From chocolate to coffee: Mucin interactions with vanillin, caffeine, and gallic acid under food-relevant conditions

Abstract

Vanillin (VAN), caffeine (CAF), and gallic acid (GA) are common food-derived compounds that contribute to the bitterness and astringency of products such as chocolate, tea, and coffee. This study investigated their interactions with porcine gastric mucin (PGM) under food- and physiology-relevant conditions (pH 3 and 7; 298-318 K). Fluorescence spectroscopy showed concentration-dependent quenching of PGM intrinsic fluorescence for all ligands. The bimolecular quenching rate constants (Kq) exceeded the diffusion-controlled limit, indicating a predominantly static quenching mechanism, while binding constants (Ka) ranged from 4 & times; 10 3 to 3.7 & times; 104 M-1 depending on ligand type, pH, and temperature. VAN induced a red shift of up to 6 nm under acidic conditions, suggesting ligand-induced conformational changes in PGM. Comparative evaluation showed stronger interactions for GA and VAN than for CAF, consistent with the higher hydroxyl-group content and hydrogenbonding capacity of the phenolic compounds. Thermodynamic analysis indicated that hydrogen bonding and van der Waals forces were the main contributors to complex formation in most systems, with Delta H values reaching -84.6 kJ mol-1 and Delta S values as low as -0.18 kJ K-1 mol-1 . Complementary 3D fluorescence and UV-Vis analyses supported ligand-induced structural changes and complex formation. Surface adsorption decreased from approximately 80-30 & micro;g mL-1 when pH was lowered from 7 to 3. SEM revealed ligand-dependent microstructural changes, with pronounced aggregation in the PGM-GA system. Overall, these results provide a consistent mechanistic understanding of mucin-ligand interactions under food-relevant conditions.