Strand Length-Dependent Antimicrobial Activity and Membrane-Active Mechanism of Arginine- and Valine-Rich β-Hairpin-Like Antimicrobial Peptides

Antimicrobial peptides with amphipathic beta-hairpin-like structures have potent antimicrobial properties and low cytotoxicity. The effect of VR or RV motifs on beta-hairpin-like antimicrobial peptides has not been investigated. In this study, a series of beta-hairpin-like peptides, Ac-C(VR)(n)(D)PG (RV)(n)C-NH2 (n = 1, 2, 3, 4, or 5), were synthesized, and the effect of chain length on antimicrobial activity was evaluated. The antimicrobial activity of the peptides initially increased and then decreased with chain length. Longer peptides stimulated the toxicity to mammalian cells. VR3, a 16-mer peptide with seven amino acids in the strand, displayed the highest therapeutic index and represents the optimal chain length. VR3 reduced bacterial counts in the mouse peritoneum and increased the survival rate of mice at 7 days after Salmonella enterica serovar Typhimurium infection in vivo. The circular dichroism (CD) spectra demonstrated that the secondary structure of the peptides was a beta-hairpin or beta-sheet in the presence of an aqueous and membrane-mimicking environment. VR3 had the same degree of penetration into the outer and inner membranes as melittin. Experiments simulating the membrane environment showed that Tip-containing VRW3 (a VR3 analog) tends to interact preferentially with negatively charged vesicles in comparison to zwitterionic vesicles, which supports the biological activity data. Additionally, VR3 resulted in greater membrane damage than melittin as determined using a flow cytometry-based membrane integrity assay. Collectively, the data for synthetic lipid vesicles and whole bacteria demonstrated that the VR3 peptide killed bacteria via targeting the cell membrane. This assay could be an effective pathway to screen novel candidates for antibiotic development.