Effect of Ultrasonic Power on the Structural and Functional Properties of Gliadin Protein-Dietary Polyphenol Conjugates

In this study, free radical oxidation was chosen to induce covalent reaction of gliadin (GL) with epigallocatechin gallate. The effects of different ultrasound (US) powers on the enhancement of the covalent reaction were compared with the aim of improving the bioavailability of polyphenols in protein modification. Covalent modification of polyphenols was verified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), high-performance liquid chromatography (HPLC) and the Folin-Ciocalteu assay. The structural changes of the covalent complexes were analyzed by ultraviolet (UV) spectroscopy and circular dichroism (CD) spectroscopy. The results showed that US disrupted weak intra-protein forces, which caused molecular depolymerization. The hydrophilic/hydrophobic amino acid residues of the protein were shifted and the tertiary structure tended to be looser, and this process altered the distribution of groups on the molecule surface and increased the water solubility of the protein. Furthermore, the shift in the spatial structure of GL resulted in the exposure of more enzyme cleavage sites, making its digestion easier. The synergistic effect of US and free radical oxidation increased the loading of antioxidant molecules onto the covalent complexes, resulting in stronger free radical scavenging activity.