Abstract
Phosphorus NMR spectroscopy was used to characterize the importance of electrostatic interactions in the lytic activity of melittin, a cationic peptide. The micellization induced by melittin has been characterized for several lipid mixtures composed of saturated phosphatidylcholine (PC) and a limited amount of charged lipid. For these systems, the thermal polymorphism is similar to the one observed for pure PC: small comicelles are stable in the gel phase and extended bilayers are formed in the liquid crystalline phase. Vesicle surface charge density influences strongly the micellization. Our results show that the presence of negatively charged lipids (phospholipid or unprotonated fatty acid) reduces the proportion of lysed vesicles. Conversely, the presence of positively charged lipids leads to a promotion of the lytic activity of the peptide. The modulation of the lytic effect is proposed to originate from the electrostatic interactions between the peptide and the bilayer surface. Attractive interactions anchor the peptide at the surface and, as a consequence, inhibit its lytic activity. Conversely, repulsive interactions favor the redistribution of melittin into the bilayer, causing enhanced lysis. A quantitative analysis of the interaction between melittin and negatively charged bilayers suggests that electroneutrality is reached at the surface, before micellization. The surface charge density of the lipid layer appears to be a determining factor for the lipid/peptide stoichiometry of the comicelles; a decrease in the lipid/peptide stoichiometry in the presence of negatively charged lipids appears to be a general consequence of the higher affinity of melittin for these membranes.