Abstract: The interaction between gliadin (G) and quercetin (Q) under different NaCl concentrations was studied by fluorescence spectroscopy, ultraviolet-visible (UV-Vis) spectroscopy and Fourier transform infrared (FTIR) spectroscopy. The results showed that at all NaCl concentrations, Q led to fluorescence quenching of G accompanied by a distinct blue shift (about 10 nm) of the fluorescence intensity with increasing NaCl concentration. When the concentration of Q was 50 μmol/L, 96%–98% of the protein fluorescence was quenched, indicating that strong interactions existed between G and Q. Q quenched the fluorescence intensity of G through static and static-dynamic modes. At 50 mmol/L NaCl concentration, the association constant (Ka) and the number of binding sites (n) were the largest, 4.87 × 107 L/mol and 1.477 1, respectively, indicating stronger interactions between G and Q at proper salt concentration. Thermodynamic data indicated that hydrophobic interaction was responsible for the formation of complexes at 50 mmol/L NaCl concentration, while hydrogen bonding was the main driving force at other NaCl concentrations. Synchronous fluorescence and UV-Vis spectroscopy suggested that Q changed the microenvironment surrounding the aromatic amino acid residues and the protein conformation. Tryptophan residues greatly contributed to quenching the protein fluorescence. FTIR spectroscopy and Raman spectroscopy revealed that specific interaction modes of G with Q existed at specific NaCl concentrations, and hydrogen bonding and hydrophobic interaction played important roles in the formation of the complexes, changing the secondary and tertiary structures and the microenvironment around the amino acid side chain of G. The above results proved that NaCl concentration influenced protein-polyphenol interaction; adding proper NaCl concentrations could facilitate the binding between gliadin and quercetin and changes in protein conformation. Our study provides a theoretical basis for the development of quercetin as a natural food additive in wheat products.

Key words: wheat gliadin; quercetin; NaCl concentration; interaction