Modifications of DMPA (1,2-dimyristoyl-sn-glycero-3-phosphate) monolayer at air-water interface in presence of cholesterol

DMPA (1,2-dimyristoyl-sn-glycero-3-phosphate)-cholesterol mixed monolayer are studied at the air-water interface at a subphase pH ≈ 7.0. Surface pressure(π)- specific molecular area (A) isotherm for DMPA-cholesterol mixed monolayer shows that the surface pressure start to rise at ≈ 0.90 nm2/molecule in contrast to the pure DMPA and cholesterol monolayers, for which pressure rises at ≈ 0.62 and 0.58 nm2/molecule respectively. BAM images shows expanded domains and thread-like connected patterns in case of DMPA-cholesterol mixed monolayer, whereas for pure DMPA and cholesterol less compact and more or totally compact domains are observed for lower (5 mN/m) and higher (30 mN/m) surface pressures. Surface morphology of the pure cholesterol, pure DMPA and DMPA-cholesterol mixed monolayer are shown from the AFM images after depositing them on hydrophilic Si substrates. Hydrophobic and electrostatic interactions between the negatively charged DMPA and cholesterol are responsible for such surface pressure dependent structural modifications in the mixed film.