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Background: Antimicrobial peptides (AMPs) are short, cationic, amphipathic molecules that have gained tremendous popularity as alternatives to traditional antibiotics due to their lower propensity to develop bacterial resistance. However, the clinical developability of AMPs remains impeded due to shortcomings such as proteolytic instability and poor penetration leading to low bioavailability., Aims: To improve the access of AMPs to cells and subsequent bacteria killing, we evaluated the cell-penetrating and antimicrobial properties of three novel libraries of synthetic peptoids using Minimum Inhibitory Concentration, killing efficacy and membrane permeabilization assays against mycobacteria and Staphylococcus aureus . In addition, we investigated cell selectivity using mammalian cells to assess peptoid toxicity., Results: We showed that short tetrameric Rhodamine B-labeled peptoids composed of a balance of aromatic and lipophilic residues have potent selective antimicrobial activity against a range of microorganisms. The most potent candidates were active against drug-resistant S. aureus isolates as well as mycobacterial strains, with cell penetrating capabilities reported in HeLa and RAW 264.7 macrophage cells., Conclusions: These data suggest that peptoids with novel dual functionalities may potentially be an interesting class of therapeutics and/or molecular delivery agents for anti-infective purposes., Competing Interests: The authors declare no conflict of interest., (© 2022 The Author(s). Published by IMR Press.)

Combinatorial chemistry allows for the rapid synthesis of large compound libraries for high throughput screenings in biology, medicinal chemistry, or materials science. Especially compounds from a highly modular design are interesting for the proper investigation of structure-to-activity relationships. Permutations of building blocks result in many similar but unique compounds. The influence of certain structural features on the entire structure can then be monitored and serve as a starting point for the rational design of potent molecules for various applications. Peptoids, a highly diverse class of bioinspired oligomers, suit perfectly for combinatorial chemistry. Their straightforward synthesis on a solid support using repetitive reaction steps ensures easy handling and high throughput. Applying this modular approach, peptoids are readily accessible, and their interchangeable side-chains allow for various structures. Thus, peptoids can easily be tuned in their solubility, their spatial structure, and, consequently, their applicability in various fields of research. Since their discovery, peptoids have been applied as antimicrobial agents, artificial membranes, molecular transporters, and much more. Studying their three-dimensional structure, various foldamers with fascinating, unique properties were discovered. This non-comprehensive review will state the most interesting discoveries made over the past years and arouse curiosity about what may come.

Controlled delivery of molecules interfering specifically with target activities in a cell of interest can be a powerful tool for experimental manipulation, because it can be administered at a defined time point and does not require genetic transformation, which in some systems is difficult and time consuming. Peptides as versatile tools that can be tailored for binding numerous binding partners, are of special interest. However, their passage through membranes, their intracellular targeting, and their sensitivity to proteases is limiting. The use of peptoids, where cationic amino-acid side chains are linked to nitrogen (rather than to carbon) of the peptide bond, can circumvent these limitations, because they are not cleavable by proteases. In the current work, we provide a proof-of-concept that such Trojan Peptoids, the plant PeptoQ, can be used to target a functional cargo (i.e. a rhodamine-labelled peptoid and a coenzyme Q10 derivative) into mitochondria of tobacco BY-2 cells as experimental model. We show that the uptake is specific for mitochondria, rapid, dose-dependent, and requires clathrin-mediated endocytosis, as well as actin filaments, while microtubules seem to be dispensable. Viability of the treated cells is not affected, and they show better survival under salt stress, a condition that perturbs oxidative homeostasis in mitochondria. In congruence with improved homeostasis, we observe that the salt induced accumulation of superoxide is mitigated and even inverted by pretreatment with PeptoQ. Using double labelling with appropriate fluorescent markers, we show that targeting of this Trojan Peptoid to the mitochondria is not based on a passage through the plasma membrane (as thought hitherto), but on import via endocytotic vesicles and subsequent accumulation in the mitochondrial intermembrane space, from where it can enter the matrix, e.g. when the permeability of the inner membrane is increased under salt stress.

Here, the combinatorial synthesis of molecule arrays via a laser-assisted process is reported. Laser-transferred polymer nanolayers with embedded monomers, activators, or bases can be reliably stacked on top of each other, spot-by-spot, to synthesize molecule arrays. These various chemicals in the nanometer-thin layers are mixed by heat or solvent vapor, inducing coupling reactions. As an example, peptoid arrays with a density of 10 000 spots per cm 2 with the sub-monomer or monomer method are generated. Moreover, successful reactions spot-by-spot are verified by laser-transferring MALDI-matrix (Matrix-assisted laser desorption/ionization) followed by MALDI mass spectrometry imaging., (© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Cell-penetrating peptoids are an important class of peptidomimetics, which can replace highly biodegradable cell penetrating peptides for enhanced drug delivery. Typically, they contain positively charged amino side chains which are synthesized via their protected analogues. To avoid the use of amine protecting groups a Click-chemistry based modular synthesis of novel hydrophilic as well as amphiphilic cell penetrating peptoids was developed to generate novel structures for drug delivery in cells.

N-substituted glycine oligomers or peptoids with charged side chains are a novel class of cell penetrating peptide mimetics and have been shown to serve as drug delivery agents. Here, we investigated by NMR spectroscopy and quantum chemical calculations whether a Rhodamine B labelled peptoid [RhoB(Spiro)-Ahx]-[But](6A)NH(2) with lysine-like side chains adopts structural motifs similar to regular peptides. Due to a low chemical shift dispersion, high resolution structure determination with conventional NMR-derived distance restraints and J-couplings was not possible. Instead, a combined assignment and structure refinement strategy using the QM/MM force field COSMOS-NMR was developed to interpret the highly ambiguous chemical shift and distance constraints and obtain a medium resolution three-dimensional structural model. This allowed us to select for the all cis-amide conformation of the peptide with a pseudo-helical arrangement of extended side chains as a faithful representative structure of [RhoB(Spiro)-Ahx]-[But](6A)NH(2). We tested the biological activity of the peptoid by live-cell imaging, which showed that the cellular uptake of the peptoid was comparable to conventional cell-penetrating peptides.

Fluorescently-labeled biomolecules are often utilized in biochemical or cellular experiments without further detailed spectroscopical characterization. This report is intended to narrow this gap and therefore presents the photophysical investigation of a library of 17 fluorescently-labeled molecules, namely peptoid transporters. First, one peptoid structure is labeled with seven different fluorophores and the spectroscopical properties are examined. Absorption and fluorescence maxima are almost identical for free dyes and conjugated dyes, suggesting free choice of a spectrally suitable fluorophore for different applications. Otherwise, extinction coefficients and quantum yields, and therefore the brightness of all seven dyes are strongly influenced. For the fluorophores, e.g. rhodamine B, the extent of this influence depends on the peptoid itself. This is shown by comparing different structures in the second part of this report. Especially the side chain functionalities influence the brightness. And finally, peptoids having two identical fluorescent labels are presented, which show decreased quantum yields. Possible reasons for the observed photophysical properties are discussed., (Copyright © 2011 Elsevier Masson SAS. All rights reserved.)

The fluorophore rhodamine B is often used in biological assays. It is inexpensive, robust under a variety of reaction conditions, can be covalently linked to bioactive molecules, and has suitable spectral properties in terms of absorption and fluorescence wavelength. Nonetheless, there are some drawbacks: it can readily form a spirolactam compound, which is nonfluorescent, and therefore may not be the dye of choice for all fluorescence microscopy applications. Herein this spirolactam formation was observed by purifying such a labeled peptoid with high performance liquid chromatography (HPLC) and monitored in detail by making a series of analytical HPLC runs over time. Additionally, a small library of eight peptoids with rhodamine B as label was synthesized. Analysis of the absorption properties of these molecules demonstrated that the problem of fluorescence loss can be overcome by coupling secondary amines with rhodamine B.

For many therapeutic applications, it has become more and more important to find synthetic compounds that have the ability to transport a variety of drugs and cargo molecules into cells and tissues. Like arginine-rich cell-penetrating peptides (CPPs), it is already known that peptide mimetics such as beta-peptides and peptoids can also express a transport function. In this study, ten fluorophore-labeled chiral and achiral peptoids with different backbone lengths and side chains as well as three peptoids coupled to a therapeutically active porphyrin moiety were prepared using a highly modular solid-phase synthesis (SPP) approach. To compare the structural determinants with the cellular uptake efficiency, all peptoids were analyzed by live cell imaging. All cells show an even vesicular distribution of the internalized peptoids, also revealing that a vesicular escape into the cytosol was stronger for peptoids with longer backbones. Moreover, the uptake efficiency correlated with both the incubation time and the given concentration. Toxicology tests and uptake experiments with porphyrin-coupled peptoids indicate their suitability for application as robust and readily available drug delivery systems or intracellular probes.