Peptoids that mimic the structure, function, and mechanism of helical antimicrobial peptides

Abstract

Antimicrobial peptides (AMPs) and their mimics are emerging as promising antibiotic agents. We present a library of "ampetoids" (antimicrobial peptoid oligomers) with helical structures and biomimetic sequences, several members of which have low-micromolar antimicrobial activities, similar to cationic AMPs like pexiganan. Broad-spectrum activity against six clinically relevant BSL2 pathogens is also shown. This comprehensive structure-activity relationship study, including circular dichroism spectroscopy, minimum inhibitory concentration assays, hemolysis and mammalian cell toxicity studies, and specular x-ray reflectivity measurements shows that the in vitro activities of ampetoids are strikingly similar to those of AMPs themselves, suggesting a strong mechanistic analogy. The ampetoids' antibacterial activity, coupled with their low cytotoxicity against mammalian cells, make them a promising class of antimicrobials for biomedical applications. Peptoids are biostable, with a protease-resistant N-substituted glycine backbone, and their sequences are highly tunable, because an extensive diversity of side chains can be incorporated via facile solid-phase synthesis. Our findings add to the growing evidence that nonnatural foldamers will emerge as an important class of therapeutics.

Fig. 1.

NMR structure of magainin-2 in DPPC micelles [Protein Data Bank (PDB) ID Code 2MAG]. (A and B) Parallel (A) and perpendicular (B) to its helical axis. (C and D) Similar views of a model structure of ampetoid 1, using backbone φ and ψ angles from a published NMR structure of a peptoid helix.(37, 41) Because NLys is achiral, the structure of 1 is expected to be significantly dynamic in solution. Residues are color coded: cationic, blue; hydrophobic, orange; all others, gray.

Fig. 2.

Peptoid monomer side-chain structures, with full and shorthand names.

Fig. 3.

Cytotoxicity data for selected peptoids and comparator peptides against A549 lung epithelial cells.

Fig. 4.

CD spectra of variants of 1 (A) and variants of 2 (B) in 10 mM Tris buffer (pH 7.4).

Fig. 5.

CD spectra of variants of 1 (A) and variants of 2 (B) in 5 mM POPC/cholesterol (1:1) SUVs suspended in 10 mM Tris buffer (pH 7.4).

Fig. 6.

CD spectra of variants of 1 (A) and variants of 2 (B) in 5 mM POPE/POPG (7:3) SUVs suspended in 10 mM aqueous Tris buffer (pH 7.4).

Fig. 7.

X-ray reflectivity data and corresponding fit for DPPG monolayer before (circles; Inset Upper) and after (squares; Inset Lower) peptoid 1 was injected into the subphase.