Effect of Secondary Structure and Side Chain Length of Hydrophobic Amino Acid Residues on the Antimicrobial Activity and Toxicity of 14‐Residue‐Long de novo AMPs

Herein we report the efficacy and toxicity of three de novo designed cationic antimicrobial peptides (AMPs) LL‐14, VV‐14 and ββ‐14, where side chains of the hydrophobic amino acids were reduced gradually. The AMPs showed broad‐spectrum antimicrobial activity against three pathogens from the ESKAPE group and two fungal strains. This study showed that side chains which are either too long or too short increase toxicity and lower antimicrobial activity, respectively. VV‐14 was found to be non‐cytotoxic and highly potent under physiological salt concentrations against several pathogens, especially Salmonella typhi TY2. These AMPs acted via membrane deformation, depolarization, and lysis. The activity of the AMPs is related to their ability to take on amphipathic helical conformations in the presence of microbial membrane mimics. Among AMPs with the same charge, hydrophobic interactions between the side chains of the residues with cell membrane lipids determine their antimicrobial potency and cytotoxicity. Strikingly, an optimum hydrophobic interaction is the crux of generating highly potent non‐cytotoxic AMPs. Antimicrobial peptides LL‐14, VV‐14, and ββ‐14, with progressively diminishing hydrophobic side chain lengths, were found to have broad‐spectrum antimicrobial activity. Antimicrobial potency and cytotoxicity of these AMPs is proportional to the length of the hydrophobic side chain. The AMPs were found to be unstructured in water and attained an amphipathic helical conformation in the presence of microbial membrane mimics. Their antimicrobial activity is effected via membrane destabilization and cell lysis. The optimum side chain length yielded the best potency and lowest cytotoxicity.