Comparison of bilayer and monolayer properties of phospholipid systems containing dipalmitoylphosphatidylglycerol and dipalmitoylphosphatidylinositol

Bilayer and monolayer phase behavior at the air-water interface of dipalmit oylphosphatidylglycerol (DPPG), dipalmitoylphosphatidylinositol (DPPI), and their binary mixtures with dipalmitoylphosphatidylcholine (DPPC), in the c ontext of their possible roles in lung surfactant function, have been syste matically compared. Surface properties, as a function of temperature and co mposition, were evaluated and analyzed by measuring equilibrium surface pre ssure-area isotherms (pi -A), equilibrium monolayer collapse pressure (pi ( c)), and minimum area per molecule at the monolayer collapse pressure (A(c) ) and by direct observation using fluorescence microscopy. Bilayer properti es were evaluated by measuring the main bilayer phase transition temperatur e (T-m) and the bilayer equilibrium spreading pressure (pi (e)) as a functi on of temperature and composition. Through thermodynamic analysis, it was f ound that DPPC/DPPG mix ideally in both the monolayer and bilayer states, w hereas DPPC/DPPI (further supported with DPPC/Soy PI) are phase-separated w ith partial miscibility in bilayers and miscible with very significant posi tive deviations from ideality in monolayers. This behavior is attributed to the distinct differences between PG and PI headgroup properties as reflect ed in their physical size, state of hydration, and possible conformational flexibility, despite identical net negative charge and identical acyl chain headgroup properties. PC and PG exhibit very similar headgroup properties, which allow the dipalmitoyl chain properties to dominate over headgroup ef fects in bilayers and monolayers. In contrast, the unique PI headgroup prop erties dominate over the dipalmitoyl chain effects giving rise to very diff erent surface and bilayer phase behavior.