Dual-Pathway Glyoxal–Peptide Reaction Mechanisms under Acidic and Alkaline Conditions for Camellia oleifera Protein-based Adhesive Performance Optimization

The pH-mediated regulation of the glyoxal-dipeptide reaction pathway was systematically investigated via electrospray ionization mass spectrometry (ESI-MS) and nuclear magnetic resonance (13C-NMR) spectroscopy. The resulting mechanistic insights were then applied to optimize Camellia oleifera protein-based adhesive performance. Under alkaline conditions, glyoxal undergoes an intramolecular Cannizzaro reaction, where one aldehyde group is reduced to an alcohol hydroxyl group, and the other aldehyde group is oxidized to a carboxyl group, resulting in the salt form of glycolic acid (HOCH₂COO-). Glycolic acid enables extensive cross-linking via bifunctional, cooperative condensation with peptide amino and amide groups. A critical pH threshold of 11 was established for this process. In contrast, under acidic conditions, intramolecular cyclization of glyoxal to cyclic ether structures was observed. Simultaneously, the dipeptide’s aliphatic amino groups were protonated and inactivated, leaving only weakly nucleophilic amide groups available for reaction, which led to a significant reduction in overall efficiency. When applied to these adhesives, bond strength was shown to exhibit a distinct pH dependency. In the glyoxal-only system, a bond strength of 0.76 MPa was attained at pH 11, corresponding to a ~65% increase relative to acidic conditions. For the melamine-glyoxal modification, this value was further optimized to 0.95 MPa at pH 13, a result ascribed to the synergistic cross-linking effect of the triazine ring.