Understanding membrane interactions and cell-wall permeation of Gram-negative bacteria is of great importance, owing to increasing bacterial resistance to existing drugs and therapeutic treatments. Here we use biomimetic lipid vesicles to analyse membrane association and penetration by synthetic derivatives of polymyxin B (PMB), a potent naturally occurring antibacterial cyclic peptide. The PMB analogues studied were PMB nonapeptide (PMBN), in which the hydrophobic alkyl residue was cleaved, PMBN diastereomer containing d-instead of l-amino acids within the cyclic ring (dPMBN) and PMBN where the hydrophobic alkyl chain was replaced with an Ala6 repeat (Ala6-PMBN). Peptide binding measurements, colorimetric transitions induced within the vesicles, fluorescence quenching experiments and ESR spectroscopy were applied to investigate the structural parameters underlying the different membrane-permeation profiles and biological activities of the analogues. The experiments point to the role of negatively charged lipids in membrane binding and confirm the prominence of lipopolisaccharide (LPS) in promoting membrane association and penetration by the peptides. Examination of the lipid interactions of the PMB derivatives shows that the cyclic moiety of PMB is not only implicated in lipid attachment and LPS binding, but also affects penetration into the inner bilayer core. The addition of the Ala6 peptide moiety, however, does not significantly promote peptide insertion into the hydrophobic lipid environment. The data also indicate that the extent of penetration into the lipid bilayer is not related to the overall affinity of the peptides to the membrane.
Abbreviations used: Ala6-PMBN, polymyxin B nonapeptide (PMBN) where the hydrophobic alkyl chain is replaced with an Ala6 repeat; DMPC, dimyristoyl phosphatidylcholine; DMPG, dimyristoyl phosphatidylglycerol; dPMBN, PMBN diastereomer containing d-instead of l-amino acids within the cyclic ring; LPS, lipopolysaccharide; NBD-PE, N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)-1,2-dihexadecanoyl-sn-glycero-3-phosphoethanolamine; OM, outer membrane; PDA, polydiacetylene; PMB, polymyxin B; THF, tetrahydrofuran; 5-DS, 5-doxyl stearic acid; Uv-vis, UV–visible.