4.7 Article

Covalent bonding between polyphenols and proteins: Synthesis of caffeic acid-cysteine and chlorogenic acid-cysteine adducts and their quantification in dairy beverages

Journal

FOOD CHEMISTRY
Volume 403, Issue -, Pages -

Publisher

ELSEVIER SCI LTD
DOI: 10.1016/j.foodchem.2022.134406

Keywords

Phenolic acids; Polyphenol oxidation; Milk proteins; Polyphenol-protein adducts; Michael addition; Protein -phenol interaction

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Protein-polyphenol interactions have significant effects on the structure and function of food. This study focuses on the synthesis and characterization of caffeic acid-cysteine (CA-Cys) and chlorogenic acid-cysteine (CGA-Cys) adducts, and develops a quantitative method to detect these adducts in food and beverages. Results show the covalent bonding between milk proteins and CA/CGA, as well as the presence of these adducts in coffee-containing beverages.
Protein-polyphenol interactions affect the structure, stability, and functional properties of proteins and poly -phenols. Oxidized polyphenols (o-quinones) react rapidly with the sulfhydryl group of cysteine (Cys) residues, inducing covalent bonding between proteins and polyphenols. However, quantitative data on such reactions remain elusive, despite the importance of depicting the significance of such interactions on food structure and function. This work reports the synthesis, purification, and characterization of caffeic acid-cysteine (CA-Cys) and chlorogenic acid-cysteine (CGA-Cys) adducts and their stable isotope analogs, CA-[13C3,15N]Cys and CGA-[13C3,15N]Cys. A sensitive LC-MS/MS isotope dilution method was developed to simultaneously quantify these adducts in foods and beverages. Protein-bound CA-Cys and CGA-Cys were detected in the micro-molar range in milk samples with added CA and CGA, confirming covalent bonding between milk proteins and CA/CGA. These adducts were detected in commercial coffee-containing beverages but not in cocoa-containing drinks. Further-more, the adducts were found to be partially stable during enzymatic protein hydrolysis.

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