Comparing the interaction of four structurally similar coumarins from Fraxinus Chinensis Roxb. with HSA through multi-spectroscopic and docking studies

The interaction between drugs and biomacromolecule such as proteins has always been a research hotspot in the field of intersections. After the drugs entering the human body, they undergo pharmacological effects through the storage and transportation of plasma. Serum albumin is the most rich and important carrier in plasma. Therefore, it is of great significance to study the interaction between drugs and protein molecules for elucidating drug delivery and pharmacological action in vivo. This research is mainly to explore the affinities of these four coumarins (fraxin, fraxetin, esculin, and esculetin) with human serum albumin (HSA), and the binding strength and the effect of molecular structure on the binding of molecular-protein complex were compared. Initially, we used spectroscopic methods to analyze the affinity, interaction mechanism, and relationship between structure and activity of these coumarins binding to HSA. Subsequently, the four coumarin ligands were molecularly docked with HSA using docking tools such as AUTODOCK and CDOCKER. With the addition of coumarins, the fluorescence of HSA weakened gradually. For the HSA-coumarins system, the Van't Hoff equation and docking results indicated that hydrogen bond and hydrophobic interaction were the main ways in which the four coumarins combined with HSA. The circular dichroism (CD) and 3D fluorescence experiments proved that the conformation of HSA was changed by the addition of coumarins. Through calculation, the distance r between HSA and fraxin/fraxetin/esculetin/esculin were 2.222, 2.092, 2.673, and 2.939 nm, respectively. Fluorescence spectroscopy and Stern-Volmer formula indicated that the order of interaction energy and binding strength between coumarins and HSA were as follows: esculin > fraxin > fraxetin > esculetin. This work compared the interaction mechanisms between four differences structured coumarins with HSA and the effect of their structural differences on the binding with HSA. It also provided valuable information for the development of coumarin drug delivery systems.