Peptide Helicity and Membrane Surface Charge Modulate the Balance of Electrostatic and Hydrophobic Interactions with Lipid Bilayers and Biological Membranes

Abstract

An amphipathic model peptide, KLALKLALKALKAAKLA-NH₂, and its complete double d-amino acid replacement set was used to analyze the process of peptide binding at lipid vesicles of different surface charge and to determine the structure of the lipid-bound peptides using CD spectroscopy. The relationship between peptide helicity, model membrane permeability, and biological activity has been studied by dye release from liposomes and investigation of antibacterial and hemolytic activity. The accumulation of cationic KLAL peptides at and the membrane-disturbing effect on bilayers of high negative surface charge were found to be dominated by charge interactions. Independent of any structural propensity, the cationic peptide side chains bind to the anionic phosphatidylglycerol moieties. The charge interactions hold the peptides at the bilayer surface, where they may disturb preferentially lipid headgroup organization by formation of peptide−lipid clusters. In contrast, KLAL peptide interaction with bilayers of low negative surface charge is highly dependent on peptide helicity. With decreasing amounts of anionic phosphatidylglycerol in the bilayer the membrane-disturbing effect of KLAL and other helical analogs substantially increases despite drastically reduced binding affinity. Less helical peptides exhibit reduced bilayer-disturbing activity, showing that the hydrophobic helix domain is decisive for binding at and inducing permeability in membranes of low negative surface charge. It is suggested that hydrophobic interactions drive the penetration of the amphipathic peptide structure into the inner membrane region, thus disturbing the arrangement of the lipid acyl chains and causing local disruption. On the basis of the proposed model for membrane disturbance, interactions modulating antibacterial and hemolytic activity are discussed.