Lipid fluidity in lung surfactant: monolayers of saturated and unsaturated lecithins

Monolayers of saturated or unsaturated lecithins (PCs) could be compressed to near-zero surface tension (gamma) when they were below the gel-to-liquid crystalline transition temperature (Tc) of the PC, whereas, when above Tc, they collapsed at gamma of 15-23 mN X m-1. Thus PCs with ordered, but not necessarily saturated, chains are required to achieve low gamma. At 37 degrees C the minimum gamma reached by monolayers containing dipalmitoyl PC (DPPC; Tc = 41 degrees C) plus 1-stearoyl-2-oleoyl PC (SOPC; Tc = 6 degrees C) depended on the proportions of the two lipids and the compression rate. When compressed at 1 cm2 X s-1, monolayers of mixtures containing less than 50% SOPC were capable of achieving gamma less than 9 mN X m-1. When the proportion of unsaturated lecithin was high (70%), compression at 1 cm2 X s-1 was insufficient to form a monolayer capable of reaching a very low surface tension. Exclusion of fluid lipid during compression could account for these observations.