A rationally designed metal-binding helical peptoid for selective recognition processes

A Peptoid-Chelator Selective to Cu2+ That Can Extract Copper from Metallothionein-2 and Lead to the Production of ROS

Copper is an essential metal ion that is involved in critical cellular processes, but which can also exhibit toxic effects through its ability to catalyze reactive oxygen species (ROS) formation. Dysregulation of copper homeostasis has been implicated in the progression of several diseases, including cancer. A novel therapeutic approach, extensively studied in recent years, is to capitalize on the increased copper uptake and dependency exhibited by cancer cells and to promote copper-associated ROS production within the tumor microenvironment, leading to the apoptosis of cancer cells. Such an effect can be achieved by selectively chelating copper from copper-bearing metalloproteins in cancer cells, thereby forming a copper-chelator complex that produces ROS and, through this, induces oxidative stress and initiates apoptosis. Herein, we describe a peptoid chelator, TB, that is highly suitable to carry this task. Peptoids are N-substituted glycine oligomers that can be efficiently synthesized on a solid support and are also biocompatible; thus, they are considered promising drug candidates. We show, by rigorous spectroscopic techniques, that TB is not only selective for Cu(II) ions, but can also effectively extract copper from metallothionein-2, and the formed complex CuTB can promote ROS production. Our findings present a promising first example for the future development of peptoid-based chelators for applications in anti-cancer chelation therapy, highlighting the potential for the prospect of peptoid chelators as therapeutics.

The First Cu(I)-Peptoid Complex: Enabling Metal Ion Stability and Selectivity via Backbone Helicity

Stabilization of Cu(I) is ubiquitous within native copper proteins. Understanding how to stabilize Cu(I) within synthetic biomimetic systems is therefore desired towards biological applications. Peptoids are an important class of peptodomimetics, that can bind metal ions and stabilize them in their high oxidation state. Thus, to date, they were not used for Cu(I) binding. Here we show how the helical peptoid hexamer, having two 2,2'-bipyridine (Bipy) groups that face the same side of the helix, forms the intramolecular air stable Cu(I) complex. Further study of the binding site by rigorous spectroscopic techniques suggests that Cu(I) is tetracoordinated, binding to only three N atoms from the Bipy ligands and to the N-terminus of the peptoid's backbone. A set of control peptoids and experiments indicates that the Cu(I) stability and selectivity are dictated by the intramolecular binding, forced by the helicity of the peptoid, which can be defined as the second coordination sphere of the metal center.

Amide bond hydrolysis of peptoids

Incorporating a chiral non-coordinating substitution at the N-terminal end within peptoids facilitates regio-selective amide bond hydrolysis mediated by a transition metal ion and/or an acidic buffer as evident by X-ray crystallographic analysis, supported by ESI-MS. This opens up a new direction for peptidomimetic compounds towards future application in chemistry, biology and medicine.

Roles of Metal Ions in Foldamers and Other Conformationally Flexible Supramolecular Systems

Conformational control is a key prerequisite for much molecular function. As chemists seek to create complex molecules that have applications beyond the academic laboratory, correct spatial positioning is critical. This is particularly true of flexible systems. Conformationally flexible molecules show potential because they resemble in many cases naturally occurring analogues such as the secondary structures found in proteins and peptides such as α-helices and β-sheets. One of the ways in which conformation can be controlled in these molecules is through interaction with or coordination to metal ions. This review explores how secondary structure (i.e., controlled local conformation) in foldamers and other conformationally flexible systems can be enforced or modified through coordination to metal ions. We hope to provide examples that illustrate the power of metal ions to influence this structure toward multiple different outcomes.

A Water-Soluble Peptoid Chelator that Can Remove Cu2+ from Amyloid-β Peptides and Stop the Formation of Reactive Oxygen Species Associated with Alzheimer's Disease

Cu bound to amyloid-β (Aβ) peptides can act as a catalyst for the formation of reactive oxygen species (ROS), leading to neuropathologic degradation associated with Alzheimer's disease (AD). An excellent therapeutic approach is to use a chelator that can selectively remove Cu from Cu-Aβ. This chelator should compete with Zn²⁺ ions (Zn) that are present in the synaptic cleft while forming a nontoxic Cu complex. Herein we describe P3, a water-soluble peptidomimetic chelator that selectively removes Cu²⁺ from Cu-Aβ in the presence of Zn and prevent the formation of ROS even in a reductive environment. We demonstrate, based on extensive spectroscopic analysis, that although P3 extracts Zn from Cu,Zn-Aβ faster than it removes Cu, the formed Zn complexes are kinetic products that further dissociate, while CuP3 is formed as an exclusive stable thermodynamic product. Our unique findings, combined with the bioavailability of peptoids, make P3 an excellent drug candidate in the context of AD.

A Di-Copper-Peptoid in a Noninnocent Borate Buffer as a Fast Electrocatalyst for Homogeneous Water Oxidation with Low Overpotential

Water electrolysis is a promising approach toward low-cost renewable fuels; however, the high overpotential and slow kinetics limit its applicability. Studies suggest that either dinuclear copper (Cu) centers or the use of borate buffer can lead to efficient catalysis. We previously demonstrated the ability of peptoids-N-substituted glycine oligomers-to stabilize high-oxidation-state metal ions and to form self-assembled di-copper-peptoid complexes. Capitalizing on these features herein we report on a unique Cu-peptoid duplex, Cu₂(BEE)₂, that is a fast and stable homogeneous electrocatalyst for water oxidation in borate buffer at pH 9.35, with low overpotential and a high turnover frequency of 129 s^-1 (peak current measurements) or 5503 s^-1 (FOWA); both are the highest reported for Cu-based water electrocatalysts to date. BEE is a peptoid trimer having one 2,2'-bipyridine ligand and two ethanolic groups, easily synthesized on solid support. Cu₂(BEE)₂ was characterized by single-crystal X-ray diffraction and various spectroscopic and electrochemical techniques, demonstrating its ability to maintain stable in four cycles of controlled potential electrolysis, leading to a high overall turnover number of 51.4 in a total of 2 h. Interestingly, the catalytic activity of control complexes having only one ethanolic side chain is 2 orders of magnitude lower than that of Cu₂(BEE)₂. On the basis of this comparison and on mechanistic studies, we propose that the ethanolic side chains and the borate buffer have significant roles in the high stability and catalytic activity of Cu₂(BEE)₂; the -OH groups facilitate protons transfer, while the borate species enables oxygen transfer toward O-O bond formation.

Sequence-function relationship within water-soluble Peptoid Chelators for Cu2

Chelation of Cu²⁺ by synthetic molecules is an emerging therapeutic approach for treating several illnesses in human body such as Wilson disease, cancer and more. Among synthetic metal chelators, those based on peptoids - N-substituted glycine oligomers - are advantageous due to their structural similarity to peptides, ease of synthesis on solid support and versatile controlled sequences. Tuning peptoid sequences, via systematically changing at least one side chain, can facilitate and control their function. Along these lines, this work aims to explore the role of the non-coordinating side chain within peptoid chelators in order to understand the factors that control the selectivity of these chelators to Cu²⁺ in water medium. To this aim, a set of peptoid trimers having a pyridine group at the acetylated N-terminal, a 2,2'-bipyridine group at the second position and a non-coordinating group at the C-terminus, where the latter is systematically varied between aromatic, aliphatic, chiral or non-chiral, were investigated as selective chelators for Cu²⁺. The effect of the position of the non-coordinating group on the selectivity of the peptoid to Cu²⁺ was also tested. Based on extensive spectroscopic data, we found that the choice of the non-coordinating group along with its position dramatically influences the selectivity of the peptoids to Cu²⁺. We showed that peptoids having bulky chiral groups at the C-terminus enable high selectivity to Cu²⁺. We further demonstrated the ability of one of the selective chelators to remove Cu²⁺ from the natural copper binding protein metallothionein in HEPES (4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid) buffer medium.

Peptidomimetics - An infinite reservoir of metal binding motifs in metabolically stable and biologically active molecules

The involvement of metal ions in interactions with therapeutic peptides is inevitable. They are one of the factors able to fine-tune the biological properties of antimicrobial peptides, a promising group of drugs with one large drawback - a problematic metabolic stability. Appropriately chosen, proteolytically stable peptidomimetics seem to be a reasonable solution of the problem, and the use of D-, β-, γ-amino acids, unnatural amino acids, azapeptides, peptoids, cyclopeptides and dehydropeptides is an infinite reservoir of metal binding motifs in metabolically stable, well-designed, biologically active molecules. Below, their specific structural features, metal-chelating abilities and antimicrobial potential are discussed.

A unique Co(iii)-peptoid as a fast electrocatalyst for homogeneous water oxidation with low overpotential

A peptoid trimer incorporating terpyridine and ethanol forms an intermolecular cobalt(iii) complex, which performs as a soluble electrocatalyst for water oxidation with a minimal overpotential of 350 mV and a high turnover frequency of 108 s⁻¹.

Metal-Binding Foldamers

Sequence-defined oligomeric molecules with discrete folding propensities, termed foldamers, are a versatile source of agents with tailored structure and function. An inspiration for the development of the foldamer paradigm are natural biomacromolecules, the sequence-encoded folding of which is the basis of life. Metal ions and clusters are common features in proteins, where the role of metal varies from supporting structure to enabling function. The ubiquity of metals in natural systems suggests promise for metals in the context of folded artificial backbones. In this Minireview, we highlight efforts to realize this potential through a survey of published work on the design, synthesis, and characterization of metal-binding foldamers.

A rationally designed peptoid for the selective chelation of Zn2+ over Cu2

The selective removal of Zn²⁺ from proteins by using a synthetic chelator is a promising therapeutic approach for the treatment of various diseases including cancer. Although the chelation of Zn²⁺ is well known, its removal from a protein in the presence of potential competing biologically relevant ions such as Cu²⁺ is hardly explored. Herein we present a peptoid - N-substituted glycine trimer - incorporating a picolyl group at the N-terminus, a non-coordinating but structurally directing benzyl group at the C-terminus and a 2,2':6',2''-terpyridine group in the second position, that selectively binds Zn²⁺ ions in the presence of excess Cu²⁺ ions in water. We further demonstrate that this chelator can selectively bind Zn²⁺ from a pool of excess biologically relevant and competitive ions (Cu²⁺, Fe³⁺, Ca²⁺, Mg²⁺, Na⁺, and K⁺) in a simulated body fluid (SBF), and also its ability to remove Zn²⁺ from a natural zinc protein domain (PYKCPECGKSFSQKSDLVKHQRTHTG) in a SBF.

Heteroleptic Coordination Environments in Metal-Mediated DNA G-Quadruplexes

The presence of metal centers with often highly conserved coordination environments is crucial for roughly half of all proteins, having structural, regulatory, or enzymatic function. To understand and mimic the function of metallo-enzymes, bioinorganic chemists pursue the challenge of synthesizing model compounds with well-defined, often heteroleptic metal sites. Recently, we reported the design of tailored homoleptic coordination environments for various transition metal cations based on unimolecular DNA G-quadruplex structures, templating the regioselective positioning of imidazole ligandosides L^I. Here, we expand this modular system to more complex, heteroleptic coordination environments by combining L^I with a new benzoate ligandoside L^B within the same oligonucleotide. The modifications still allow the correct folding of parallel tetramolecular and antiparallel unimolecular G-quadruplexes. Interestingly, the incorporation of L^B results in strong destabilization expressed in lower thermal denaturation temperatures T_m. While no transition metal cations could be bound by G-quadruplexes containing only L^B, heteroleptic derivatives containing both L^I and L^B were found to complex Cu^II, Ni^II, and Zn^II. Especially in case of Cu^II we found strong stabilizations of up to ΔT_m = +34°C. The here shown system represents an important step toward the design of more complex coordination environments inside DNA scaffolds, promising to culminate in the preparation of functional metallo-DNAzymes.

Peptide science: A "rule model" for new generations of peptidomimetics

Peptides have been heavily investigated for their biocompatible and bioactive properties. Though a wide array of functionalities can be introduced by varying the amino acid sequence or by structural constraints, properties such as proteolytic stability, catalytic activity, and phase behavior in solution are difficult or impossible to impart upon naturally occurring α-L-peptides. To this end, sequence-controlled peptidomimetics exhibit new folds, morphologies, and chemical modifications that create new structures and functions. The study of these new classes of polymers, especially α-peptoids, has been highly influenced by the analysis, computational, and design techniques developed for peptides. This review examines techniques to determine primary, secondary, and tertiary structure of peptides, and how they have been adapted to investigate peptoid structure. Computational models developed for peptides have been modified to predict the morphologies of peptoids and have increased in accuracy in recent years. The combination of in vitro and in silico techniques have led to secondary and tertiary structure design principles that mirror those for peptides. We then examine several important developments in peptoid applications inspired by peptides such as pharmaceuticals, catalysis, and protein-binding. A brief survey of alternative backbone structures and research investigating these peptidomimetics shows how the advancement of peptide and peptoid science has influenced the growth of numerous fields of study. As peptide, peptoid, and other peptidomimetic studies continue to advance, we will expect to see higher throughput structural analyses, greater computational accuracy and functionality, and wider application space that can improve human health, solve environmental challenges, and meet industrial needs. STATEMENT OF SIGNIFICANCE: Many historical, chemical, and functional relations draw a thread connecting peptides to their recent cognates, the "peptidomimetics". This review presents a comprehensive survey of this field by highlighting the width and relevance of these familial connections. In the first section, we examine the experimental and computational techniques originally developed for peptides and their morphing into a broader analytical and predictive toolbox. The second section presents an excursus of the structures and properties of prominent peptidomimetics, and how the expansion of the chemical and structural diversity has returned new exciting properties. The third section presents an overview of technological applications and new families of peptidomimetics. As the field grows, new compounds emerge with clear potential in medicine and advanced manufacturing.

Strengthening Peptoid Helicity through Sequence Site-Specific Positioning of Amide cis-Inducing NtBu Monomers

The synthesis of biomimetic helical secondary structures is sought after for the construction of innovative nanomaterials and applications in medicinal chemistry such as the development of protein-protein interaction modulators. Peptoids, a sequence-defined family of oligomers, enable a peptidomimetic strategy, especially considering the easily accessible monomer diversity and peptoid helical folding propensity. However, cis-trans isomerization of the backbone tertiary amides may impair the peptoid's adoption of stable secondary structures, notably the all-cis polyproline I-like helical conformation. Here, we show that cis-inducing NtBu achiral monomers strategically positioned within chiral sequences may reinforce the degree of peptoid helicity, although with a reduced content of chiral side chains. The design principles presented here will undoubtedly help achieve more conformationally stable helical peptoids with desired functions.

Zinc-dysprosium functionalized amyloid fibrils

The heterometallic Zn2Dy2 entity bearing partially saturated metal centres covalently decorates a highly ordered amyloid fibril core and the functionalised assembly exhibits catalytic Lewis acid behaviour.

Exploring the Functional Consequences of Protein Backbone Alteration in Ubiquitin through Native Chemical Ligation

Ubiquitin (Ub) plays critical roles in myriad protein degradation and signaling networks in the cell. We report herein Ub mimetics based on backbones that blend natural and artificial amino acid units. The variants were prepared by a modular route based on native chemical ligation. Biological assays show that some are enzymatically polymerized onto protein substrates, and that the resulting Ub tags are recognized for downstream pathways. These results advance the size and complexity of folded proteins mimicked by artificial backbones and expand the functional scope of such agents.

Combinatorial design of multimeric chelating peptoids for selective metal coordination

The combinatorial synthesis of a new library of tetrameric peptoid ligands is introduced, enabling coordination and characterization of f-block metals.

Current methods for metal chelation are generally based on multidentate organic ligands, which are generated through cumbersome multistep synthetic processes that lack flexibility for systematically varying metal-binding motifs. Octadentate ligands incorporating hydroxypyridinone or catecholamide binding moieties onto a spermine scaffold are known to display some of the highest affinities towards f-elements. Enhancing binding affinity for specific lanthanide or actinide ions however, necessitates ligand architectures that allow for modular and high throughput synthesis. Here we introduce a high-throughput combinatorial library of 16 tetrameric N-substituted glycine oligomers (peptoids) containing hydroxypyridinone or catecholamide chelating units linked via an ethylenediamine bridge and, for comparison, we also synthesized the corresponding mixed ligands derived from the spermine scaffold: 3,4,3-LI(1,2-HOPO)₂(CAM)₂ and 3,4,3-LI(CAM)₂(1,2-HOPO)₂. Coordination-based luminescence studies were carried out with Eu³⁺ and Tb³⁺ to begin probing the properties of the new ligand architecture and revealed higher sensitization efficiency with the spermine scaffold as well as different spectroscopic features among the structural peptoid isomers. Solution thermodynamic properties of selected ligands revealed different coordination properties between the spermine and peptoid analogues with Eu³⁺ stability constants log β₁₁₀ ranging from 28.88 ± 3.45 to 43.97 ± 0.49. The general synthetic strategy presented here paves the way for precision design of new specific and versatile ligands, with a variety of applications tailored towards the use of f-elements, including separations, optical device optimization, and pharmaceutical development.

Peptoids advance multidisciplinary research and undergraduate education in parallel: Sequence effects on conformation and lipid interactions

Peptoids are versatile peptidomimetic molecules with wide-ranging applications from drug discovery to materials science. An understanding of peptoid sequence features that contribute to both their three-dimensional structures and their interactions with lipids will expand functions of peptoids in varied fields. Furthermore, these topics capture the enthusiasm of undergraduate students who prepare and study diverse peptoids in laboratory coursework and/or in faculty led research. Here, we present the synthesis and study of 21 peptoids with varied functionality, including 19 tripeptoids and 2 longer oligomers. We observed differences in fluorescence spectral features for 10 of the tripeptoids that correlated with peptoid flexibility and relative positioning of chromophores. Interactions of representative peptoids with sonicated glycerophospholipid vesicles were also evaluated using fluorescence spectroscopy. We observed evidence of conformational changes effected by lipids for select peptoids. We also summarize our experiences engaging students in peptoid-based projects to advance both research and undergraduate educational objectives in parallel.

Fluorinated Aromatic Monomers as Building Blocks To Control α-Peptoid Conformation and Structure

Peptoids are peptidomimetics of interest in the fields of drug development and biomaterials. However, obtaining stable secondary structures is challenging, and designing these requires effective control of the peptoid tertiary amide cis/trans equilibrium. Herein, we report new fluorine-containing aromatic monomers that can control peptoid conformation. Specifically, we demonstrate that a fluoro-pyridine group can be used to circumvent the need for monomer chirality to control the cis/trans equilibrium. We also show that incorporation of a trifluoro-methyl group ( N^CF₃Rpe) rather than a methyl group ( NRpe) at the α-carbon of a monomer gives rise to a 5-fold increase in cis-isomer preference.

Folding of unstructured peptoids and formation of hetero-bimetallic peptoid complexes upon side-chain-to-metal coordination

Helices are key structural features in biopolymers, enabling a variety of biological functions. Mimicking these secondary structure motifs has wide potential in the development of biomimetic materials. Peptoids, N-substituted glycine oligomers, are an important class of peptide mimics that can adopt polyproline type helices if the majority of their sequence consists of chiral bulky pendent groups. Such side-chains are structure inducers but they have no functional value. We present here the inclusion of several metal-binding groups in one peptoid oligomer as a new platform towards the development of functional helical peptoids. Thus, we describe the coordination of two metal ions to unstructured peptoids incorporating four 8-hydroxyquinoline (HQ) ligands at fixed positions as two (HQ, HQ) metal binding sites, and a mixture of chiral benzyl and alkyl substituents in varied positions along the peptoid backbone. For the first time, we demonstrate by circular dichroism spectroscopy, solution NMR techniques and high-level DFT calculations that some of these unstructured peptoids can fold upon metal binding to form helical structures. Replacing one HQ ligand with a terpyridine (Terpy) ligand resulted in unique sequences that can selectively coordinate Cu2+ to the (Terpy, HQ) and Zn2+ (or Co2+) to the (HQ, HQ) sites from a solution mixture containing Cu2+ and Zn2+ (or Co2+) ions. Interestingly, the binding of Cu2+ to the (Terpy, HQ) site in one of these peptoids can initiate a conformational change that in turn facilitates the coordination of Zn2+ (or Co2+) ions to the (HQ, HQ) site, demonstrating a unique example of positive allosteric cooperativity in peptide mimics.

A bio-inspired approach to ligand design: folding single-chain peptoids to chelate a multimetallic cluster

Synthesis of biomimetic multimetallic clusters is sought after for applications such as efficient storage of solar energy and utilization of greenhouse gases. However, synthetic efforts are hampered by a dearth of ligands that are developed for multimetallic clusters due to current limitations in rational design and organic synthesis. Peptoids, a synthetic sequence-defined oligomer, enable a biomimetic strategy to rapidly synthesize and optimize large, multifunctional ligands by structural design and combinatorial screening. Here we discover peptoid oligomers (≤7 residues) that fold into a single conformation to provide unprecedented tetra- and hexadentate chelation by carboxylates to a [Co4O4] cubane cluster. The structures of peptoid-bound cubanes were determined by 2D NMR spectroscopy, and their structures reveal key steric and side-chain-to-main chain interactions that work in concert to rigidify the peptoid ligand. This efficient ligand design strategy holds promise for creating new scaffolds for the abiotic synthesis and manipulation of multimetallic clusters.

Metalloporphyrin Dimers Bridged by a Peptoid Helix: Host-Guest Interaction and Chiral Recognition

Co-facial porphyrins have been designed to construct porphyrin tweezers with versatile molecular recognition capabilities. In this study, we synthesized metalloporphyrin–peptoid conjugates (MPPCs) displaying two metalloporphyrins on a peptoid scaffold with either achiral unfolded (1) or helical (2 and 3) secondary structures. Host–guest complexation of MPPCs was realized with various guests of different lengths and basicities, and the extent of complexation was measured by UV-vis and circular dichroism (CD) spectroscopic titration. Intermolecular and intramolecular chirality induction were observed on achiral and chiral peptoid backbones, respectively. Spectroscopic data indicated that a broad scope of achiral guests can be recognized by chiral 2; in particular, longer and more flexible guests were seen to bind more tightly on 2. In addition, chiral 2 provided a distinct CD couplet with dl-, d-, or l-Lys-OMe, which was a result of the diastereomeric host–guest complex formation. Our results indicated that MPPCs can recognize, contrast, and analyze various achiral, chiral, or racemic molecules. Based on co-facial metalloporphyrins present on peptoid scaffolds, we developed a novel class of porphyrin tweezers, which can be further utilized in asymmetric catalysis, molecular sensing, and drug delivery.

Stabilising Peptoid Helices Using Non-Chiral Fluoroalkyl Monomers

Stability towards protease degradation combined with modular synthesis has made peptoids of considerable interest in the fields of chemical biology, medicine, and biomaterials. Given their tertiary amide backbone, peptoids lack the capacity to hydrogen-bond, and as such, controlling secondary structure can be challenging. The incorporation of bulky, charged, or chiral aromatic monomers can be used to control conformation but such building blocks limit applications in many areas. Through NMR and X-ray analysis we demonstrate that non-chiral neutral fluoroalkyl monomers can be used to influence the Kcis/trans equilibria of peptoid amide bonds in model systems. The cis-isomer preference displayed is highly unprecedented given that neither chirality nor charge is used to control the peptoid amide conformation. The application of our fluoroalkyl monomers in the design of a series of linear peptoid oligomers that exhibit stable helical structures is also reported.

Solid-Phase Synthesis of Azole-Comprising Peptidomimetics and Coordination of a Designed Analog to Zn2

Peptidomimetics that can coordinate transition metals have a variety of potential applications as catalysts, sensors, or materials. A new modular peptidomimetic scaffold, the “azole peptoid”, is introduced here. We report methods for the solid-phase synthesis of eleven examples of trimeric N-substituted oligoamides that include oxazole- or thiazole-functionalized backbones. The products prepared comprise a diversity of functionality, including a metal-coordinating terpyridine group. The modular synthetic approach enables ready preparation of analogs for specific applications. To highlight a potential use of this new synthetic scaffold, a trimeric azole peptoid functionalized with a terpyridine residue was prepared and studied. The characteristic 2:1 ligand:metal binding of this terpyridine-functionalized azole peptoid to Zn²⁺ in aqueous solution was observed. These studies introduce azole peptoids as a useful class of biomimetic molecules for further study and application.

A supramolecular lanthanide separation approach based on multivalent cooperative enhancement of metal ion selectivity

Multivalent cooperativity plays an important role in the supramolecular self-assembly process. Herein, we report a remarkable cooperative enhancement of both structural integrity and metal ion selectivity on metal-organic M4L4 tetrahedral cages self-assembled from a tris-tridentate ligand (L1) with a variety of metal ions spanning across the periodic table, including alkaline earth (CaII), transition (CdII), and all the lanthanide (LnIII) metal ions. All these M4L14 cages are stable to excess metal ions and ligands, which is in sharp contrast with the tridentate (L2) ligand and bis-tridentate (L3) ligand bearing the same coordination motif as L1. Moreover, high-precision metal ion self-sorting is observed during the mixed-metal self-assembly of tetrahedral M4L4 cages, but not on the M2L3 counterparts. Based on the strong cooperative metal ion self-recognition behavior of M4L4 cages, a supramolecular approach to lanthanide separation is demonstrated, offering a new design principle of next-generation extractants for highly efficient lanthanide separation.

A Systematic Review of Antiamyloidogenic and Metal-Chelating Peptoids: Two Structural Motifs for the Treatment of Alzheimer's Disease

Alzheimer's disease (AD) is an incurable form of dementia affecting millions of people worldwide and costing billions of dollars in health care-related payments, making the discovery of a cure a top health, societal, and economic priority. Peptide-based drugs and immunotherapies targeting AD-associated beta-amyloid (Aβ) aggregation have been extensively explored; however, their therapeutic potential is limited by unfavorable pharmacokinetic (PK) properties. Peptoids (N-substituted glycine oligomers) are a promising class of peptidomimetics with highly tunable secondary structures and enhanced stabilities and membrane permeabilities. In this review, the biological activities, structures, and physicochemical properties for several amyloid-targeting peptoids will be described. In addition, metal-chelating peptoids with the potential to treat AD will be discussed since there are connections between the dysregulation of certain metals and the amyloid pathway.

Chiral Cu(ii), Co(ii) and Ni(ii) complexes based on 2,2′-bipyridine modified peptoids

Helical peptoids bearing 2,2′-bipyridine, varied in their chiral bulky side chains and their N-terminus form chiral complexes with Cu(ii), Co(ii) and Ni(ii) via intramolecular binding.

Water soluble hydrophobic peptoids via a minor backbone modification

The incorporation of piperazine or homopiperazine within hydrophobic peptoid scaffolds leads to their water solubility while increasing their overall conformational order in water.

Homogeneous and Robust Polyproline Type I Helices from Peptoids with Nonaromatic α-Chiral Side Chains

Peptoids that are oligomers of N-substituted glycines represent a class of peptide mimics with great potential in areas ranging from medicinal chemistry to biomaterial science. Controlling the equilibria between the cis and trans conformations of their backbone amides is the major hurdle to overcome for the construction of discrete folded structures, particularly for the development of all-cis polyproline type I (PPI) helices, as tools for modulating biological functions. The prominent role of backbone to side chain electronic interactions (n → π*) and side chains bulkiness in promoting cis-amides was essentially investigated with peptoid aromatic side chains, among which the chiral 1-naphthylethyl (1npe) group yielded the best results. We have explored for the first time the possibility to achieve similar performances with a sterically hindered α-chiral aliphatic side chain. Herein, we report on the synthesis and detailed conformational analysis of a series of (S)-N-(1-tert-butylethyl)glycine (Ns1tbe) peptoid homo-oligomers. The X-ray crystal structure of an Ns1tbe pentamer revealed an all-cis PPI helix, and the CD curves of the Ns1tbe oligomers also resemble those of PPI peptide helices. Interestingly, the CD data reported here are the first for any conformationally homogeneous helical peptoids containing only α-chiral aliphatic side chains. Finally we also synthesized and analyzed two mixed oligomers composed of NtBu and Ns1tbe monomers. Strikingly, the solid state structure of the mixed oligomer Ac-(tBu)₂-(s1tbe)₄-(tBu)₂-COOtBu, the longest to be solved for any linear peptoid, revealed a PPI helix of great regularity despite the presence of only 50% of chiral side chain in the sequence.

Cyclic Peptoids as Mycotoxin Mimics: An Exploration of Their Structural and Biological Properties

Cyclic peptoids have recently emerged as important examples of peptidomimetics for their interesting complexing properties and innate ability to permeate biological barriers. In the present contribution, experimental and theoretical data evidence the intricate conformational and stereochemical properties of five novel hexameric peptoids decorated with N-isopropyl, N-isobutyl, and N-benzyl substituents. Complexation studies by NMR, in the presence of sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (NaTFPB), theoretical calculations, and single-crystal X-ray analyses indicate that the conformationally stable host/guest metal adducts display architectural ordering comparable to that of the enniatins and beauvericin mycotoxins. Similarly to the natural depsipeptides, the synthetic oligolactam analogues show a correlation between ion transport abilities in artificial liposomes and cytotoxic activity on human cancer cell lines. The reported results demonstrate that the versatile cyclic peptoid scaffold, for its remarkable conformational and complexing properties, can morphologically mimic related natural products and elicit powerful biological activities.

Heterogeneous-Backbone Foldamer Mimics of Zinc Finger Tertiary Structure

A variety of oligomeric backbones with compositions deviating from biomacromolecules can fold in defined ways. Termed "foldamers," these agents have diverse potential applications. A number of protein-inspired secondary structures (e.g., helices, sheets) have been produced from unnatural backbones, yet examples of tertiary folds combining several secondary structural elements in a single entity are rare. One promising strategy to address this challenge is the systematic backbone alteration of natural protein sequences, through which a subset of the native side chains is displayed on an unnatural building block to generate a heterogeneous backbone. A drawback to this approach is that substitution at more than one or two sites often comes at a significant energetic cost to fold stability. Here we report heterogeneous-backbone foldamers that mimic the zinc finger domain, a ubiquitous and biologically important metal-binding tertiary motif, and do so with a folded stability that is superior to the natural protein on which their design is based. A combination of UV-vis spectroscopy, isothermal titration calorimetry, and multidimensional NMR reveals that suitably designed oligomers with >20% modified backbones can form native-like tertiary folds with metal-binding environments identical to the prototype sequence (the third finger of specificity factor 1) and enhanced thermodynamic stability. These results expand the scope of heterogeneous-backbone foldamer design to a new tertiary structure class and show that judiciously applied backbone modification can be accompanied by improvement to fold stability.

A modular platform to develop peptoid-based selective fluorescent metal sensors

Despite the reduction in industrial use of toxic heavy metals, there remain contaminated natural water sources across the world. Herein we present a modular platform for developing selective sensors for toxic metal ions using N-substituted glycine, or peptoid, oligomers coupled to a fluorophore. As a preliminary evaluation of this strategy, structures based on previously identified metal-binding peptoids were synthesized with terminal pyrene moieties. Both derivatives of this initial design demonstrated a turn-off response in the presence of various metal ions. A colorimetric screen was designed to identify a peptoid ligand that chelates Hg(ii). Multiple ligands were identified that were able to deplete Hg(ii) from a solution selectively in the presence of an excess of competing ions. The C-terminal fluoropeptoid derivatives demonstrated similar selectivity to their label-free counterparts. This strategy could be applied to develop sensors for many different metal ions of interest using a variety of fluorophores, leading to a panel of sensors for identifying various water source contaminants.

Control of porphyrin interactions via structural changes of a peptoid scaffold

A template to control porphyrin interactions is constructed by displaying porphyrins at defined positions on a helical peptoid.