Acyl chain length tuning improves antimicrobial potency and biocompatibility of short designed lipopeptides

Designed antimicrobial lipopeptides (ALPs) offer the attractive benefits of short peptide sequences and flexible tuning of amphiphilicity by altering the acyl chain length. These lipopeptides kill microbes by forming intriguing in-membrane nanostructures and causing the leakage of internal contents. However, how subtle differences in the molecular structures of the lipopeptides affect their antimicrobial efficacy and biocompatibility to host cells is still under-investigated.

This work focuses on assessing changes in the acyl chain length of CH₃(CH₂)_n-2CO-KKKIII-NH₂ (n = 10, 12 and 14, K = lysine, I = isoleucine, denoted as C_nKI3) on the antimicrobial potency and cytotoxicity by combining biological assays with physical measurements. Aggregation properties were characterized by changes in critical aggregation concentration (CAC) from surface tension measurements. Antimicrobial susceptibility tests, cytotoxic MTT assays, haemolytic tests, and dynamic bactericidal experiments were employed to reveal their bioactive potency toward different types of cells. To further investigate lipopeptides' underlying antimicrobial and cytotoxic mechanisms, lipid monolayer and lipid small unilamellar vesicle (SUV) models were established and biophysically characterized.

An increase in n led to the decrease in the CAC of C_nKI3, showing a rising membrane-lytic power. Subsequent bioactive measurements revealed the optimal performance of C₁₂KI3 from this series of lipopeptides. The selective membrane binding behaviour was well supported by neutron reflection data from charged lipid monolayer models, revealing membrane-supported nanostructures of ALPs. However, increased membrane-lytic actions in C₁₄KI3 led to notably increased toxicity and reduced selectivity. On the other hand, C₁₄KI3 can impose faster dynamic killing than natural lipopeptide polymyxin B, showing the distinct impact of the amphiphilic balance from the designed lipopeptide. In contrast, the distinctly weaker binding to zwitterionic membrane models (monolayers and SUVs) provided direct nanoscale structural evidence to the mildness of the designed ALPs on host cells. This work demonstrates the high selectivity and fast killing of rationally designed short ALPs to microbes via in-membrane nanostructuring.