Effects of pH on the stability and compressibility of DPPC/cholesterol monolayers at the air–water interface

This study investigates pH effects on membrane compressibility of and molecular interactions between dipalmitoylphosphatidylcholine (DPPC) and cholesterol within the lipid monolayer at the air–water interface. Surface pressure versus molecular area isotherms of the DPPC/cholesterol monolayers of various molar ratios were measured under acidic, neutral or alkaline conditions, respectively. The isotherms provided information for the quantitative analysis of the miscibility and stability of the mixed monolayers, molecular surface area condensation, area compression modulus and cohesive surface pressure. Our results showed that the area condensing effect of cholesterol on the DPPC monolayer was pronounced at low surface pressures and was most significant for monolayers spread on a neutral subphase. The most stable state of the mixed monolayer was observed at the molar ratio of cholesterol Xchol=0.6 for the neutral subphase, at Xchol=0.5 or 0.6 for the acidic subphase, and at Xchol=0.8 for the alkaline subphase. At high surface pressures, monolayers spread on an acidic or alkaline subphase were found to be more compressible than those on the neutral subphase. However, weak molecular interactions and poor miscibility between DPPC and cholesterol were found in monolayers spread over the alkaline subphase solution.