Interaction of CAP18-derived peptides with membranes made from endotoxins or phospholipids

Antimicrobial peptides with alpha-helical structures and positive net charges are in the focus of interest with regard to the development of new antibiotic agents, in particular against Gram-negative bacteria. Interaction between seven polycationic alpha-helical CAP18-derived peptides and different types of artificial membranes composed of phosphatidylcholine or lipopolysaccharide of the Gram-negative bacterium Escherichia coli were investigated using different biophysical techniques. Results obtained from fluorescence energy transfer spectroscopy with liposomes, monolayer measurements on a Langmuir trough, and electrophysiological measurements on planar reconstituted asymmetric bilayer membranes including the lipid matrix of the outer membrane of E. coli were correlated, and these data were, furthermore, correlated with structural parameters of the peptides (net charge, alpha-helical content, hydrophobic moment, and hydrophobicity). All peptides induced current fluctuations in planar membranes due to the formation of transient lesions above a peptide- and lipid-specific minimal clamp voltage. Antibacterial activity was exhibited only by those peptides that induced lesion formation in the reconstituted outer membrane at clamp voltages below the transmembrane potential of the natural membrane. Thus, we propose that the physicochemical properties of both the peptides as well as of the target membranes are important for antibacterial activity.