Phase diagrams of systems with cationic alpha-helical membrane-spanning model peptides and dioleoylphosphatidylcholine

Ternary phase diagrams have been constructed of systems with dioleoylphosph atidylcholine (DOPC) and water, and two a-helical membrane-spanning model p eptides, KKLAKK16[KK(LA)(6)KK] and KKLAKK20[KK(LA)(8)KK]. It was found that these peptides induced non-lamellar liquid crystalline phases. The amount of peptide needed for this phase transition depended on the water content a nd the temperature; and for KKLAKK16, a smaller amount of peptide was neede d to induce non-lamellar phases than for KKLAKK20. Both peptides were found to induce an isotropic phase, and KKLAKK16 also induced a reversed hexagon al phase. Both peptides may also reside in a lamellar (L-alpha) phase. When magic angle spinning (MAS) P-31 NMR experiments were performed on samples containing the L-alpha phase and an isotropic phase, four different isotrop ic chemical shifts were observed. The isotropic chemical shifts could be as signed to the phases, using spinning sidebands to calculate the chemical sh ift anisotropy (CSA) corresponding to each isotropic shift. MAS C-13 NMR al so indicated a difference in the aggregational state of the peptides betwee n the L-alpha and isotropic phases. The phase diagrams were compared to the phase diagram of a similar model peptide, AWW(LA)(5)WWA in systems with DO PC and water. It was concluded that the phase behaviour was influenced by b oth electrostatic interactions between the peptides and the lipid headgroup s, and the difference between the hydrophobic length of the peptide and the hydrophobic thickness of the lipid bilayer. (C) 2001 Elsevier Science B.V. All rights reserved.