Comparison of Cyclic and Linear Antimicrobial Peptides*

The problem of world-wide bacterial resistance to conventional antibiotics motivates the study of antimicrobial peptides (AMPs) as alternative antibiotics.  Our work compares two linear forms of AMPs (LE-53 (12-mer) and LE-55 (16-mer)) with their cyclic versions (CE-03 and CE-05).  These synthetic AMPs are designed de novo in the Deslouches lab at the University of Pittsburgh, where Minimum Inhibitory Concentration (MIC) and toxicity tests are performed.  The Tristram-Nagle lab carries out circular dichroism (CD) to determine the AMP secondary structure, and X-ray diffuse scattering (XDS) to determine elastic properties, chain order and membrane structure.  For bacterial killing activity, CE-03 is slightly more effective than LE-53, but CE-05 is dramatically more effective than LE-55. All four are non-toxic to both red and white blood cells.  We find that none of these AMPs is alpha-helical, as they were designed not to be helical. Rather they all adopt random coil and beta-sheet structures in lipid model membranes (LMMs) mimicking Gram-postivie G(+) and Gram-negative G(-) membranes.  For elastic properties, all four AMPs cause a slight softening (lowering of K_C) with increasing concentration.  For chain order, all four AMPs cause some chain disorder in G(+) and G(-) LMMs, but have a smaller disordering effect in Euk33 (eukaryotic) LMMs.  Membrane thickness decreases for all four AMPs in G(+) and G(-) LMMs, but increases in Euk33 LMMs, suggesting that membrane thinning is associated with destabilization, and membrane thickening with stabilization (no toxicity).  Small-angle x-ray scattering (SAXS) informs that all except LE-55 are fusogenic to G(+) and G(-) ULVs, but none is fusogenic to Euk33 ULVs.  While all four AMPs locate in the headgroup region in G(+) and G(-) LMMs, they all locate in the hydrocarbon region in Euk33 LMMs.  These biophysical studies give insights into the molecular mechanisms of antimicrobial activity.