Study on the complexation of isoquercitrin with beta-cyclodextrin and its derivatives by spectroscopy.

The inclusion complexes of isoquercitrin (IQ) with cyclodextrins (CDs) including beta-cyclodextrin (beta-CD), hydroxypropyl-beta-cyclodextrin (HP-beta-CD) and dimethyl-beta-cyclodextrin (DM-beta-CD) have been investigated using the methods of steady-state fluorescence, UV-vis absorption and induced circular dichroism. The stoichiometric ratio of the three complexes was found to be 1:1 and the stability constants (K) were estimated from spectrofluorometric titrations, as well as the thermodynamic parameters. Maximum inclusion ability was measured in the case of DM-beta-CD due to the increased hydrophobicity of the host cavity, followed by HP-beta-CD and beta-CD. The effect of pH on the complexation process was also quantitatively assessed. IQ exists in different molecular forms depending on pH and beta-CDs were most suitable for inclusion of the neutral form of IQ. The phase-solubility diagrams obtained with beta-CD, HP-beta-CD and DM-beta-CD were all classical A(L) type. And DM-beta-CD provided the best solubility enhancement, 12.3-fold increase compared to 2.8- and 7.5-fold increase for beta-CD and HP-beta-CD. The apparent stability constants obtained from the solubility data at 25 degrees C were comparable with those obtained from the fluorescence assays. Moreover, (1)H NMR was carried out, which revealed that the IQ favorably inserted into the inner cavity from the chromone part instead of the phenyl part, which was in agreement with molecular modeling studies.