Lipid-Protein Interaction in Mixed Monolayers from Phospholipids and Proteins

The thermotropic behavior of different proteins with phospholipids in monolayers was investigated by measuring the surface pressure and the surface viscosity. When phospholipids, such as dipalmitoylphosphatidylcholine (DPPC) and dimyristoylphosphatidylcholine (DMPC), were mixed with the hydrophobic proteins, such as hemoglobin and β-casein, below their phase transition temperatures (Tc), a considerable expansion of monolayers was observed. On the other hand, when phospholipids were mixed with the hydrophilic proteins, such as bovine serum albumin (BSA) and lysozyme below Tc, no expansion was observed. When DMPC was mixed with different proteins at Tc or above, all mixtures formed slightly deviated monolayers from ideal mixing, independent of a variety of proteins. This type of behavior suggests that the hydrophobic proteins perturb the structure of boundary lipids near the protein molecule below Tc, while the hydrophobic proteins do not perturb the phospholipid structure below Tc because of their low hydrophobicities. At Tc or above the membrane properties are independent of a variety of proteins, because all lipid hydrocarbon chains cause a disordering, even without proteins. In addition, when DPPC was mixed with a hydrophobic protein, such as hemoglobin, the fluidity of monolayer increased significantly.