Effect of β-cyclodextrin nanocavity confinement on the photophysics of robinetin

We have studied the confinement of robinetin, a therapeutically active plant flavonol, in cyclodextrin (CDx) nanocavities, using steady state and time resolved fluorescence spectroscopy. Enhanced tautomer emission (arising from excited state intramolecular proton transfer (ESIPT)) as well as dramatically blue shifted (∼10 nm in β-CDx and ∼33 nm in SHP β-CDx) normal fluorescence observed upon addition of the β-CDxs indicate that robinetin readily enters the doughnut-shaped hydrophobic cavity of β-CDx where the chromone moiety is well shielded from external hydrogen bonding perturbations. Detailed analyses of the fluorescence data (emission profile, anisotropy, decay times) indicate that robinetin forms 1:1 inclusion complexes with both natural and chemically modified β-cyclodextrins (β-CDx and SHP β-CDx) with affinity constant values K = 195 ± 17 M−1 and 1055 ± 48 M−1 respectively, indicating the prospective utility of SHP β-CDx in particular as an effective drug carrier. Unlike β-CDxs, α-CDxs do not form inclusion complexes with robinetin. To further characterize the robinetin/β-CDxs complexes, circular dichroism (CD) spectroscopic studies have been performed, which reveal that incorporation of robinetin molecules in the chiral environment of the β-CDxs strongly affects the electronic transitions of robinetin leading to the occurrence of positive induced circular dichroism (ICD) bands in the near ultra-violet (UV) region. Molecular mechanics calculations show that the inclusion complex with the chromone ring inserted into the β-CDx cavity is most favorable, in agreement with our spectroscopic data.