Cyclization of Two Antimicrobial Peptides Improves Their Activity

One promising strategy to combat worldwide antimicrobial resistance involves using cyclic peptides as antibacterial agents. Cyclization of peptides can confer several advantages, including enhanced stability to proteolysis, decreased toxicity and increased bactericidal efficacy. This paper examines two cyclic peptides CE-03 (12 AAs) and CE-05 (16 AAs) and evaluates their effectiveness in combating bacterial infections, their stability and toxicity. We compare them to their linear versions. Circular dichroism (CD) reveals that CE-03 and CE-05 both adopt random coil and β-sheet structures in lipid model membranes (LMMs) mimicking G(-) and G(+) bacteria, where they are both bactericidal. Using X-ray diffuse scattering (XDS), their effects on lipid model membranes show a deep penetration of both peptides into eukaryotic LMMs where they are nontoxic, while a headgroup location in bacterial LMMs correlates with bacterial killing. Neutron reflectometry (NR) confirms the AMP locations determined using XDS. Further, solution small-angle X-ray scattering demonstrates that both peptides induce vesicle fusion in bacterial LMMs without affecting eukaryotic LMMs. Proteolytic degradation studies show that both CE-05 and CE-03 do not lose activity when incubated with the elastase enzyme, while the helical E2-35 AMP becomes inactive upon proteolysis.