Structure-function relationships governing the multimodal antioxidant protection of quinoa protein hydrolysates

Plant protein hydrolysates have emerged as promising natural antioxidants but their bioactivity remains poorly understood due to the lack of integrated structure-function analyses under physiologically relevant conditions. In this context, this work aimed to elucidate the relationship between the structure of quinoa protein hydrolysates (QpH), environmental pH, and antioxidant performance using an integrated approach combining Fourier Transform Infrared spectroscopy (FTIR), chemometric analysis [Multivariate Curve Resolution (MCR)], and validation in a bacterial membrane model. QpH were produced using the broad-specificity protease Protamex [Enzyme-to-Substrate (E/S) 4% w/w, 200 min], yielding a highly hydrolyzed peptide pool (44% hydrolysis degree) dominated by low-molecular-weight species (<5000 Da, ∼95%). FTIR and chemometric analyses revealed distinct pH-dependent structural populations associated with protonated, deprotonated, aggregated, and solvated peptide states. These transitions were consistent with changes in surface charge (from -1.03 mV at pH 2 to -21.58 mV at pH 9) and were accompanied by a marked increase in antioxidant activity (from 114.9 to 1428.7 μmol TE/g). In a biologically relevant model based on Lactiplantibacillus plantarum WCFS1, QpH exhibited a strong dose-dependent capacity to delay lipid peroxidation kinetics [Time constant (Ʈ) increased from 2.0 to 6.9 min] and to protect up to 54.8% of membrane lipids from oxidation, without affecting bacterial viability. The antioxidant mechanism likely involves a multimodal action combining radical scavenging, iron chelation by deprotonated carboxylate groups, and physical shielding at the membrane interface by amphipathic peptides. Compared to previous studies, this work provides a novel integrated framework linking molecular structure, environmental conditions, and biological functionality, enabling mechanistic interpretation beyond a merely descriptive analysis. These findings highlight the importance of pH as a key modulator of peptide functionality and support the potential application of QpH as sustainable functional ingredients for mitigating oxidative stress in food systems and bacterial membrane models.