Comparative interaction of alpha-helical and beta-sheet amphiphilic isopeptides with phospholipid monolayers

The two sequential amphiphilic peptide isomers, (Leu-Lys-Lys-Leu)4 and (Leu-Lys)8, were chosen as models for alpha-helical and beta-sheet peptides, respectively. In order to evaluate the contribution of the secondary structure of a peptide to its penetration into cellular membranes, interactions of these isopeptides with L-alpha-dimyristoyl phosphatidylcholine (DMPC) monolayers were studied. Both isopeptides penetrate into DMPC monolayers up to an exclusion pressure of approximately 27 mN/m, but a discontinuity is observed in the penetration profile of the alpha-helical (LKKL)4. The main parameters extracted from the compression isotherms of the mixed peptide/DMPC monolayers-namely, transition pressure, mean area, elasticity modulus, and energy of mixing-were analyzed. These analyses indicate that the alpha-helical isomer interacts strongly with DMPC by forming a 1:32 (LKKL)4-DMPC complex. When engaged in this complex, (LKKL)(4) behaves as an hydrophobic helix and has a tendency to become vertically oriented in the course of the compression process. The beta-sheet (LK)8 interacts more weakly with DMPC and no complex can be detected.