INTERACTIONS OF MODEL HUMAN PULMONARY SURFACTANTS WITH A MIXED PHOSPHOLIPID-BILAYER ASSEMBLY - RAMAN-SPECTROSCOPIC STUDIES

The temperature dependence and acyl chain packing properties of the bi nary lipid mixtures of dipalmitoylphosphatidylcholine-d62 (DPPC-d62)/d ipalmitoylphosphatidylglycerol (DPPG) multilayers, reconstituted with two synthetic peptides for modeling the membrane behavior of the SP-B protein associated with human pulmonary surfactant, were investigated by vibrational Raman spectroscopy. The synthetic peptides consisted of 21 amino acid residues representing repeating charged units of either lysine or aspartic acid separated by hydrophobic domains consisting o f four leucines (KL4 or DL4, respectively). These peptides were design ed to mimic the alternating hydrophobic and hydrophilic sequences defi ning the low molecular weight SP-B protein. Raman spectroscopic parame ters consisting of integrated band intensities, line widths, and relat ive peak height intensity ratios were used to probe the bilayer order/ disorder characteristics of the liposomal perturbations reflected by t he reconstituted membrane assemblies. Temperature profiles derived fro m the various Raman intensity parameters for the 3100-2800-cm-1 carbon -hydrogen (C-H) and the 2000-2300-cm-1 carbon-deuterium (C-D) stretchi ng mode regions, spectral intervals representative of acyl chain vibra tions, reflected lipid reorganizations specific to peptide interaction s with either the DPPC-d62 or DPPG component of the liposome. For the multilamellar surfactant systems composed of either KL4 Or DL4 reconst ituted with the binary DPPG/DPPC-d62 lipid mixture, the breadth of the gel to liquid crystalline phase transition temperatures T(M), defined by acyl chain C-H and C-D stretching mode order/disorder parameters, increased from about 1-degrees-C in the peptide-free systems to over 1 0-degrees-C. This breadth in T(M) indicates an increased lipid disorde r and a distinct noncooperative chain melting process for the model li posomes. In comparing the interactions of the synthetic peptides with DPPG/DPPC mixtures and with DPPC liposomes alone, the negatively charg ed DL4 peptide perturbs the DPPG component of the lipid mixture more s trongly than the DPPC-d62 component; moreover, the DL4 peptide disrupt s the structure of the DPPG lipid domains in the binary mixture to a g reater extent than the KL4 peptide. The microdomain heterogeneity of t he binary lipid mixture arising from lipid-peptide interactions is dis cussed in terms of the Raman spectral properties of the multilayers. T he Raman data in conjunction with previous bubble surfactometer and an imal studies (Cochrane & Revak, 1991) suggest that lipid domain struct ures are present in functional surfactants and that the dynamic bilaye r microheterogeneity induced by the surfactant peptide or protein is e ssential for pulmonary mechanics.