Surface properties of polyene glycol phospholipid monolayers

We studied the surface properties of monolayers composed of polyunsaturated conjugated ethylene glycol phospholipids (carotenoid lipids), compared the data with monolayers of dipalmitoylphosphatidylcholine (DPPC) to which carotenoids were added and evaluated the impact of the unsaturated glycol lipids on monolayers with the glycerolipid DPPC. The carotenoid based glycol lipids formed monolayers at the air/water interface. Using the Langmuir method we obtained series of pressure-area (π-A) isotherms and determined the limiting area A per molecule of three glycol lipids, C30:9-C₀A=42.6±1.4Ų, C30:9-C₂A=76.1±2.5Ǻ² and C30:9-C₁₂A=354.0±12.0Ų and their mixtures with DPPC at various mole fraction X. C30:9-C₀ and C30:9-C₂ did not affect significantly the shape of the isotherm, but caused their slight shift toward a lower and larger molecular area, respectively. C30:9-C₁₂ at mole fractions X>0.02 affected the shape of isotherm. The compressibility modulus C_s^-1 of monolayers depended on the surface pressure. C_s^-1 value was substantially higher for DPPC monolayers in comparison with those of pure glycol lipids. At low surface pressure π=5-10mN/m and low mole fractions X<0.02 the glycol lipids formed complexes with DPPC; at higher surface pressure the separation of pure components took place. The dipole potential of the monolayers composed of cationic glycol lipids C30:9-C₂ and C30:9-C₁₂ was higher in comparison with those of zwitterionic DPPC and C30:9-C₀. This may be connected with various contributions of dipole moments of the molecules and their orientation in the monolayer.