Nano-Antimicrobial Peptides Based on Constitutional Isomerism-Dictated Self-Assembly

Self-assembly has been identified as an innovative strategy for improving the antimicrobial efficacy and bioavailability of short peptides. However, the detailed molecular information of short peptides linking to the self-assembly structures and antimicrobial activity remains to be more clearly understood. This work reported that the constitutional isomeric sequences of cationic peptides showed a significant impact on their antimicrobial activity. We investigated the self-assembly structures of two constitutional isomeric peptides Ac-RFSFSFR-NH₂ and Ac-SFRFRFS-NH₂, which contained the same serine, alkaline, and phenylalanine residues but in a different order. Transmission electron microscopy (TEM) and atomic force microscopy (AFM) revealed that the constitutional isomers self-assembled into different morphologies in an aqueous solution. The sequence with alkaline residues located at both termini of the peptide favored the formation of β-sheet conformation and nanofibers, while irregular nanospheres were observed when positioning the alkaline residues at the center of the isomeric peptide. The ζ-potential measurements showed that the Ac-RFSFSFR-NH₂ nanofibers had a net potential of +17.4 mV, whereas the apparent potential of Ac-SFRFRFS-NH₂ nanospheres dropped steeply to +1.0 mV. These differences of the constitutional isomeric peptides were directly reflected in their antimicrobial activities. In comparison with the peptide Ac-SFRFRFS-NH₂, the constitutional isomer Ac-RFSFSFR-NH₂ exhibited much higher antimicrobial efficacy against Gram-positive Staphylococcus aureus and Bacillus subtilis and Gram-negative Escherichia coli and Pseudomonas aeruginosa. Moreover, several pairs of constitutional isomeric peptides with a similar sequence layout yielded the same outcome. These collective results not only highlight the importance of the isomeric sequence on the antimicrobial efficacy of short peptides but also increase further potential in optimizing the design of self-assembled nano-antimicrobial peptides (AMPs).