Inducing a Helical Kink in a Model Peptide Antibiotic Reduces Peptide-Membrane Interaction Favorability in Vesicles

Abstract

Peptide antibiotics offer promise as robust alternatives to conventional small molecule antibiotics. While the mechanism of function of peptide antibiotics remains an active area of research, it is generally understood that these peptide interact with and disrupt cell membranes. We are studying the interactions of model peptide antibiotics with large unilamellar vesicles (LUVs) in order to understand how helical shape influences the peptide-membrane interaction. Our model antibiotic peptides are octamers composed primarily of the hydrophobic dialkylated amino acid Aib (α -aminoisobutyric acid), which imparts a strong 310-helical bias due to steric hindrance at the α-carbon. Positively charged lysine residues are placed in either adjacent positions (KK45) or separated by one helical turn (KK36) in the sequence. Using NMR spectroscopy, we are measuring the binding constant for these two peptides with zwitterionic (DMPC) and anionic (DMPG) LUVs by measuring the disappearance of the amide 1H NMR signal as LUVs are added to peptide solution. The large size of the peptide-LUV complex results in a long rotational correlation time, and thus dramatic broadening (disappearance) of the peptide signal. Preliminary experiments indicate that neither peptide binds appreciably to DMPC LUVs, while KK36 has a greater affinity for DMPG LUVs than KK45 (KD, KK36=0.34 +/- 0.09 mM versus KD,KK45=1.1 +/- 0.1 mM). Circular dichroism experiments indicate that KK45 remains largely 310-helical in water and when bound to DMPG LUVs, while KK36 undergoes a transition from largely [α]-helical in water to largely 310-helical when bound to DMPG LUVs. These results together suggest that both charge and structure are important factors in peptide-lipid association.