Rheology and microscopy study of lung surfactant monolayers

Lung surfactant is a lipid-protein mixture that lines the alveolar air-liquid interface of air-breathing vertebrates and acts to lower surface tension. Lack of lung surfactant can result in respiratory distress syndrome both in neonatal infants and in adults. To find a better way to treat RDS needs a better understanding of lung surfactant. We built a magnetic needle viscometer and used it with Langmuir isotherms, fluorescence microscope, Brewster angle microscope, atomic force microscope as well as gracing incidence X-ray diffraction to determine the role of each individual component in model or nature lung surfactant and the interactions between those components. Lung surfactant protein SP-B and peptides based on SP-B induce a reversible folding transition at monolayer collapse that allows all components of surfactant to be retained at the interface during respreading. SP-B also interacts with POPG specifically and induces a 3-D aggregation in fluid phase above certain surface pressure. Palmitic acid, as well as n-hexadecanol, makes the monolayer rigid at low surface tension and fluid at high surface tension and modifies SP-C function, and decreases the tilt angle of monolayer at a certain surface pressure. Shear viscosity of model lung surfactant mixtures strongly relates to the morphology of monolayer and particularly to the fraction of solid domain. The surface viscosity scales as $msmso=1-AAc-1$ in which μ_so is the viscosity of the liquid phase fraction of the monolayer, A is the area fraction of solid phase measured by fluorescence microscopy, and A_c is a critical solid phase fraction. This scaling relationship is directly analogous to that derived for three-dimensional colloidal dispersions in a solvent with long-range repulsive interactions between the solids (with area fraction replacing volume fraction). The repulsion between solid domains is consistent with the known repulsive dipole-dipole interaction between solid phase domains in monolayers. Identifying the function of lung surfactant proteins and lipids is essential to the rational design of replacement surfactants for treatment of respiratory distress syndrome.