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This study has been carried out to understand the nature of conformational flexibility and electrostatic properties of polyhydroxyl coumarins derivatives. When these compounds present in the active site of S. cerevisiae α-glucosidase, the lactone rings of the molecules are flanking out, while all benzene rings are embedded deep inside the binding cavity. These hydroxyl groups can interact with the surrounding amino acids by hydrogen bonds easily. When the hydroxyl groups at the C9 of benzene ring are replaced by methoxy groups, there is no evident influence on the hydrogen bonding interactions with the surrounding amino acid Asp68 and Lys155. However, their Laplacian values of electron densities of hydroxyl O–H bonds are obviously decreased in the active site, which suggests concentrated electron densities. In general, most of the electron densities of chemical bonds become more depleted after docking with the S. cerevisiae α-glucosidase, implying strong interactions with the surrounding amino acids. For polyhydroxyl coumarin derivatives, the global maximum values of the molecular electrostatic potential on molecular vdW surfaces stem from hydrogen atoms of the hydroxyl groups. However, the values are decreased evidently and stem from the different atoms in both phases while methoxy group is introduced. These fine details at electronic level allow to better understand the exact interactions between natural coumarins derivatives and target protein.

Display Omitted With the increasing of length of the alkyl chain in carboxyalkyl group, B ring of the apigenin derivatives is embedded much more deeply into the binding cavity while carboxyalkyl stretches to the neighboring cavity.The electron density values of the carbonyl in the carboxyl group become higher than the solution status due to the strong molecular interactions.All of the HOMO and LUMO are distributed throughout the whole apigenin ring in solution phase, whereas the disappeared phenomenon happened on the B rings of some molecules (IIVI), leading to higher energy gaps. As one of the most investigated flavonoids, apigenin, is considered to be a strong -glucosidase inhibitor. However, the clinical utility of apigenin is limited due to its low solubility. It was reported that the solubility and biological activity can be improved by introducing sole carboxyalkyl group into apigenin, especially the 7-substitution. With the increase of length of the alkyl chain in carboxyalkyl group, B ring of the apigenin derivative is embedded much more deeply into the binding cavity while the carboxyalkyl stretches to the neighboring cavity. All of the terminal carboxyl groups form hydrogen bonding interactions easily with the surrounding polar amino acids, such as His239, Ser244, Arg312 and Asp349. Thus, the electron density values of the carbonyl in the carboxyl group become higher than the solution status due to the strong molecular interactions. In fact, electron densities of most of the chemical bonds are decreased after molecular docking procedure. On compared with the solution phase, however, dipole moments of most of these molecules are increased, and their vectors are reoriented distinctly in the active sites. It is noticed that all of the Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) are distributed throughout the whole parent apigenin ring in solution phase, whereas the disappeared situation happened on the B rings of some molecules (IIIV) in the active site, leading to higher energy gaps.

Apigenin is an important flavonoids due to its antidiabetic bioactivity. It was reported experimentally that the 7-substituent derivative of apigenin has higher biological activity than 4′- and 5-substituted derivatives while introducing sole carboxyalkyl group -(CH2)7COOH into the parent structure. Molecular docking studies indicated that the other two derivatives have lower binding affinities than the 7-substituent derivative (-7.52 kcal mol−1), which is considered to be a better inhibitor than the parent molecule. Almost all of the carbon atoms and oxygen atoms are coplaner for all three molecules in solution phase, however, all carboxyalkyl groups bend inside into the parent molecules in the active site, and the jagged geometries of the carbon chains are destroyed correspondingly. In addition, most of the electron densities of the chemical bonds for all molecules are decreased, especially the 7-substituent derivative. In contrast, most of the Laplacian values for three molecules are increased in the active site, which suggests that the charge densities at the bond critical point (bcp) are much more depleted than the solution phase. Dipole moments of derivatives are all increased in the active site, suggesting strong intermolecular interactions. After interacting with the S. cerevisiae α-glucosidase, only the 7-substituent derivative has the lowest energy gap ΔE HOMO-LUMO, which indicates the lowest stability and the highest inhibition activity.