Antimicrobial properties and interaction of two Trp-substituted cationic antimicrobial peptides with a lipid bilayer

Tryptophan (Trp) residues reportedly exhibit a strong membrane-disruptive activity, and this property endows Trp-containing antimicrobial peptides (AMPs) with a unique ability to interact with the surface of bacterial cell membranes, possibly improving antimicrobial properties. In this study, we investigated the influence of Trp residues engineered to have a distinct preference for the interface region of lipid bilayers on antimicrobial activity. We designed two Trp-substituted AMPs (I1WL5W and I4WL5W) by replacing Ile or Leu residues with two Trp residues at different positions in the L-K6 peptide, and determined their antimicrobial activity and mechanism of membrane action. Both I1WL5W and I4WL5W exhibited significantly higher antimicrobial activity and lower cytotoxicity against Gram-negative and Gram-positive bacteria compared with L-K6. The Trp-substituted peptides had a disordered structure in aqueous solution and adopted an α-helical structure in solutions of 50% trifluoroethanol/water and 30 mM SDS. I1WL5W and I4WL5W caused a significant leakage of calcein from liposomes containing membranes that mimicked those of Escherichia coli and Staphylococcus aureus. Scanning electron microscopy analysis suggested that I1WL5W and I4WL5W killed bacteria by disrupting bacterial cell membranes. Furthermore, fluorescence and quenching data from a variety of liposomes, which mimic different cell membranes, indicated that the Trp-substituted peptides could insert into the lipid bilayers and induce blue shifts in the emission spectra of the Trp residues. I1WL5W and I4WL5W were also less susceptible to acrylamide or KI quenchers. The current work may be important for designing novel Trp-containing peptides exhibiting strong antimicrobial abilities by penetrating bacterial membranes.