pH-dependent interactions between low-methoxyl pectin and lysozyme: Characterization of soluble complexes and antimicrobial films with enhanced activity

This study investigates the interactions between low methoxyl pectin (an anionic polysaccharide) and lysozyme (a cationic protein) under different pH conditions and their application in antimicrobial films. By gradually adding LM pectin (0–4 g/L) to a constant concentration (0.714 g/L) of lysozyme solution, it was observed that the positive charge of lysozyme was gradually neutralized, forming soluble complexes. The properties of the complexes, including appearance, charge, and size, were found to be strongly dependent on pH. At low pH, a higher concentration of pectin was required to form the complex. As the pH increased from 3.0 to 9.0, the zeta potential of the complexes decreased from −10 mV to −35 mV, indicating that the electrostatic repulsion of the complexes increased at higher pH. Circular dichroism (CD) and Fourier-transform infrared spectroscopy (FTIR) elucidated the molecular mechanisms of the complexes under different pH conditions. Molecular dynamics (MD) simulations further confirmed the significant effect of pH on the binding energy and interaction energy between molecules, with the strongest binding and interaction occurring at pH 7. The binding energy absolute value was 39.3368, significantly higher than at other pH values, and corresponded to a greater number of active sites. Additionally, antimicrobial films based on the pectin/lysozyme complex were developed and characterized. Scanning electron microscopy (SEM) results showed that lysozyme was evenly distributed in the film in the form of complexes, with the size influenced by the pH of the preparation solution, in agreement with the particle size measurement results. The antimicrobial activity of the films was enhanced by two mechanisms: enzymatic degradation of pectin and dissociation of electrostatic complexes in the presence of divalent ions. Antibacterial tests demonstrated that the composite films exhibited significant antimicrobial effects under simulated food contamination and salty food matrix conditions, especially when pectinase and CaCl₂ were added. This study provides new insights for the development of natural polysaccharide/protein composite films with enhanced antimicrobial properties and lays a foundation for their potential application in food packaging. • The increasing concentrations of low-methoxyl pectin neutralize the positive charges of lysozyme, facilitating the formation of soluble complexes. • The characteristics of these complexes are significantly influenced by the pH level of the solution. • Molecular dynamics (MD) simulations validate that the stability and efficacy of the binding interactions are highly pH-dependent. • Antimicrobial films based on pectin-lysozyme have been prepared and characterized. • The antimicrobial activity of the films can be enhanced through two mechanisms: enzymatic degradation of pectin and dissociation of electrostatic complexes in the presence of divalent ions.