Preparation, Characterization and Water Solubility of Inclusion Complexes of Daidzein with Amino-Modified β-Cyclodextrins

To improve the water solubility of daidzein, solid inclusion complexes of daidzein with two amino-modified β-cyclodextrins (ACDs), i.e., mono-6-amino-6-deoxy-β-cyclodextrin (NCD) and mono-6-ethylenediamino-6-deoxy-β-cyclodextrin (ENCD), were prepared by the saturated solution method in water with yields of 83% and 67%, respectively, under the optimized conditions including the ratio of daidzein/ACD of 3:1 and the stirring time of 72 h. The formation of the two inclusion complexes was confirmed by X-ray diffractometry (XRD) and thermo gravimetric (TG) analysis. The inclusion stoichiometry of the inclusion complexes was 1:1 from the Job plot and their complexation stability constants (KS) were 899.2 and 203.8 M−1 from fluorescence titration, respectively. After formation of inclusion complexes with NCD and ENCD, the water solubility of daidzein was dramatically raised to 15.2 and 13.2 mg mL−1 at 25 °C, increasing by 1800 and 1500-fold compared to native daidzein (8.31 μg mL−1).