High potency and broad-spectrum antimicrobial peptides synthesized via ring-opening polymerization of α-Aminoacid-N-carboxyanhydrides

Antimicrobial peptides (AMPs), particularly those effective against methicillin-resistant Staphylococcus aureus (S. aureus) and antibiotic-resistant Pseudomonas aeruginosa (P. aeruginosa), are important alternatives to antibiotics. Typical peptide synthesis methods involving solid-phase sequential synthesis are slow and costly, which are obstacles to their more widespread application. In this paper, we synthesize peptides via ring-opening polymerization of α-amino acid N-carboxyanhydrides (NCA) using a transition metal initiator. This method offers high potential for inexpensive synthesis of substantial quantities of AMPs. Lysine (K) was chosen as the hydrophilic amino acid and alanine (A), phenylalanine (F), and leucine (L) as the hydrophobic amino acids. We synthesized five series of AMPs (i.e., P(KA), P(KL), P(KF), P(KAL), and P(KFL)), varied the hydrophobic amino acid content from 0 to 100%, and determined minimal inhibitory concentrations (MICs) against clinically important Gramnegative and Gram-positive bacteria and fungi (i.e., Escherichia coli (E. coli), P. aeruginosa, Serratia marcescens (S. marcescens), and Candida albicans (C. albicans). We found that P(K₁₀F _7.5L_7.5) and P(K₁₀F₁₅) show the broadest activity against all five pathogens and have the lowest MICs against these pathogens. For P(K₁₀F_7.5L_7.5), the MICs against E. coli, P. aeruginosa, S. marcescens, S. aureus, and C. albicans are 31 μg/mL, 31 μg/mL, 250 μg/mL, 31 μg/mL, and 62.5 μg/mL, while for P(K₁₀F₁₅) the respective MICs are 31 μg/mL, 31 μg/mL, 250 μg/mL, 31 μg/mL, and 125 μg/mL. These are lower than the MICs of many naturally occurring AMPs. The membrane depolarization and SEM assays confirm that the mechanism of microbe killing by P(K₁₀F _7.5L_7.5) copeptide includes membrane disruption, which is likely to inhibit rapid induction of AMP-resistance in pathogens.