Raman spectroscopic studies of model human pulmonary surfactant systems: phospholipid interactions with peptide paradigms for the surfactant protein SP-B.

The temperature dependence of dipalmitoylphosphatidylcholine (DPPC)/phosphatidylglycerol (PG) multilayers, reconstituted with various synthetic peptides for modeling human lung surfactant, was monitored by vibrational Raman spectroscopy. The synthetic peptides consisted, respectively, of residues 59-81 of the human surfactant protein SP-B and 21 amino acid residue peptides containing repeating units of arginine separated by either four or eight leucines (RL4 or RL8). Each peptide demonstrated the ability to reduce significantly the surface tension of analogues of the phospholipid mixture used in the Raman studies. Raman spectroscopic integrated band intensities and relative peak height intensity ratios, two spectral parameters used to determine bilayer disorder, provided sensitive probes for characterizing multilayer perturbations in the reconstituted liposomes. Temperature profiles derived from the various Raman intensity parameters for the 3100-2800-cm-1 carbon-hydrogen (C-H) stretching mode region, a spectral interval representative of acyl chain vibrations, reflected lipid reorganizations due to the bilayer interactions of these peptides. For the three reconstituted multilamellar surfactant systems, the gel-to-liquid-crystalline phase-transition temperatures Tm, defined by acyl chain C-H stretching mode order/disorder parameters, increased from 35 degrees C in the peptide free system to 37-38 degrees C, indicating increased lipid headgroup constraints for the model liposomes. Although the values of Tm were similar for the three recombinant lipid/peptide assemblies, individual phase-transition cooperativities varied significantly between systems and between spectroscopically derived order/disorder parameters.