STRUCTURAL INVESTIGATIONS OF BASIC AMPHIPATHIC MODEL PEPTIDES IN THE PRESENCE OF LIPID VESICLES STUDIED BY CIRCULAR-DICHROISM, FLUORESCENCE, MONOLAYER AND MODELING

A cationic amphiphilic peptide made of 10 leucine and 10 lysine residu es, and four of its fluorescent derivatives in which leucines were sub stituted by Trp residues at different locations on the primary sequenc e have been synthesized. The interactions of these five peptides with neutral anionic or cationic vesicles were investigated using circular dichroism, steady state and time-resolved fluorescence with a combinat ion of Trp quenching by brominated lipid probes, monolayers, modeling with minimization and simulated annealing procedures. We show that all the five peptides interact with neutral and anionic DMPC, DMPG, DOPC or egg yolk PC vesicles. The binding takes place whatever the peptide conformation in solution is. In the case of DMPC bilayers the binding free energy Delta G is estimated at -8 kcal mole(-1) and the number of phospholipid molecules involved is about 20-25 per peptide molecule. Peptides are bound as single-stranded alpha helices orientated paralle l to the bilayer surface. In the anchoring of phospholipid head groups around the peptides, the lipid molecules are not smeared out in a pla ne parallel to the membrane surface but are organized around the hydro philic face of the alpha helices like 'wheat grains around an ear' and protrude outside the bilayer rewards the solvent. We suggest that suc h a lipid arrangement generates transient structural defects responsib le for the membrane permeability enhancement. When an electrical poten tial is applied, the axis of the peptide helices remains parallel to t he membrane surface and does not reorient to give rise to a bundle of helix monomers that forms transmembrane channels via a 'barrel stave' mechanism. The penetration depth of alpha helices in relation to the p osition of phosphorus atoms in the unperturbed lipid leaflet is estima ted at 3.2 Angstrom. (C) 1998 Elsevier Science B.V. All rights reserve d.