Surface chemistry of binary mixtures of phospholipids in monolayers. Infrared studies of surface composition at varying surface pressures in a pulmonary surfactant model system

Phospholipid monomolecular films at the air/water interface were studied using Langmuir-Blodgett (L-B) surface chemistry, 31P NMR spectroscopy, and infrared (IR) spectroscopy. These monolayers were composed of binary mixtures of acyl chain perdeuterated 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (i.e., DPPC-d62) with 1,2-dipalmitoyl-sn-glycero-3-phosphoglycerol (i.e., DPPG). This particular PC-PG binary mixture was chosen for study since this lipid system has been used as a model for pulmonary surfactant, especially in conjunction with the so-called "squeezing-out" hypothesis of pulmonary mechanics. This theory predicts that upon successive compression-expansion cycles, a surfactant surface film will reorganize to exclude all components except DPPC, thus resulting in a stable, low surface tension film. Several general results were obtained from these experiments. First, we have developed a combined spectroscopic assay using high-resolution 31P NMR spectroscopy in combination with the C-H and C-D vibrational intensities obtained from the IR spectroscopy of binary mixtures in which one component is acyl chain perdeuterated. Using attenuated total reflectance IR spectroscopy of transferred L-B films, this combined spectroscopic approach allows us to quantitatively describe the fractional composition of each component in the binary monomolecular film. Second, when these methods are applied to transferred monolayer films of DPPC-d62 and DPPG (at an initial PC:PG mole ratio of 7:1), we find no evidence for a "squeezing-out" of the DPPG monolayer component at high surface pressure resulting in an enrichment of the DPPC component in the transferred monolayer film.(ABSTRACT TRUNCATED AT 250 WORDS)