Phase transitions of phospholipid monolayers penetrated by apolipoproteins

Experiments for adsorbing apolipoproteins (CI and AII) on a phospholipid (DPPC, rac-1,2-dipalmitoyl-snglycero-3-phosphocholine) monolayer were made. Our results indicate that lipoproteins in fact did not adsorb underneath the DPPC monolayer; instead lipoproteins actually penetrate the DPPC monolayer to form part of the monolayer at the air/water interface. The binary monolayers were isothermically compressed and their textures observed with Brewster angle microscopy. These monolayers that are rich in DPPC present two clear first-order phase transitions between condensed phases, as well as one between a condensed phase and a gas phase. At very high lateral pressures, condensed domains rich in protein present a high reflectivity. These domains melt away as pressure increases, leaving them indistinguishable from the rest of the low-reflectivity optically isotropic monolayer. Apparently, they lose density or thickness as if proteins were expelled from the air/water interface. A model for understanding the phase transitions in these binary systems is presented, which could have important implications in the understanding of lipoprotein physiology.