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.antibiotics ͉ peptidomimetics ͉ structure-activity studies N atural antimicrobial peptides (AMPs) defend a wide array of organisms against bacterial pathogens and show potential as supplements for or replacements of conventional antibiotics, because few bacteria have evolved resistance to them (1-3). Many AMPs kill bacteria by permeabilization of the cytoplasmic membrane, causing depolarization, leakage, and death (4), whereas others target additional anionic bacterial constituents (e.g., DNA, RNA, or cell wall components) (2, 5). Amphipathic secondary structures in which residues are segregated into hydrophobic and cationic regions ( Fig. 1 A and B) are the hallmark of most AMPs (6). Regardless of their final target of killing, AMPs must interact with the bacterial cytoplasmic membrane, and their amphipathicity is integral to such interactions (1, 2, 7). Additionally, their cationic nature imparts AMPs with some measure of selectivity, because mammalian cell membranes are largely zwitterionic. The precise nature of AMP-membrane interactions remains controversial and actively debated; a variety of mechanisms have been proposed, including the carpet (4), barrel-stave pore (4), toroidal pore (8), and aggregate (9) models. Nevertheless, a considerable number of structure-activity investigations have elucidated how the physicochemical properties of these molecules relate to their biological activities (4,7,(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22).Although AMPs have been actively studied for decades (23-25), they have yet to see widespread clinical use (1). This is due in part to the vulnerability of many peptide therapeutics to rapid in vivo degradation, which dramatically reduces their bioavailability. Nonnatural mimics of AMPs...

The antimicrobial peptide protegrin-1 (PG-1) interacts with membranes in a manner that strongly depends on membrane lipid composition. In this research we use an approach representing the outer layers of bacterial and red blood cell membranes with lipid monolayers and using a combination of insertion assay, epifluorescence microscopy, and surface x-ray scattering to gain a better understanding of antimicrobial peptide's mechanism of action. We find that PG-1 inserts readily into anionic dipalmitoyl-phosphatidylglycerol, palmitoyl-oleoyl-phosphatidylglycerol, and lipid A films, but significantly less so into zwitterionic dipalmitoyl-phosphatidylcholine, palmitoyl-oleoyl-phosphatidylcholine, and dipalmitoyl-phosphatidylethanolamine monolayers under similar experimental conditions. Epifluorescence microscopy shows that the insertion of PG-1 into the lipid layer results in the disordering of lipid packing; this disordering effect is corroborated by grazing incidence x-ray diffraction data. X-ray reflectivity measurements further point to the location of the peptide in the lipid matrix. In a pathologically relevant example we show that PG-1 completely destabilizes monolayer composed of lipid A, the major component in the outer membrane of Gram-negative bacteria, which is likely to be the mechanism by which PG-1 disrupts the outer membrane, thus allowing it to reach the target inner membrane.

Interaction of the human antimicrobial peptide LL-37 with lipid monolayers has been investigated by a range of complementary techniques including pressure-area isotherms, insertion assay, epifluorescence microscopy, and synchrotron x-ray scattering, to analyze its mechanism of action. Lipid monolayers were formed at the air-liquid interface to mimic the surface of the bacterial cell wall and the outer leaflet of erythrocyte cell membrane by using phosphatidylglycerol (DPPG), phosphatidylcholine (DPPC), and phosphatidylethanolamine (DPPE) lipids. LL-37 is found to readily insert into DPPG monolayers, disrupting their structure and thus indicating bactericidal action. In contrast, DPPC and DPPE monolayers remained virtually unaffected by LL-37, demonstrating its nonhemolytic activity and lipid discrimination. Specular x-ray reflectivity data yielded considerable differences in layer thickness and electron-density profile after addition of the peptide to DPPG monolayers, but little change was seen after peptide injection when probing monolayers composed of DPPC and DPPE. Grazing incidence x-ray diffraction demonstrated significant peptide insertion and lateral packing order disruption of the DPPG monolayer by LL-37 insertion. Epifluorescence microscopy data support these findings.

We are grateful to Ada Yonath and her group of the Max-Planck-Insitut fur strukturelle Molekularbiologie, Hamburg for use of laboratory facilities and to the staff of HASYLAB for assistance. We thank Meir Lahav, Isabelle Weissbuch, and Ivan Kuzmenko for discussions.

A study of the interaction of gramicidin A (gA), tert-butyloxycarbonyl-gramicidin (g-BOC), and desformyl gramicidin (g-des) with dioleoyl phosphatidylcholine (DOPC) and DOPC/phosphatidylserine (PS) mixed monolayers on a mercury electrode is reported in this paper. Experiments were carried out in electrolytes KCl (0.1 mol dm(-3)) and Mg(NO3)2 (0.05 mol dm(-3)). The channel-forming properties of the gramicidins were studied by following the reduction of Tl(I) to Tl(Hg). The frequency dependence of the complex impedance of coated electrode surfaces in the presence and absence of the gramicidins was estimated between 65,000 and 0.1 Hz at potentials of -0.4 V versus Ag/AgCl with 3.5 mol dm(-3) KCl. Epifluorescence microscopy was used to qualitatively correlate the interaction of the gramicidin peptides with dipalmitoyl phosphatidylcholine (DPPC) and dipalmitoyl phosphatidylglycerol (DPPG) at the air-water interface. gA was shown to form Tl+ conducting channels in a DOPC monolayer, while g-BOC and g-des did not. In DOPC-30% PS (DOPC-0.3PS) layers, there is a marked increase in channel activity of all three gramicidin derivatives. None of the peptides facilitate the permeability of the DOPC-0.3PS layer to Cd2+. All three peptides interact with the layer as shown by capacitance-potential curves and impedance spectroscopy indicated by penetration of the peptide into the dielectric, an increase in surface "roughness", and an increased significance of low-frequency relaxations. The order of interaction is gA> g-des > g-BOC. The epifluorescence study of DPPC and DPPG layers at the air-water interface shows a selective action of the different gramicidins.

A promising class of potential new antibiotics are the antimicrobial peptides or their synthetic mimics. Herein we assess the effect of the type of cationic side chain (i.e., guanidino vs. amino groups) on the membrane perturbing mechanism of antimicrobial α-peptide–β-peptoid chimeras. Two separate Langmuir monolayers composed of 1,2-dipalmitoyl-sn-glycero-3-phosphatidylglycerol (DPPG) and lipopolysaccharide Kdo2-lipid A were applied to model the outer membranes of Gram-positive and Gram-negative bacteria, respectively. We report the results of the measurements using an array of techniques, including high-resolution synchrotron surface X-ray scattering, epifluorescence microscopy, and in vitro antimicrobial activity to study the molecular mechanisms of peptidomimetic interaction with bacterial membranes. We found guanidino group-containing chimeras to exhibit greater disruptive activity on DPPG monolayers than the amino group-containing analogues. However, this effect was not observed for lipopolysaccharide monolayers where the difference was negligible. Furthermore, the addition of the nitrobenzoxadiazole fluorophore did not reduce the insertion activity of these antimicrobials into both model membrane systems examined, which may be useful for future cellular localization studies.

Oligomers of acylated lysines (OAKs) are synthetic mimics of host defense peptides (HDPs) with promising antimicrobial properties. Here we challenged the OAK concept for its ability to generate both systemically efficient and economically viable lead compounds for fighting multidrug-resistant bacteria. We describe the design and characterization of a miniature OAK composed of only 3 lysyls and 2 acyls (designated C(12(omega7))K-beta(12)) that preferentially targets gram-positive species by a bacteriostatic mode of action. To gain insight into the mechanism of action, we examined the interaction of OAK with various potential targets, including phospholipid bilayers, using surface plasmon resonance, and Langmuir monolayers, using insertion assays, epifluorescence microscopy, and grazing incidence X-ray diffraction, in a complementary manner. Collectively, the data support the notion that C(12(omega7))K-beta(12) damages the plasma-membrane architecture similarly to HDPs, that is, following a near-classic 2-step interaction including high-affinity electrostatic adhesion and a subsequent shallow insertion that was limited to the phospholipid head group region. Notably, preliminary acute toxicity and efficacy studies performed with mouse models of infection have consolidated the potential of OAK for treating bacterial infections, including systemic treatments of methicillin-resistant Staphylococcus aureus. Such simple yet robust chemicals might be useful for various antibacterial applications while circumventing potential adverse effects associated with cytolytic compounds.

When structural flexibility is a plus: A link between structural flexibility of biomimetic antimicrobials and their ability to penetrate into the hydrophobic core and disrupt the integrity of bacterial lipid model membranes has been established using liquid surface X‐ray scattering techniques. Results indicate that the modes of interaction of flexible and conformationally restrained antimicrobials with the bacterial membranes are different (see picture).