H-nuclear magnetic resonance (NMR) spectroscopy. Molecular modeling was used to help establish the mode of interaction of the β-CDs with quercetin. Solubility enhancements of quercetin obtained with the three β-CDs followed the rank order: SBE-β-CD > HP-β-CD > β-CD. The stability of quercetin at alkaline pHs also showed substantial improvement. NMR spectroscopic analysis suggested that the B-ring, C-ring, and part of the A-ring of quercetin display favorable interaction with the hydrophobic cavity of the β-CDs, which was confirmed by molecular dynamics (MD) simulations using a solvated model of the quercetin-β-CD complex. An inclusion complex model has been established for explaining the observed augmentation of solubility and stability of quercetin in water by β-CDs. © 2005 Wiley-Liss, Inc. and the American Pharmacists Association