Abstract
In this work, hydrogen-bonding interactions between apigenin and water/ethanol were investigated from a theoretical perspective using quantum chemical calculations. Two conformations of apigenin molecule were considered in this work. The following results were found. (1) For apigenin monomer, the molecular structure is non-planar, and all of the hydrogen and oxygen atoms can be hydrogen-bonding sites. (2) Eight and seven optimized geometries are obtained for apigenin (I)–H2O/CH3CH2OH and apigenin (II)–H2O/CH3CH2OH complexes, respectively. In apigenin, excluding the aromatic hydrogen atoms in the phenyl substituent, all other hydrogen atoms and the oxygen atoms form hydrogen-bonds with H2O and CH3CH2OH. (3) In apigenin–H2O/CH3CH2OH complexes, the electron density and the E(2) in the related localized anti-bonding orbital are increased upon hydrogen-bond formation. These are the cause of the elongation and red-shift of the X−H bond. The sum of the charge change transfers from the hydrogen-bond acceptor to donor. The stronger interaction makes the charge change more intense than in the less stable structures. (4) Most of the hydrogen-bonds in the complexes are electrostatic in nature. However, the C4−O5···H, C9−O4···H and C13−O2···H hydrogen-bonds have some degree of covalent character. Furthermore, the hydroxyl groups of the apigenin molecule are the preferred hydrogen-bonding sites.
Highlights
IntroductionPropolis (or bee glue) has been used as a popular natural remedy in folk medicine for centuries all over the world
Propolis has been used as a popular natural remedy in folk medicine for centuries all over the world1
In apigenin (II), the binding distance between H7 and O4 is approximately 1.7 Å, which is less than the sum of the van der Waals atomic radii of hydrogen and oxygen22
Summary
Propolis (or bee glue) has been used as a popular natural remedy in folk medicine for centuries all over the world. It is reported that propolis possesses a broad array of biological properties, such as anti-septic, anti-mycotic, anti-viral, spasmolytic, anti-inflammatory and immunostimulatory activities2 Due to these characteristics, which can yield health benefits, propolis has attracted much attention in recent years as an important substance for use in foods and medicines to improve health and to prevent diseases. More than 300 compounds, including aromatic acids, esters, waxy acids, flavonoids and terpenoids have been isolated from raw propolis2 Among those constituents, flavonoids are one of the most important bioactive compounds and are reported to be responsible for some of the medicinal properties of propolis such as anti-microbial, anti-cancer, anti-oxidant and anti-viral activities. The investigation of the hydrogen-bonding interactions in mixtures of flavonoids and water/ethanol is important to understand the mechanism of the extraction process and the physical essence of the mixture. Density functional theory (DFT) and MP2 methods were used to reveal the hydrogen-bonding interactions from a theoretical viewpoint
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