Influence of lipid chain unsaturation on melittin-induced micellization.

It is well known that melittin, an amphipathic helical peptide, causes the micellization of phosphatidylcholine vesicles. In the present work, we conclude that the extent of micellization is dependent on the level of unsaturation of the lipid acyl chains. We report the results obtained on two systems: dipalmitoylphosphatidylcholine (DPPC), containing 10(mol)% saturated or unsaturated fatty acid (palmitic, oleic, or linoleic), and DPPC, containing 10(mol)% positively charged diacyloxy-3-(trimethylammonio)propane bearing palmitic or oleic acyl chains. For both systems, the presence of unsaturation in the lipid acyl chains inhibits melittin-induced micellization. Conversely, the addition of saturated palmitic acid to the DPPC matrix enhances the micellization. This modulation is proposed to be associated with the cohesion of the hydrophobic core. When the lipid chain packing of the gel-phase bilayer is already perturbed by the presence of unsaturation, it seems easier for the membrane to accommodate melittin at the interface, and the distribution of the peptide in the bilayer could be the origin of the inhibition of the micellization. The cohesion of the apolar core is shown to play an unquestionable role in melittin-induced micellization; however, this contribution does not appear to be as important as the electrostatic interactions between melittin and positively or negatively charged lipids.