Abstract
Small mole fractions of cholesterol segregate into 10 - 100 nm diameter nanodomains in dipalmitoylphosphatidylcholine (DPPC) monolayers. The nanodomains segregate to DPPC domain boundaries, reducing the line tension, Δ, which alters domain shapes. The nanodomains consist of a 6:1 lipid: cholesterol “complex” and the surface viscosity, ηs, decreases exponentially with the area fraction of the complex at a given surface pressure. ηs increases exponentially with surface pressure, independent of cholesterol content, as predicted by a free area model that relates ηs to monolayer compressibility and collapse pressure. G', the elastic modulus, decreases with Δ at low cholesterol fractions, in analogy to 3-D emulsions. Increasing cholesterol further causes a sharp increase in G', likely due to a transition from a tilted to untilted molecular packing. Understanding the effects of small mole fractions of cholesterol should resolve the role of cholesterol in human lung surfactants and may give clues as to how cholesterol influences raft formation in cell membranes.
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