Structural effects of a basic peptide on the organization of dipalmitoylphosphatidylcholine/dipalmitoylphosphatidylserine membranes: a fluorescent resonance energy transfer study.

We studied the effect of a model basic peptide, hexalysiltryptophan, on the organization of dipalmitoylphosphatidylcholine/dipalmitoylphosphatidylserine unilamellar vesicles by means of fluorescent resonance energy transfer (FRET) between fluorescently labeled phospholipids. Several FRET theoretical models assuming different bilayer geometries and probe distributions were fitted to the time-resolved data. The experiments were carried out at two temperatures in different regions of the lipid mixture phase diagram. At 45 degrees C, the expected gel/fluid phase separation was verified by model fitting in peptide-free vesicles, which from the FRET approach means that domains are larger than approximately 200 A. No noticeable alteration of membrane organization was detected upon increasing the peptide concentration. At variance, for the single fluid phase at 60 degrees C, there was a large increase in FRET efficiency upon peptide addition to the lipid vesicles, mainly caused by peptide-induced vesicle aggregation. The system gradually changed from unilamellar lipid vesicles to a multibilayer geometry, and a limit lamellar repeat distance of approximately 57 A was recovered. Furthermore, no evidence for lateral domain formation on the FRET length scale was found at this temperature, the cationic peptide being only able to induce local lipid demixing, causing a short-range sequestration of 2-3 acidic lipids around each surface-adsorbed peptide.