INFLUENCE OF MEMBRANE-SPANNING ALPHA-HELICAL PEPTIDES ON THE PHASE-BEHAVIOR OF THE DIOLEOYLPHOSPHATIDYLCHOLINE WATER SYSTEM/

The effect of solubilized hydrophobic peptides on the phase behavior o f dioleoylphosphatidylcholine (DOPC)/ water system was studied by H-2- and P-31-NMR spectroscopy and by x-ray diffraction, and partial phase diagrams were constructed. The utilized peptides were HCO-AWW(LA)(5)W WA-NHCH2CH2OH (WALP16), which is an artificial peptide designed to res emble a transmembrane part of a membrane protein; and VEYAGIALFFVAAVLT LWSMLQYLSAAR (Pgs peptide E), a peptide that is identical to one of th e putative transmembrane segments of the membrane-associated protein p hosphatidylglycerophosphate synthase (Pgs) in Escherichia coli. Circul ar dichroism spectroscopy suggests that both peptides are mostly or-he lical in DOPC vesicles. The most striking features in the phase diagra m of the WALP16/DOPC/water system are 1) a single lamellar liquid crys talline (L-alpha) phase forms only at very low peptide concentrations. 2) At low water content and above a peptide/lipid molar ratio of simi lar to 1:75 a reversed hexagonal liquid crystalline (H-II) phase coexi sts with an L-alpha phase, while in excess water this phase forms at a peptide/lipid molar ratio of similar to 1:25. 3) At peptide/lipid rat ios greater than or equal to 1:6 a single H-II phase is stable. Also, the Pgs peptide E strongly affects the phase behavior, and a single L- alpha phase is only found at low peptide concentrations (peptide/lipid molar ratios <1:50), and water concentrations (45% (w/w). Higher pept ide content results in coexistence of L-alpha and isotropic phases. Ge nerally, the fraction of the isotropic phase increases with increasing temperature and water concentration, and at 80% (w/w) water content o nly a single isotropic phase is stable at 55 degrees C. Thus, both pep tides were found to be able to induce nonlamellar phases, although dif ferent in structure, in the DOPC/water system. The phase transitions, the extensions of the one-phase regions, and the phase structures obse rved for the two systems are discussed in terms of the molecular struc ture of the two peptides and the matching between the hydrophobic leng ths of the peptides and the bilayer thickness of DOPC.