Deposition of egg-PC to an Air/Water and Triolein/Water Interface

Abstract

Phospholipid monolayers play a critical role in stabilization of biological interfaces including the alveoli of the lung, fat droplets in adipose tissue, and apolipoproteins. Behavior of phospholipids at an air-water interface is well understood. However, work at oil-water interfaces is limited due to technical challenges associated with a Langmuir trough. In this study, egg-phosphatidylcholine(PC) was deposited onto a drop of either air or triolein(TO) formed in a low salt buffer and the surface tension was measured using a drop tensiometer. The egg-PC was deposited by constituting it into SUVs and then allowing molecules to absorb to the surface. We observed that egg-PC binds irreversibly to both interfaces and at equilibrium exerts 15 and 12 mN/m at an air and triolein interface, respectively. To determine the surface concentration, which cannot be measured directly, compression isotherms from a Langmuir trough were compared to that of the drop tensiometer. The air-water interfaces had identical characteristics so the surface concentration of the drop can be determined by simply overlaying the two isotherms. Since TO is also surface active there will be triolein incorporated into the monolayer. Since TO is less surface active than PC as the pressure(Π) increases the triolein is progressively ejected. To understand the Π/area isotherm of PC on the TO drop a variety of TO-PC mixtures were spread at the air-water interface. The isotherms show an abrupt break in the curve at a specific Π caused by the ejection of TO from the monolayer into the bulk phase. A plot of these surface transition points against Π gives the monolayer surface composition at any Π. The oil drop experiment always contains bulk phase of TO, thus the 2-D phase rule predicts the monolayer composition of the droplet over a range of Π.