Antimicrobial Peptide Interactions with Phospholipid Membranes : Effects of Peptide and Lipid Composition on Membrane Adsorption and Disruption

Sammanfattning: The interactions between antimicrobial peptides and phospholipid membranes were investigated, in terms of lipid headgroup variations and the role of cholesterol, as well as peptide composition and structure. Also strategies for increasing proteolytic stability were evaluated. The interactions were studied on model membranes in the form of liposomes and supported bilayers, through a combination of ellipsometry, fluorescence spectroscopy, circular dichroism, dynamic light scattering, electrophoresis, electron cryomicroscopy, and bacterial/cell culture experiments. The findings showed that membrane tolerance against the lytic activity of melittin, was increased on anionic membranes by electrostatic arrest in the headgroup region, and was reduced by hydration repulsion. The presence of cholesterol caused a reduction in melittin adsorption, while at the same time reducing membrane tolerance per adsorbed peptide. Differences in membrane leakage mechanisms were also attributed to cholesterol, where large scale structural effects contributed to the leakage, while other membranes followed the pore formation model. Substituting specific amino acids for tryptophan on an LL-37 derivative, was shown to increase stability against bacterial proteases, while at the same time significantly increasing antibacterial properties. These substitutions, as well as terminal modifications, increased adsorption and membrane lytic properties in a way that was less dependent on electrostatics. Furthermore, by comparing short cationic peptides with oligotryptophan end-tagged versions, the lytic mechanism of end-tagged peptides, and the different contributions of arginine and lysine to membrane adsorption and disruption were demonstrated. This thesis is a contribution to the development of antimicrobial peptides as therapeutic alternatives to conventional antibiotics.