Central β-turn increases the cell selectivity of imperfectly amphipathic α-helical peptides

Although membrane lytic antimicrobial peptides (AMPs) show enormous potential for addressing mounting global antibiotic resistance, therapeutic applications are hindered by their weak antimicrobial activity, high toxicity, salt sensitivity and poor understanding of structure-activity relationships. To investigate the effects of different parameters on the biological activities of AMPs, a rational approach was adopted to design a series of short cationic α-helical peptides comprising the Ac-WxKyWxzzyKxWyK-NH₂ sequence, where x: cationic residues (Arg or Lys), y: hydrophobic residues (Ala, Val, Ile or Leu), and zz: β-turn (rigid D-Pro-Gly turn or flexible Gly-Gly turn). The peptides showed a more helical structure as the concentration of membrane-mimetic solution increased. The peptide RL with a central D-Pro-Gly turn (x: Arg, y: Lys, zz = D-Pro-Gly) exhibited broad-spectrum antimicrobial activities (2-8 μM) against ten types of clinically relevant microorganisms and even maintained its activity in the presence of physiological salts and showed excellent selectivity toward bacterial cells over human red blood cells and mammalian cells. However, the toxicity was increased after the removal of D-Pro-Gly turn. Additionally, the bactericidal activity was reduced when the D-Pro-Gly turn was replaced by a Gly-Gly turn. Fluorescence spectroscopy and electron microscopy analyses indicated that RL and its derivatives killed microbial cells by permeabilizing the cell membrane and damaging membrane integrity. In conclusion, these findings clearly generalized a potential method for designing or optimizing AMPs, and the peptide RL is a promising therapeutic candidate to combat antibiotic resistance.

Statement of significance: We proposed a rational approach to design imperfectly amphiphilic peptides and identified RL (Ac-WRKLWRpGLKRWLK-NH₂) in particular that shows strong antibacterial properties, low toxicity and high salt resistance. The β-turn unit inserted into the central position of cationic α-helical peptides, especially the D-Pro-Gly turn, significantly increase the cell selectivity of the synthetic amphiphiles. The findings demonstrate a potential method for designing and/or optimizing AMPs, which would facilitate the development of strategies to design peptide-based antimicrobial biomaterials in a variety of biotechnological and clinical applications.