Effect of hydrophobic surfactant peptides SP-B and SP-C on binary phospholipid monolayers. I. Fluorescence and dark-field microscopy

The influence of the hydrophobic proteins SP-B and SP-C, isolated from pulm onary surfactant, on the morphology of binary monomolecular lipid films con taining phosphocholine and phosphoglycerol (DPPC and DPPG) at the air-water interface has been studied using epifluorescence and dark-field microscopy . In contrast to previously published studies, the monolayer experiments us ed the entire hydrophobic surfactant protein fraction (containing both the SP-B and SP-C peptides) at physiologically relevant concentrations (similar to 1 wt %). Even at such low levels, the SP-B/C peptides induce the format ion of a new phase in the surface monolayer that is of lower intrinsic orde r than the liquid condensed (LC) phase that forms in the pure lipid mixture . This presumably leads to a higher structural flexibility of the surface m onolayer at high lateral pressure. Variation of the subphase pH indicates t hat electrostatic interaction dominates the association of the SP-B/C pepti des with the lipid monolayer. As evidenced from dark-field microscopy, mono layer material is excluded from the DPPC/DPPG surface film on compression a nd forms three-dimensional, surface-associated structures of micron dimensi ons. Such exclusion bodies formed only with SP-B/C peptides. This observati on provides the first direct optical evidence for the squeeze-out of pulmon ary surfactant material in situ at the air-water interface upon increasing monolayer surface pressures.