Structure–activity study of the antibacterial peptide fallaxin

Abstract Fallaxin is a 25‐mer antibacterial peptide amide, which was recently isolated from the West Indian mountain chicken frog Leptodactylus fallax . Fallaxin has been shown to inhibit the growth of several Gram‐negative bacteria including Enterobacter cloacae , Escherichia coli , Klebsiella pneumoniae , and Pseudomonas aeruginosa. Here, we report a structure–activity study of fallaxin based on 65 analogs, including a complete alanine scan and a full set of N‐ and C‐terminal truncated analogs. The fallaxin analogs were tested for hemolytic activity and antibacterial activity against methicillin‐resistant Staphylococcus aureus (MRSA), vancomycin‐intermediate resistant S. aureus, ( VISA ), methicillin‐susceptible S. aureus ( MSSA ), E. coli, K. pneumoniae , and P. aeruginosa . We identified several analogs, which showed improved antibacterial activity compared to fallaxin. Our best candidate was FA12, which displayed MIC values of 3.12, 25, 25, and 50 μM against E. coli , K. pneumoniae, MSSA , and VISA, respectively. Furthermore, we correlated the antibacterial activity with various structural parameters such as charge, hydrophobicity 〈H〉, mean hydrophobic moment 〈μ H 〉, and α‐helicity. We were able to group the active and inactive analogs according to mean hydrophobicity 〈H〉 and mean hydrophobic moment 〈μ H 〉. Far‐UV CD‐spectroscopy experiments on fallaxin and several analogs in buffer, in TFE, and in membrane mimetic environments (small unilamellar vesicles) indicated that a coiled‐coil conformation could be an important structural trait for antibacterial activity. This study provides data that support fallaxin analogs as promising lead structures in the development of new antibacterial agents.