Theoretical studies on the hydrogen-bonding interactions between luteolin and water: a DFT approach

Flavonoids are among the most important bioactive compounds responsible for the medical properties of honey and propolis. Water is the solvent most commonly used to extract flavonoids from honey and propolis. Hydrogen-bonding interactions are of great importance in the extraction process. In this work, hydrogen-bonding interactions between a representative flavonoid, luteolin, and water were investigated by density functional theory (DFT) from a theoretical viewpoint. The following conclusions were drawn: first, the molecular structure of luteolin is non-planar. Second, nine optimized geometries for the luteolin-H₂O complex were obtained. With the exception of the aromatic hydrogen atoms in the phenyl substituent, the other hydrogen and oxygen atoms formed hydrogen-bonds with H₂O. Third, luteolin-H₂O complexation is accompanied by charge rearrangement. The electron density and the second-order perturbation stabilization energy [E(2)] in the related anti-bonding orbital of the hydrogen-bond donors were increased, causing elongation and a red-shift of the X-H bond in X-H···Y. The stronger interaction makes the electron density and the E(2) increase more in the more stable geometries. The sum of the electron density is transferred from hydrogen-bond acceptors to donors. Fourth, the hydrogen-bonds in the luteolin-H₂O complex are weak and basically electrostatic in nature. In addition, O-H···O hydrogen-bonds are stronger than C-H···O hydrogen-bonds in the luteolin-H₂O complex.