The efficiency of
pulmonary surfactant to stabilize the respiratory surface depends critically on the ability of
surfactant to form highly packed films at the air-liquid interface. In the present study we have compared the packing and hydration properties of
lipids in native
pulmonary surfactant and in several
surfactant models by analyzing the pressure and temperature dependence of the fluorescence emission of the
LAURDAN (1-[6-(dimethylamino)-2-naphthyl]dodecan-1-one) probe incorporated into
surfactant interfacial films or free-standing membranes. In interfacial films, compression-driven changes in the fluorescence of
LAURDAN, evaluated from the generalized polarization function (GPF), correlated with changes in packing monitored by surface pressure. Compression isotherms and GPF profiles of films formed by native
surfactant or its organic extract were compared at 25 or 37 °C to those of films made of
dipalmitoylphosphatidylcholine (DPPC),
palmitoyloleoylphosphatidylcholine (POPC), DPPC/
phosphatidylglycerol (PG) (7:3, w/w), or the mixture DPPC/POPC/palmitoyloleoylphosphatidylglycerol (POPG)/
cholesterol (Chol) (50:25:15.10), which simulates the
lipid composition of
surfactant. In general terms, compression of
surfactant films at 25 °C leads to
LAURDAN GPF values close to those obtained from pure DPPC monolayers, suggesting that compressed
surfactant films reach a dehydrated state of the
lipid surface, which is similar to that achieved in DPPC monolayers. However, at 37 °C, the highest GPF values were achieved in films made of full
surfactant organic extract or the mixture DPPC/POPC/POPG/Chol, suggesting a potentially important role of
cholesterol to ensure maximal packing/
dehydration under physiological constraints. Native
surfactant films reached high pressures at 37 °C while maintaining relatively low GPF, suggesting that the complex three-dimensional structures formed by whole
surfactant might withstand the highest pressures without necessarily achieving full
dehydration of the
lipid environments sensed by
LAURDAN. Finally, comparison of the thermotropic profiles of
LAURDAN GPF in
surfactant model bilayers and monolayers of analogous composition shows that the fluorophore probes an environment that is in average intrinsically more hydrated at the interface than inserted into free-standing bilayers, particularly at 37 °C. This effect suggests that the dependence of membrane and
surfactant events on the balance of polar/non-polar interactions could differ in bilayer and monolayer models, and might be affected differently by the access of water molecules to confined or free-standing
lipid structures.