Effect of β-cyclodextrin on the molecular properties of myricetin upon nano-encapsulation: insight from optical spectroscopy and quantum chemical studies.

Myricetin, a bioactive plant flavonol, readily forms inclusion complex with the drug delivery vehicle beta-cyclodextrin (β-CD). Appearance of typical "dual emission", consisting of normal (470 nm) and ESIPT tautomer (530 nm) bands, with concomitant rise in fluorescence intensity and dramatically blue shifted normal fluorescence of myricetin with increasing β-CD concentration, indicates facile entry of myricetin into the cavity of β-CD. The stoichiometry of the inclusion complex has been established to be equimolar (1:1), with an equilibrium constant of 439 ± 18 M(-1) at 25 °C. The driving force of inclusion is attributed to strong van der Waals interaction and formation of hydrogen bond between host (β-CD) and guest (myricetin). Both experimental and theoretical studies indicate that myricetin possibly incorporates within β-CD through its benzoyl moiety. Inclusion in β-CD increases the antioxidant potency of myricetin which has been attributed to the less delocalised HOMO and reduced HOMO-LUMO energy gap in the confined state.