Activation mechanism of whey protein isolate mediated by free radicals generated in the ascorbic acid/hydrogen peroxide system.

Free radical grafting is a promising method that has been used to modify whey protein isolate (WPI) using polyphenol and provides desirable functionalities, but the reaction mechanism is unclear. This work investigated the grafting mechanism of WPI - (-)-Epigallocatechin-3-gallate (EGCG) through experimental techniques and molecular docking. The results showed that only the ascorbic acid radical anion (Asc) is generated in the Asc/ HO system. Asc removed hydrogen atoms from amino and sulfhydryl groups, resulting in a decrease in their content. Cystine, proline, methionine, and histidine were discerned to be the grafting sites between WPI and EGCG for more sensitiveness. Furthermore, Asc and •OH changed the secondary structure of WPI, but Asc further induced oxidative deamination of amino acids. Therefore, the grafting mechanism is presumed that the Asc catches hydrogen atoms in protein-sensitive amino acids and generates macromolecular radicals that induce the formation of WPI-EGCG graft.