DPPC Monolayers Exhibit an Additional Phase Transition at High Surface Pressure

Abstract

Pulmonary surfactant forms a monolayer at the air/aqueous interface within the lung. During the breath process, the surface pressure (Π) periodically varies from ∼40mN/m up to ∼70mN/m. The film is mechanically stable during this rapid and reversible expansion.Pulmonary surfactant consists of ∼90% of lipid with 10% integrated proteins. Among its lipid compounds, di-palmitoyl-phosphatidylcholine (DPPC) dominates (∼45wt%). DPPC is the only known lipid that can be compressed to very high surface pressure (∼70mN/m) before its monolayer collapses. Most probably, this feature contributes to the mechanical stability of the alveoli monolayer. Still, to the best of our knowledge, some details of the compression isotherm presented here and the related structures of the DPPC monolayer were not studied so far.The liquid-expanded/liquid-condensed phase transition of the DPPC monolayer at ∼10mN/m is well known. Here, we report a second phase transition at elevated surface pressure (∼50mN/m). The lateral structure of the monolayer at selected states (8mN/m, 20mN/m, 30mN/m, 40mN/m, 50mN/m, 60mN/m, 70mN/m; covering the whole pressure range of the isotherm) was investigated by grazing incidence X-ray diffraction (GIXD). The results report on the 2D packing lattice and on the inter-chain distance dxy. Moreover, the tilt angle of the palmitoyl chains was calculated combining the lattice parameters and the geometrical boundary conditions. The course of the inter-chain distance versus surface pressure exhibits two regimes, separated by the phase transition.