Anion Recognition by Aliphatic Helical Oligoureas

Anion-Dependent Hydrogen-Bond Polarity Switching in Ethylene-bridged Urea Oligomers

The reversible coordination of anions to an N,N'-disubstituted 3,5-bis(trifluoromethyl)phenylurea located at a terminus of a linear chain of ethylene-bridged hydrogen-bonded ureas triggers a cascade of conformational changes. A series of hydrogen-bond polarity reversals propagates along the oligomer, leading to a global switch of its hydrogen-bond directionality. The induced polarity switch, transmitted through four reversible urea groups, results in a change in emission and excitation wavelengths of a fluorophore located at the opposite terminus of the oligomer. The molecule thus behaves as a chemical sensor with a relayed remote spectroscopic response to variations in anion concentration. The polarity switch induced by anion concentration constitutes an artificial communication mechanism for conveying information through oligomeric structures.

Insight into Substrate Recognition by Urea-Based Helical Foldamer Catalysts Using a DFT Global Optimization Approach

Peptides and foldamers have recently gained increasing attention as chiral catalysts to achieve challenging (asymmetric) transformations. We previously reported that short helically folded aliphatic oligoureas in combination with achiral Brønsted bases are effective H-bonding catalysts for C-C bond-forming reactions─i.e., the conjugate addition of 1,3-dicarbonyl pronucleophiles to nitroalkenes─with high reactivity and selectivity and at remarkably low chiral catalyst/substrate molar ratios. This theoretical investigation at the density functional theory level of theory, aims to both analyze how the substrates of the reaction interact with the foldamer catalyst and rationalize a chain-length dependence effect on the catalytic properties. We confirm that the first two ureas are the only H-bond donors available to interact with external molecules. Moreover, each urea site interacts with one of the two reactants allowing a short distance between the two reacting carbons, thus facilitating the conjugated addition. Additionally, it was observed that the molecular recognition and catalyst-substrate interactions are mainly governed by electrostatic interactions but not orbital interactions (see from NBO if this is finally true). On these grounds, an electrostatic potential (ESP) analysis showed an important internal charge separation in the catalyst, the positive ESP region being concentrated around the first two ureas, with its area extending as the number of residues increases.

Urea based foldamers

N,N'-linked oligoureas are a class of enantiopure, sequence-defined peptidomimetic oligomers without amino acids that form well-defined and predictable helical structures akin to the peptide α-helix. Oligourea-based foldamers combine a number of features-such as synthetic accessibility, sequence modularity, and folding fidelity-that bode well for their use in a range of applications from medicinal chemistry to catalysis. Moreover, it was recently recognized that this synthetic helical backbone can be combined with regular peptides to generate helically folded peptide-oligourea hybrids that display additional features in terms of helix mimicry and protein-surface recognition properties. Here we provide detailed protocols for the preparation of requested monomers and for the synthesis and purification of homo-oligoureas and peptide-oligourea hybrids.

Delivery of siRNA by tailored cell-penetrating urea-based foldamers

Cell-penetrating foldamers (CPFs) have recently shown promise as efficient and safe nucleic acid delivery systems. However, the application of CPFs to siRNA transport remains scarce. Here, we report helical CPFs tailored with specific end-groups (pyridylthio- or n-octyl-ureas) as effective molecular systems in combination with helper lipids to intracellularly deliver biologically-relevant siRNA.

Catalytic Foldamers: When the Structure Guides the Function

Enzymes are predominantly proteins able to effectively and selectively catalyze highly complex biochemical reactions in mild reaction conditions. Nevertheless, they are limited to the arsenal of reactions that have emerged during natural evolution in compliance with their intrinsic nature, three-dimensional structures and dynamics. They optimally work in physiological conditions for a limited range of reactions, and thus exhibit a low tolerance for solvent and temperature conditions. The de novo design of synthetic highly stable enzymes able to catalyze a broad range of chemical reactions in variable conditions is a great challenge, which requires the development of programmable and finely tunable artificial tools. Interestingly, over the last two decades, chemists developed protein secondary structure mimics to achieve some desirable features of proteins, which are able to interfere with the biological processes. Such non-natural oligomers, so called foldamers, can adopt highly stable and predictable architectures and have extensively demonstrated their attractiveness for widespread applications in fields from biomedical to material science. Foldamer science was more recently considered to provide original solutions to the de novo design of artificial enzymes. This review covers recent developments related to peptidomimetic foldamers with catalytic properties and the principles that have guided their design.

Peptide Stapling with Anion-π Catalysts

We report design, synthesis and evaluation of a series of naphthalenediimides (NDIs) that are bridged with short peptides. Reminiscent of peptide stapling technologies, the macrocycles are conveniently accessible by a chromogenic nucleophilic aromatic substitution of two bromides in the NDI core with two thiols from cysteine sidechains. The dimension of core-bridged NDIs matches that of one turn of an α helix. NDI-stapled peptides exist as two, often separable atropisomers. Introduction of tertiary amine bases in amino-acid sidechains above the π-acidic NDI surface affords operational anion-π catalysts. According to an enolate chemistry benchmark reaction, anion-π catalysis next to peptides occurs with record chemoselectivity but weak enantioselectivity. Catalytic activity drops with increasing distance of the amine base to the NDI surface, looser homocysteine bridges, mismatched, shortened and elongated α-helix turns, and acyclic peptide controls. Elongation of isolated turns into short α helices significantly increases activity. This increase is consistent with remote control of anion-π catalysis from the α-helix macrodipole.

Helical Anion Foldamers in Solution

Using anions to induce molecular structure is a rapidly growing area of dynamic and switchable supramolecular chemistry. The emphasis of this review is on helical anion foldamers in solution, and many of the beautiful complexes described herein are accentuated by their crystal structures. Anion foldamers are defined as single- or multistrand complexes-often helical-that incorporate one or more anions. The review begins by discussing foldamer structure and nomenclature and follows with discourse on the anions which are employed. Recent advances in functional foldamers that bind a single anion are examined, including: induced chirality, stimuli-responsive dynamics, fluorescence changes, organocatalysis, anion transport, and halogen bonding. The review then inspects multianion foldamers, and this section is organized by the number of strands within the foldamer-from single- to triple-strand foldamers. Finally, the review is punctuated by recent hydrogen- and halogen-bonding triple-strand anion foldamers.

Sugar-derived oxazolone pseudotetrapeptide as γ-turn inducer and anion-selective transporter

The intramolecular cyclization of a C-3-tetrasubstituted furanoid sugar amino acid-derived linear tetrapeptide afforded an oxazolone pseudo-peptide with the formation of an oxazole ring at the C-terminus. A conformational study of the oxazolone pseudo-peptide showed intramolecular C=O···HN(II) hydrogen bonding in a seven-membered ring leading to a γ-turn conformation. This fact was supported by a solution-state NMR and molecular modeling studies. The oxazolone pseudotetrapeptide was found to be a better Cl⁻-selective transporter for which an anion–anion antiport mechanism was established.

Signal transduction in oligoamide foldamers by selective non-covalent binding of chiral phosphates at a urea binding site

The transduction of biological signals depends on the spatial communication of conformational change. We report a synthetic mimic of this signal transduction process in which non-covalent binding induces a change in the position of equilibrium between two rapidly interconverting screw-sense conformers of a synthetic helical polyamide. Selectivity was achieved by incorporating at the N-terminus of the polyamide a urea-based anion recognition site capable of binding chiral phosphate anions. As a result of solvent-dependent binding, an induced conformational change propagates from the binding site through the amide chain, leading to a screw-sense preference detectable in the form of a chemical shift separation between two NMR active ¹³C labels. The remote induction of screw sense preference indicates successful communication of a signal originating solely from non-covalent binding.

Effect of replacing main-chain ureas with thiourea and guanidinium surrogates on the bactericidal activity of membrane active oligourea foldamers

Membrane-active foldamers have recently emerged as potential mimics of antimicrobial peptides (AMPs). Amphiphilic cationic helical N,N'-linked oligoureas are one such class of AMP mimics with activities in vitro against a broad range of bacteria including Bacillus anthracis, a Gram-positive sporulating bacillus and causing agent of anthrax. Here we have used site-selective chemical modifications of the oligourea backbone to gain additional insight into the relationship between structure and function and modulate anthracidal activity. A series of analogues in which urea linkages at selected positions are replaced by thiourea and guanidium surrogates have been prepared on solid support and tested against different bacterial forms of B. anthracis (germinated spores and encapsulated bacilli). Urea→thiourea and urea→guanidinium replacements close to the negative end of the helix dipole led to analogues with increased potency and selectivity for B. anthracis versus mammalian cells.

Foldamers in Medicinal Chemistry

Bio-inspired synthetic backbones leading to foldamers can provide effective biopolymer mimics with new and improved properties in a physiological environment, and in turn could serve as useful tools to study biology and lead to practical applications in the areas of diagnostics or therapeutics. Remarkable progress has been accomplished over the past 20 years with the discovery of many potent bioactive foldamers originating from diverse backbones and targeting a whole spectrum of bio(macro)molecules such as membranes, protein surfaces, and nucleic acids. These current achievements, future opportunities, and key challenges that remain are discussed in this article.

Supramolecular chirality and symmetry breaking of fluoride complexes of achiral foldamers

Fluoride binding of achiral oligoamide foldamers produces polar and chiral crystal structures and induces exceptional symmetry breaking of the bulk sample.