AbstractNatural and synthetic antimicrobial peptides (AMPs) show interesting features, and they are considered possible alternatives to common antibiotics which might induce resistance in bacteria. We present a comparative study of the interaction of two homologous AMPs with lipids mimicking the cytoplasmic membrane of prokaryotic and eukaryotic cells. The peptides were derived from the membranolytic protein NK-lysin. Phosphatidylcholine (PC) was used as a representative of human erythrocytes and phosphatidylethanolamine (PE) was chosen to build the model cytoplasmic membrane of Escherichia coli. Although the sequences of the investigated peptides vary only in one position, they show a difference in activity against E. coli. The results of small angle X-ray scattering and differential scanning calorimetry revealed that these two analogs behaved differently upon interaction with PE liposomes. One of the peptides significantly decreased the temperature of the hexagonal phase transition, inducing a negative membrane curvature, whereas the second one shifted the phase transition temperature to higher values. No influence on the lipid phase behavior or the bilayer structure was detected after mixing the peptides with PC vesicles. This observation completed the results of the hemolytic assay, where the toxic effect of the peptides on the red blood cells was not found.