Covalent conjugate of pea protein induced by cyanidin-3-O-glucoside quinone: The structural formation and functional properties

The unique structural of anthocyanins makes them vulnerable to degradation within food processing environments. Proteins commonly coexist with polyphenols in natural food matrices on a non-covalent and covalent basis, serving to stabilize protection for polyphenols. Here, three natural pea-extracted proteins, including albumin, vicilin, and legumin were bound to cyanidin-3-O-glucoside (C3G), and binding modes, conjugate structures and functions were characterized. Isothermal titration calorimetry showed a strongly exothermic interaction between C3G and proteins. The results of sodium dodecyl sulfate-polyacrylamide gel electrophoresis and fourier transform infrared spectroscopy revealed that the C3G-quinone (C3GQ) formed by C3G oxidation established covalent cross-links with C–S and C–N in the amino acid residues of proteins, and could form more large molecular weight aggregates at an addition amount of 1 mM. The free sulfhydryl and amino contents of the three protein conjugates decreased with the addition of C3GQ. Due to the presence of 4–5 times more sulfhydryl groups in albumin as compared to globulin, albumin showed a 20% higher binding capacity for C3GQ than globulin. This consequently led to the highest antioxidant activity in the C3GQ-albumin conjugate. In contrast to native pea proteins, the introduction of covalent modifications induces changes in the secondary structure of the pea proteins. Specifically, the structure of albumin tends to exhibit increased orderliness, while the molecular flexibility of globular proteins was augmented. Thus, the conjugates have enhanced thermal stability. Our results affirmed that the stability of C3G can be more effectively improved through designing it to covalently interact with proteins relative to non-covalent interactions. This study aimed at offering novel theoretical underpinnings for the functional utilization of C3G and pea proteins in the food industry.