Charge-dependent insertion of beta-lactoglobulin into monoglyceride monolayers.

The interactions between beta-lactoglobulin and 1-monostearoyl-glycerol were studied in order to gain insight into protein-gel-phase monoglyceride interactions. Using a monomolecular layer at the air-water interface, we determined the insertion of beta-lactoglobulin into the monoglycerides under different conditions of protein and surface charge by varying the pH and/or incorporating charged amphiphiles into the monolayer, respectively, and using subphases with either a low or high ionic strength. The interactions were quantified by determining the binding of 14C-labeled beta-lactoglobulin to the monolayer. Our results show the importance of electrostatics for binding of beta-lactoglobulin to condensed monoglycerides. Moreover, electrostatic interactions were found to be important for specific insertion of beta-lactoglobulin into the monolayer. A negatively charged surface in particular allowed positively charged beta-lactoglobulin to insert in a surface charge density-dependent manner, even at surface pressures as high as 36 mN/m, whereas under other conditions, the limiting insertion pressure was 32 mN/m. The rheological properties of the monolayer were not affected by the interactions with beta-lactoglobulin.