Comparing Peptide-Lipid Interactions and Antimicrobial Activities of Peptides with Similar “Core” Lengths But Variable Arginine and Tryptophan Residues

Abstract

Antimicrobial peptides offer a potential alternative therapy option for the increasing problem of microbial drug resistance. To address this issue, “hybrid” peptides with total lengths intermediate between the surface-acting antimicrobial lactoferrin-derived LfB6 (RRWQWR-NH2) and the inert model transmembrane α-helical peptide RWALP23 (acetyl-GRALW(LA)6LWLARA-NH2) have been designed with varying numbers of arginines, tryptophans and net positive charge, but with the same “core” length of five residues. The lipid interactions of the hybrid LfB-RWALP peptides having between 9 and 15 residues were investigated by circular dichroism, tryptophan fluorescence, and 31P and 2H solid-state NMR spectroscopy, and their antimicrobial activities assayed. Peptide . . . . . . . . . Sequence RRWWALP15 . . . . acetyl-GRRWWLALALWWRRA15-NH2 RRWWALP13 . . . . acetyl-RRWWLALALWWRR13-NH2 RWALP11 . . . . . . . acetyl-GRWLALALWRA11-NH2 RWALP9 . . . . . . . . RWLALALWR9-NH2 Our results indicate that despite having the same core length, the peptides exhibit different conformations, different interactions with lipids, and different antimicrobial activities. While the 13-mer is α-helical in neutral and anionic lipid mixtures, the conformations of the shorter 11-mer and 9-mer vary considerably depending on the lipid environment. Fluorescence spectroscopy suggests that the tryptophan residues of all peptides are located at the membrane-water interface, with slightly varying depths of insertion into the lipid bilayer. Despite small perturbations to the phospholipid head groups, solid-state 31P NMR spectra indicate that the lipids remain primarily in a bilayer phase. 2H NMR spectra reveal that the two shorter peptides aggregate, while the two longer (and more helical) peptides align in neutral and anionic lipid mixtures. Although all four peptides exhibit similar antimicrobial activities, some variation is observed against gram negative and gram positive bacterial strains.