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Can antimicrobial activity and peptide stability of alpha-helical peptides be increased by making them into dimers and macrocycles? Here, we explore that concept by using KR-12 as the starting point for peptide engineering. KR-12 has previously been determined as the minimalized antimicrobial fragment of the human host defense peptide LL-37. Backbone-cyclized KR-12 dimers, tethered by linkers of two to four amino acid residues, were synthesized and their antimicrobial activity, proteolytic stability and structures characterized. A modified KR-12 sequence, with substitutions at previously identified key residues, were also included in the screening panel. The backbone cyclized KR-12 dimers showed improved antimicrobial activity and increased stability compared to monomeric KR-12. The most active cyclic dimer displayed 16-fold higher antibacterial activity compared to KR-12 against  Pseudomonas aeruginosa  and  Staphylococcus aureus , and 8-fold increased fungicidal activity against  Candida albicans . It also showed increased hemolytic and cytotoxic activity. Enhanced antimicrobial activity coincided with increased membrane permeabilization of liposomes with one distinct discrepancy: monomeric KR-12 was much less disruptive of liposomes with bacterial lipid composition compared to liposomes from fungal lipid extract. Circular dichroism showed that the four-residue linked most active cyclic dimer had 65% helical content when bound to lyso-phosphatidylglycerol micelles, indicating that the helical propensity of the parent peptide is maintained in the new macrocyclic form. In conclusion, the current work on KR-12 suggests that dimerization together with backbone cyclization is an effective strategy for improving both potency and stability of linear antimicrobial peptides.

Backbone Cyclization and Dimerization of LL-37-Derived Peptides Enhance Antimicrobial Activity and Proteolytic Stability