Mechanism of antibacterial action of a synthetic peptide with an Ala-peptoid residue based on the scorpion-derived antimicrobial peptide IsCT

A novel bacterial cell-selective antimicrobial peptide, IsCT-P (ILKKIWKPIKKLF-NH₂), was designed based on the scorpion-derived α-helical antimicrobial peptide, IsCT. Here, we investigated the effect of substituting Pro⁸ of IsCT-P with the Ala-peptoid residue (N-methylglycine) on the peptide's structure and mechanism of action. Circular dichroism analysis revealed that the modified peptide, IsCT-a, has a much lower α-helicity than IsCT-P in membrane mimicking conditions, suggesting the peptoid residue provides much more structural flexibility than the proline residue. IsCT-a was also much less effective than IsCT-P at causing leakage of fluorescent dye entrapped within negatively charged vesicles and at dissipating the membrane potential of Staphylococcus aureus. Collectively, our results suggest that the antibacterial action of IsCT-a is due to the inhibition of intracellular targets rather than the disruption and depolarization of bacterial cell membranes.