Molecular Mechanism Underlying the Non-covalent Interaction between Sinapic Acid and Rice Bran Glutelin

The current study aimed to explore the dynamic binding process and molecular mechanism for the non-covalent interaction between sinapic acid (SA) and rice bran glutelin (RBG). Fluorescence spectroscopy was used to investigate the mechanism of fluorescence quenching, the number of binding sites and thermodynamic parameters. Further, the binding process of complex formation and the underlying molecular mechanism were revealed by the combination of homology modeling, molecular docking and molecular dynamic simulations. Fluorescence study showed that SA quenched the intrinsic fluorescence of RBG via static mode, indicating the formation of SA-RBG complex. The number of binding site was about 1. Thermodynamic parameters suggested that the SA could bind with RBG spontaneously, which was predominately driven by hydrophobic interactions. Molecular docking revealed that there were five potent binding sites on RBG for SA. Further molecular dynamic simulation revealed that SA could not only bind stably at binding site C2 but also exhibited the lowest binding free energy, suggesting that C2 was the most favorable binding cavity among the five predicted binding sites. Furthermore, molecular dynamic simulation results including the radius of gyrate, root mean square deviation and root mean square fluctuation further validated the binding stable between SA and RBG. The decomposition of binding free energy to per amino acid residue combined with binding mode analysis indicated that six key residues (including Ile131, Ile90, Trp149, Gln261, Tyr151 and Tyr102) of RBG and two methoxy groups of SA played critical roles in the binding process between SA an RBG. The above results would provide theoretical basis for the application and development of SA-RBG complex as functional ingredients.