Selective Binding Behaviors of β-Cyclodextrin and Its Derivatives with Azadirachtin Guests by Competitive Inclusion Method

Spectrophotometric titrations were performed in aqueous buffer solution (pH = 10.5) at 25 degrees C to determine the binding constants of beta-cyclodextrin(beta-CD), mono (6-ethylene-diamino-6-deoxy)-beta-cyclodextrin (en-beta-CD), mono (6-diethylene-triamino-6-deoxy)-beta-cyclodextrin(dien-beta-CD), heptakis-(2,6-tri-O-methyl)-beta-cyclodextrin (DM-beta-CD) heptakis (2, 3, 6-tri-O-methyl)beta-Cyclodextrin (TM-beta-CD), and (2-hydroxypropyl)-beta-cyclodextrin (HP-beta-CD) with azadirachtin guest molecules using phenolphthalein as a spectral probe. The results indicate that several weak interactions cooperatively contribute to the inclusion complexation of the beta-cyclodextrin hosts and the complex stability is dominated by the size/shape-matching between host and guest. For azadirachtin, the molecular inclusion ability is HP beta-CD > en-beta-CD approximate to dien-beta-CD > beta-CD > DM-beta-CD - TM-beta-CD. The substituent on the cyclodextrin derivatives changes its original binding ability. Furthermore, the six beta-cyclodextrin hosts can also serve as discrimination reagents toward azadirachtin, and have the similar binding constants between azadirachtin A and azadirachtin B.