Magainin 1-induced leakage of entrapped calcein out of negatively-charged lipid vesicles

Effects of magainin 1, a novel antimicrobial peptide, on the permeability of lipid vesicles were investigated by using calcein as a trapped fluorescent marker. Magainin 1 induces the leakage of calcein specifically out of negatively-charged vesicles. The peptide binds to bovine brain phosphatidylserine sonicated vesicles according to the Langmuir isotherm with a binding constant of 3.8.10(5) M-1 and a binding-site number of 0.10 per lipid molecule. The leakage seems to occur at a critical binding number of approx. 0.03 per lipid molecule. A circular dichroism study revealed that magainin 1 conforms mainly to an unordered structure both in an aqueous solution and in the presence of egg yolk phosphatidylcholine vesicles, whereas to an amphiphilic helix with the phosphatidylserine vesicles. In conclusion, magainin 1 interacts with acidic lipids through electrostatic interactions followed by hydrophobic interactions to form an amphiphilic helix, inducing the leakage.