Incorporation of ß-lactoglobulin in a lipid/porphyrin monolayer at the air-water interface

A catanionic lipid/porphyrin monolayer was formed at the air.water interface by the tetra-anionic porphyrin, tetra-sodiummeso-tetra(4-sulfonatophenyl)porphine (TSPP), mixed with the cationic lipid dioctadecyldimethylammonium bromide(DODAB) in a 1:4 molar ratio. This binary mixture (TSPP/4DODAB) was used as the incorporation matrix of β-lactoglobulin (βLG). Binary and ternary systems (TSPP/4DODAB/zβLG, where z stands for the number of protein residues per TSPP) were characterized by surface pressure versus area (π-A) measurements and by Brewster angle microscopy (BAM) observation at the air-water interface. π-A measurements and BAM images show that protein is incorporated in the expanded regime of the monolayer and is gradually expelled upon compression at high surface pressures. The successive compression-expansion cycles indicate that the protein under adsorbed to the floating film is reincorporated after the expansion of the monolayer. At low subphase pH, TSPP tends to aggregate decreasing the interaction with DODAB molecules. Electrostatic and hydrophobic interactions are responsible for the presence of βLG at the interfacial film.