Influence of the Angle Subtended by the Positively Charged Helix Face on the Membrane Activity of Amphipathic, Antibacterial Peptides
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To investigate the influence of the angle subtended by the positively charged helix face on membrane activity, six amphipathic alpha-helical peptides with angles between 80 degrees and 180 degrees, but with retained hydrophobicity, hydrophobic moment, and positive overall charge, were designed starting from the sequence of the antibacterial peptide magainin 2. CD investigations revealed that all analogs are in an alpha-helical conformation in vesicle suspension. The ability of the peptides to induce dye release from negatively charged phosphatidylglycerol (PG) vesicles decreased with increasing angle. However, peptides with a large angle of positively charged residues (140-180 degrees) exhibited a considerably higher permeabilizing activity at zwitterionic phosphatidylcholine (PC) and mixed PC/PG (3:1) vesicles than analogs with a small angle (80-120 degrees). In addition, analogs with large angles were more active in antibacterial and hemolytic assays. The antibacterial specificity of these analogs was decreased. Binding investigations showed that peptide binding is favored by a large angle and a high content of negatively charged phospholipid. In contrast, a small angle and a low negative membrane charge enhanced the membrane-permeabilizing efficiency of the bound peptide fraction. All analogs stabilized the bilayer phase of phosphatidylethanolamine over the inverted hexagonal phase. Therefore, a class L mechanism of permeabilization can be excluded. Furthermore, the analogs do not act by the induction of positive curvature strain or by a "carpet-like" mechanism. Our results are in accordance with a pore mechanism: The membrane-permeabilizing efficiency of analogs with enhanced angle of positively charged residues is reduced due to electrostatic repulsion between adjacent helices within the pore, thus resulting in a decreased pore-forming probability and/or pore destabilization.
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