Molecular insight into the inclusion of the dietary plant flavonol fisetin and its chromophore within a chemically modified γ-cyclodextrin: Multi-spectroscopic, molecular docking and solubility studies

We explored the encapsulation of dietary plant flavonols fisetin and its chromophore 3-hydroxyflavone, within 2-hydroxypropyl-gamma-cyclodextrin (HP.-CDx) nano-cavity in aqueous solution using multi-spectroscopic approaches and molecular docking. Upon addition of HP gamma-CDx, dramatic changes occur in the intrinsic 'two color' fluorescence behavior of the fluorophores. This is manifested by significant increase in the steady state fluorescence intensities, anisotropies, average fluorescence lifetimes and rotational correlation times. Furthermore, in the CDx environment, intrinsically achiral flavonols exhibit prominent induced circular dichroism bands. These findings indicate that the flavonol molecules spontaneously enter the relatively hydrophobic, chiral environment of the HP gamma-CDx nano-cavities. Molecular docking computations corroborate the spectroscopic findings, and predict selectivity in orientation of the encapsulated flavonols. HP gamma-CDx inclusion increases the aqueous solubility of individual flavonols similar to 100-1000 times. The present study demonstrates that the hydroxypropyl substituent in gamma-CDx controls the inclusion mode of the flavonols, leading to their enhanced solubilization and altered spectral signatures.