Abstract
Fatty acids, cholesterol, and phospholipids are amphiphilic compounds of biological interest, which form ordered monolayers mimicking biomembranes, and can be studied with the Langmuir technique using surface pressure-area isotherms and compressibility plots. Proteins are also components of biomembranes or are present in body fluids. In this study, the influence of lysozyme on different films of a fatty acid (stearic acid or oleic acid), cholesterol, a phospholipid (dipalmitoylphosphatidylcholine, DPPC, or palmitoyloleoylphosphatidylcholine, POPC), and mixtures of them is presented using a 0.9% saline solution as subphase. Results show that the presence of lysozyme alters the lipid monolayer formation in an important way at the beginning (low surface pressures) and the middle (intermediate surface pressures) parts of the isotherm. At high surface pressures, the phospholipids DPPC and POPC and the saturated fatty acid, stearic acid, expel lysozyme from the surface, while oleic acid and cholesterol permit the presence of lysozyme on it. The mixtures of oleic acid-DPPC also expel lysozyme from the surface at high surface pressures, while mixtures of oleic acid-POPC and cholesterol-POPC permit the presence of lysozyme on it. The compressibility of the monolayer is affected in all cases, with an important reduction in the elastic modulus values and an increase in the fluidity, especially at low and intermediate surface pressures.
Highlights
Fatty acids, cholesterol, and phospholipids are of biological interest, and they can form ordered and compact monolayers
According to surface pressure-area isotherms and compressibility plots, it is observed that lysozyme affects lipid monolayer formation
This expulsion is more important for Stearic acid (SA), DPPC, and POPC, while for Oleic acid (OA) and cholesterol, the isotherms indicate that a residual content of lysozyme remains in the monolayer
Summary
Cholesterol, and phospholipids are of biological interest, and they can form ordered and compact monolayers. They found that at lower surface pressures, a monomolecular layer of bovine serum albumin, BSA, is formed on the water surface and molecules start to lift up when increasing the surface pressure. Pasquier et al [38] studied protein interfacial layers of ovalbumin and lysozyme at a free air–water interface, using neutron reflectivity and null-ellipsometry. These authors found that the combined effect of a positive charge of protein and the presence of advection flow (convection towards the interface) induce the formation of interfacial multilayers. Lipid monolayers are considered as a model for half a membrane, being valuable to characterize protein–
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