Structural requirements for potent versus selective cytotoxicity for antimicrobial dermaseptin S4 derivatives

To better understand the structural requirements for selective cytotoxicity of antimicrobial peptides, seven dermaseptin S4 analogs were produced and investigated with respect to molecular organization in solution, binding properties to model phospholipid membranes, and cytotoxic properties. Native dermaseptin S4 displayed high aggregation in solution and high binding affinity. These properties correlated with high cytotoxicity. Yet, potency was progressively limited when facing cells whose plasma membrane was surrounded by increasingly complex barriers. Increasing the positive charge of the native peptide led to partial depolymerization that correlated with higher binding affinity and with virtually non-discriminative high cytotoxicity against all cell types. The C-terminal hydrophobic domain was found responsible for binding to membranes but not for their disruption. Truncations of the C terminus combined with increased positive charge of the N-terminal domain resulted in short peptides having similar binding affinity as the parent compound but displaying selective activity against microbes with reduced toxicity toward human red blood cells. Nuclear magnetic resonance-derived three-dimensional structures of three active derivatives enabled the delineation of a common amphipathic structure with a clear separation of two lobes of positive and negative electrostatic potential surfaces. Whereas the spatial positive electrostatic potential extended considerably beyond the peptide dimensions and was required for potency, selectivity was affected primarily by hydrophobicity. The usefulness of this approach for the design of potent and/or selective cytolytic peptides is discussed herein.