Characterization of Antimicrobial Peptides Relating to Shortened RWALP Model Peptides

Abstract

In the face of increasing bacterial drug resistance, small membrane-active peptides offer potentially attractive avenues to alternative antimicrobial agents. The goals of this project are to characterize the lipid interactions and analyze the antimicrobial efficacy of model peptides that are longer than the lactoferrin-derived, surface-acting LfB6 (RRWQWR-NH2), yet shorter than the transmembrane RWALP23 (acetyl-GRALW(LA)6LWLARA-NH2). New-generation RWALP peptides, of general sequence (ac-GRnWm(LA)jLWmRnA-NH2) were designed with varying total lengths (13-15) and numbers of Arg and Trp residues. 2H-alanines were incorporated at several positions to serve as probes for recording solid-state NMR spectra from mechanically aligned samples of the peptides in bilayers of DLPC, DMPC or DOPC. Labeled RWALP13 (j=3, n=1, m=1) exhibits partial water solubility, no antimicrobial activity, and 2H-NMR spectra characteristic of isotropic motion, even in the presence of lipid bilayers. Circular dichroism spectra of RWALP13 suggest partial α-helical character in water that is enhanced when lipids are present. The 2H-NMR spectra indicate that the longer 14- or 15-residue peptides are aligned to varying degrees in the different lipid bilayer membranes. Antimicrobial assays reveal that peptides with four arginines have higher activity than those with only two arginines. Interestingly, within the 4-Arg category, RRWALP15 (j=3, n=2, m=1) shows higher activity (MIC of 6.25 μ1/4g/ml) against E. coli than does RRWWALP15 (j=2, n=2, m=2; MIC of 25 μ1/4g/ml). The 2H-NMR spectra of RRWALP15 suggest significant alignment of the peptide with respect to lipid bilayer membranes. The combined antimicrobial and spectral features make RRWALP15 an especially good candidate for further analysis.