Bio-instructive materials on-demand - combinatorial chemistry of peptoids, foldamers, and beyond

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.

Cyclic Peptoid-Peptide Hybrids as Versatile Molecular Transporters

Addressing intracellular targets is a challenging task that requires potent molecular transporters capable to deliver various cargos. Herein, we report the synthesis of hydrophobic macrocycles composed of both amino acids and peptoid monomers. The cyclic tetramers and hexamers were assembled in a modular approach using solid as well as solution phase techniques. To monitor their intracellular localization, the macrocycles were attached to the fluorophore Rhodamine B. Most molecular transporters were efficiently internalized by HeLa cells and revealed a specific accumulation in mitochondria without the need for cationic charges. The data will serve as a starting point for the design of further cyclic peptoid-peptide hybrids presenting a new class of highly efficient, versatile molecular transporters.

Mitochondrial-targeted penetrating peptide delivery for cancer therapy

INTRODUCTION

Mitochondria are promising targeting organelles for anticancer strategies; however, mitochondria are difficult for antineoplastic drugs to recognize and bind. Mitochondria-penetrating peptides (MPPs) are unique tools to gain access to the cell interior and deliver a bioactive cargo into mitochondria. MPPs have combined or delivered a variety of antitumor cargoes and obviously inhibited the tumor growth in vivo and in vitro. MPPs create new opportunities to develop new treatments for cancer.

AREAS COVERED

We review the target sites of mitochondria and the target-penetration mechanism of MPPs, different strategies, and various additional strategies decorated MPPs for tumor cell mitochondria targeting, the decorating mattes including metabolism molecules, RNA, DNA, and protein, which exploited considered as therapeutic combined with MPPs and target in human cancer treatment.

EXPERT OPINION/COMMENTARY

Therapeutic selectivity that preferentially targets the mitochondrial abnormalities in cancer cells without toxic impact on normal cells still need to be deepen. Moreover, it needs appropriate study designs for a correct evaluation of the target delivery outcome and the degradation rate of the drug in the cell. Generally, it is optimistic that the advances in mitochondrial targeting drug delivery by MPPs plasticity outlined here will ultimately help to the discovery of new approaches for the prevention and treatment of cancers.

Mitochondria-Targeting Peptoids

Mitochondria-specific delivery methods offer a valuable tool for studying mitochondria-related diseases and provide breakthroughs in therapeutic development. Although several small-molecule and peptide-based transporters have been developed, peptoids, proteolysis-resistant peptidomimetics, are a promising alternative to current approaches. We designed a series of amphipathic peptoids and evaluated their cellular uptake and mitochondrial localization. Two peptoids with cyclohexyl residues demonstrated highly efficient cell penetration and mitochondrial localization without significant adverse effects on the cells and mitochondria. These mitochondria-targeting peptoids could facilitate the selective and robust targeted delivery of bioactive compounds, such as drugs, antioxidants, and photosensitizers, with minimal off-target effects.

CuAAC: An Efficient Click Chemistry Reaction on Solid Phase

Click chemistry is an approach that uses efficient and reliable reactions, such as Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC), to bind two molecular building blocks. CuAAC has broad applications in medicinal chemistry and other fields of chemistry. This review describes the general features and applications of CuAAC in solid-phase synthesis (CuAAC-SP), highlighting the suitability of this kind of reaction for peptides, nucleotides, small molecules, supramolecular structures, and polymers, among others. This versatile reaction is expected to become pivotal for meeting future challenges in solid-phase chemistry.

Controlling the width of nanosheets by peptide length in peptoid–peptide biohybrid hydrogels

The width of self-assembling nanosheets could be controlled by the variation of peptide length.

Synthesis of glycotriazololipids and observations on their self-assembly properties

Various carbohydrate-anchored triazole-linked lipids prepared by solvent-free mechanochemical azide-alkyne click reaction, on analysis by TEM, have been found to spontaneously self-assemble in solvents leading to structures of interesting physicochemical attributes. Interestingly, analogous compounds based on different sugars (e.g., d-glucose, and d-galactose, as also d-lactose) assemble in patterns distinctly different from each other thus reiterating the fact that the structure of the sugar as well as that of the lipid are important factors that determine the size and shape of the supramolecular assembly formed. Besides, the molecular self-assembly was also found to be solvent-as well as temperature-dependent.

Functionalized triazolopeptoids--a novel class for mitochondrial targeted delivery

Here we introduce linear 1,4-triazolopeptoids as a novel class of cell penetrating peptidomimetics suitable as organ targeting molecular transporters of bioactive cargo. Repetitive triazole moieties with up to three residues were assembled on solid supports using copper-catalyzed alkyne-azide cycloadditions (CuAAC) in a submonomer approach. Depending on the lipophilicity of their side chain appendages the 1,4-triazolopeptoids showed either endosomal localization or a strong colocalization with the mitochondria of HeLa cells with moderate toxicity. While the basic triazolopeptoids mainly target the neuromast cells in zebrafish embryos, the lipophilic ones colocalize with either cartilage in the jaws and the blood vessel system.

Highly efficient synthesis of polyfluorinated dendrons suitable for click chemistry

A new and efficient access to C_2v-symmetric dendrons with up to 72 magnetically equivalent fluorine atoms is presented. Those dendrons are well suited to act as potential ¹⁹F MRI probes.

Cell-penetrating peptoids: introduction of novel cationic side chains

During the last decade peptoid-based molecular transporters have been broadly applied. They are highly valued for their easy synthesis and their superior stability against enzymatic degradation. The special structure of peptoids generally allows introducing a variety of different side chains. Yet, the cationic side chains of cell-penetrating peptoids displayed solely lysine- or arginine-like structures. Thus this report is intended to extend the spectrum of cationic peptoid side chains. Herein, we present novel functional groups, like polyamines, aza-crown ethers, or triphenylphosphonium ions that are introduced into peptoids for the first time. In addition, the obtained peptoids were tested for their cell-penetrating properties.