Flavonoids are vital constituents of propolis that are responsible for its medicinal activity. Flavonoid extraction commonly employs ethanol and water as solvents. In the extraction reaction, hydrogen-bonding interactions play a crucial role. In this study, hydrogen-bonding interactions between myricetin-an abundant flavonoid in propolis-and ethanol or water were studied theoretically using density functional theory (DFT) methods. The molecular geometry and charge properties of the myricetin monomer were analyzed first. After careful optimization, nine stable myricetin-CH3CH2OH/H2O complex geometries were obtained. Hydrogen bonds were confirmed to exist in these optimized structures. The most stable structures were found to be those with hydrogen bonds involving the hydrogen atoms of hydroxyl groups and the oxygen atom of the keto group of myricetin. The characteristics of the hydrogen-bonding interactions in the optimized structures were carefully analyzed. The hydrogen bonds in the optimized geometries were shown to be closed-shell-type interactions. H5' in ring B of myricetin presented the strongest interaction. The hydrogen bonds were found to be Coulombic interactions. Those between the hydrogen atoms of the hydroxyl groups in myricetin and the oxygen atoms in CH3CH2OH and H2O were of moderate strength and had some covalent character, while the others were weak and were dominantly electrostatic in character.
Keywords: Atoms in molecules; Density functional theory; Flavonoid; Hydrogen bond; Myricetin.