Cell selectivity and mechanism of action of antimicrobial model peptides containing peptoid residues

To develop a useful method for designing cell-selective antimicrobial peptides and to investigate the effect of incorporating peptoid residues into an alpha-helical model peptide on structure, function, and mode of action, we synthesized a series of model peptides incorporating Nala (Ala-peptoid) into different positions of an amphipathic alpha-helical model peptide (KLW). Incorporation of one or two Nala residues into the hydrophobic helix face of KLW was more effective at disrupting the alpha-helical structure and bacterial cell selectivity than incorporation into the hydrophilic helix face or hydrophobic/hydrophilic interface. Tryptophan fluorescence studies of peptide interaction with model membranes indicated that the cell selectivity of KLW-L9-a and KLW-L9,13-a is closely correlated with their selective interactions with negatively charged phospholipids. KLW-L9,13-a, which has two Nala residues in its hydrophobic helix face, showed a random structure in membrane-mimicking conditions. KLW-L9,13-a exhibited the highest selectivity toward bacterial cells, showing no hemolytic activity and no or less cytotoxicity compared with other peptides against four mammalian cell lines. Unlike other model peptides, KLW-L9,13-a caused no or little membrane depolarization in Staphylococcus aureus or lipid flip-flop in negatively charged vesicles. In addition, KLW-L9,13-a caused very little fluorescent dye leakage from negatively charged vesicles. Furthermore, confocal laser-scanning microscopy and DNA-binding assays showed that KLW-L9,13-a probably exerts its antibacterial action by penetrating the bacterial membrane and binding to cytoplasmic compounds (e.g., DNA), resulting in cell death. Collectively, our results demonstrate that the incorporation of two Nala residues into the central position of the hydrophobic helix face of noncell-selective alpha-helical peptides is a promising strategy for the rational design of intracellular, cell-selective antimicrobial peptides.