Incorporation of ß-lactoglobulin in monolayers of dioctadecyldimethylammonium bromide studied by Brewster angle microscopy

The deposition of pure bovine β-lactoglobulin (βLG) monolayer or the incorporation in a dioctadecyldimethylammonium bromide (DODAB) monolayer, were studied by π-A measurements at the air-/water interface and by direct visualization of the interface by BAM. The co-spreading of the monolayer material dissolved in a mixed volatile solvent (chloroform-ethanol) was selected based on reproducible data and minimum consumption of protein. Conformational changes induced by the mixed solvent on the native structure of βLG, investigated by circular dicroism, disappear after the solvent evaporation at the interface. The π-A isotherms suggest and BAM images confirm, that, at low surface pressures, βLG is incorporated in the liquid expanded monolayer of DODAB, while at the plateau near 30 mN m^-1 the protein is squeezed out into micro-domains partially immersed in the subphase and strongly adsorbed under the DODAB layer. The topography of DODAB/yβLG mixed films changes with surface pressure and number of protein residues, y, incorporated per DODAB molecule. BAM observation shows that at low y DODAB molecules dominate the topography of monolayer at the interface.