Lateral Stress Profiles In Lipid Monolayers

Abstract

We have used molecular dynamics simulations to study the lateral stress profiles in lipid monolayers at the air/water interface. From the calculations, we determined the “surface of tension” in the complex interfacial layer. We identified the factors for monolayer stability, which allows explaining the maximum surface pressure sustained by a selected lipid mixture (collapse pressure). This is relevant for understanding the function of biological interfaces, such as the surfactant-covered gas exchange interface in the lungs, and designing artificial/replacement surfactant mixtures.We calculated the stress distributions for lipid monolayers of different composition under varying surface pressure, including both liquid-expanded and liquid-condensed phases. The stress distribution in the hydrocarbon chain region is most affected by the surface pressure. In the liquid-expanded phase, the stress becomes negative at the chain/air interface. In the liquid-condensed phase, the negative stress in the chains is partially compensated by positive pressure due to increased density, and the profile is characterized by multiple peaks originating from chain and head group ordering. The simulations were performed with both atomistic and coarse-grained molecular models, which led to qualitatively similar results. To test the estimated collapse pressures, the coarse-grained model was used to simulate monolayer collapse upon lateral compression. To induce 2D-3D transformations that require long time scales, small defects were introduced, which provided nucleation sites for monolayer folding.