Polysaccharide charge density regulating protein adsorption to air/water interfaces by protein/polysaccharide complex formation

Because the formation of protein/polysaccharide complexes is dominated by electrostatic interaction, polysaccharide charge density is expected to play a major role in the adsorption behavior of the complexes. In this study, pullulan (a non-charged polysaccharide) carboxylated to four different charge densities (fraction of carboxylated subunits: 0.1, 0.26, 0.51, and 0.56) was used to investigate the effect of charge density on the properties of mixed protein/polysaccharide adsorbed layers at air/water interfaces. With all pullulan samples, soluble complexes with β-lactoglobulin could be formed at low ionic strength, pH 4.5. It was shown that the higher was the pullulan charge density, the more the increase of surface pressure in time was retarded as compared to that for pure β-lactoglobulin. The retardation was even more pronounced for the development of the dilatational modulus. The lower dilatational modulus can be explained by the ability of the polysaccharides to prevent the formation of a compact protein layer at the air/water interface due to electrostatic repulsion. This ability of the polysaccharides to prevent "layer compactness" increases with the net negative charge of the complexes. If charge density is sufficient (≥0.26), polysaccharides may enhance the cohesion between complexes within the adsorbed layer. The charge density of polysaccharides is shown to be a dominant regulator of both the adsorption kinetics as well as the resulting surface rheological behavior of the mixed layers formed. These findings have significant value for the application of complex protein-polysaccharide systems. © 2007 American Chemical Society