Cyclodextrins-Kaempferol Inclusion Complexes: Spectroscopic and Reactivity Studies

The slightly water-soluble flavonoid kaempferol (KAE) and its inclusion complexes with beta-cyclodextrin (beta CD), hydroxypropyl-beta-cyclodextrin (HP beta CD) or heptakis-2,6-O-dimethyl-beta-cyclodextrin (DM beta CD) were investigated. The stoichiometric ratios and association constants describing the extent of the formation of the complexes have been determined. Binding constants, estimated from fluorescence studies at different temperatures, were analyzed so as to gain information about the mechanisms involved in the association processes. The thermodynamic data for the inclusion of KAE in DM beta CD and HP beta CD indicated that it is mainly enthalpy-driven whereas for beta CD it is an entropy-driven process. Complex formation was monitored by two-dimensional ROESY experiments through the detection of intramolecular dipolar interaction. ROESY experiments provided data indicating that the B-ring of kaempferol is immersed in the apolar cavity with the A- and C-ring protruding from the wider rim for the three cyclodextrins studied. The antioxidant studies of KAE and CDs complexes showed an increment in its antioxidant activity. The complexes behave as better antioxidants than kaempferol alone.