Alkaline Reaction Pathways of Phenolic Compounds with β-Lactoglobulin Peptides: Polymerization and Covalent Adduct Formation

A common strategy for a protein's functionality modification is the covalent binding of phenolic compounds (PCs) under alkaline conditions. Whether intentionally applied or arising during food processing and storage, such reactions are highly relevant, as alkaline pH promotes oxidation, covalent adduct formation, and polymerization, thereby altering both PC and protein properties. However, the interplay of these reactions and the impact of PC structure remain insufficiently understood. This study aimed at characterizing covalent binding products of structurally related PCs with tryptic peptides of the model protein β-lactoglobulin (β-Lg) at pH 9. Emphasis was given on substitution patterns and steric effects influencing polymerization and peptide adduct building. Hydroxycinnamic acid and flavonoid derivatives differing in hydroxyl substitution and carrying polar (glycosidic) groups were selected. Incubation products were characterized by HPLC-DAD and high-resolution mass spectrometry. Results showed that both mono- and dihydroxy PC undergo oxidation under alkaline conditions, but with distinct reactivity. Monohydroxy PCs form only limited peptide adducts due to resonance stabilization and steric hindrance. In contrast, dihydroxy PCs displayed a higher reactivity, producing more polymerization products and covalent adducts. Their enhanced reactivity is linked to the ability of quinone formation with reduced electrostatic repulsion, while additional polar substituents promote interactions with polar amino acids. At the same time, these substituents impose steric constraints on PC polymerization, modulating oligomer size and thereby influencing peptide binding. Overall, the findings highlight structural determinants of PC reactivity and provide mechanistic insight into the balance between polymerization and covalent peptide modification under alkaline conditions.