Interaction forces and membrane charge tunability: Oleic acid containing membranes in different pH conditions

Oleic acid is known to interact with saturated lipid molecules and increase the fluidity of gel phase lipid membranes. In this work, the thermodynamic properties of mixed monolayers of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and oleic acid at the air-water interface were determined using Langmuir isotherms. The isotherm study revealed an attractive interaction between oleic acid and DPPC. The incorporation of oleic acid also monotonically decreased the elastic modulus of the monolayer indicative of higher fluidity with increasing oleic acid content. Using the surface force apparatus, intermembrane force–distance profiles were obtained for substrate supported DPPC membranes containing 30 mol% oleic acid at pH 5.8 and 7.4. Three different preparation conditions resulted in distinct force profiles. Membranes prepared in pH 5.8 subphase had a low number of nanoscopic defects ≤ 1% and an adhesion magnitude of ~ 0.6 mN/m. A slightly higher defect density of 1–4% was found for membranes prepared in a physiological pH 7.4 subphase. The presence of the exposed hydrophobic moieties resulted in a higher adhesion magnitude of 2.9 mN/m. Importantly, at pH 7.4, some oleic acid deprotonates resulting in a long-range electrostatic repulsion. Even though oleic acid increased the DPPC bilayer fluidity and the number of defects, no membrane restructuring was observed indicating that the system maintained a stable configuration.