FTIR external reflectance studies of lipid monolayers at the air-water interface: Applications to pulmonary surfactant

FTIR external reflectance spectra of monomolecular films of natural products and model mixtures relevant to pulmonary surfactant physiology were collected concurrently with surface measurements directly at the air-water interface. Films studied were calf long surfactant extract (CLSE) and its phospholipid fraction (PPL) along with 2:1 DPPC-d(62):DPPG and 2:1 DPPC-d(62):DOPG containing 0, 1 or 2 wt% of the hydrophobic surfactant proteins SP-E and SP-C (SP-B+C). The CH2 antisymmetric and symmetric stretching bands (similar to 2920 and 2852 cm(-1)) along with the analogous CD2 stretching bands (similar to 2194 and 2089 cm(-1)) were analyzed, and band heights, integrated intensities and peak frequency positions were plotted as a function of measured surface pressure. Data suggest that 2:1 DPPC-d62:DPPG + 2wt% SP-B+C is the most ordered and stable of the films and can be compressed to the highest sustainable surface pressure. Data from the model mixtures indicate that the surfactant protein interacts differently with each of the lipid components. Plots of the CH/CD intensity ratios versus surface pressure show an increase in this ratio upon the addition of SP-BI-C as the protein apparently orders the CH component (DPPG or DOPG) and slightly disorders the CD component (DPPC-d(62)).