Building β-peptide H10/12 foldamer helices with six-membered cyclic side-chains: fine-tuning of folding and self-assembly

6-Strand to Stable 10/12 Helix Conformational Switch by Incorporating Flexible β-hGly in the Homooligomers of Camphor Derived β-Amino Acid: NMR and X-Ray Crystallographic Evidence

Rational design of unnatural amino acid building blocks capable of stabilizing predictable secondary structures similar to protein fragments is pivotal for foldamer chemistry/catalysis. Here, we introduce novel β-amino acid building blocks: [1S,2R,4R]exoCDA and [1S,2S,4R]endoCDA, derived from the abundantly available R(+)-camphor, which is traditionally known for its medicinal value. Further, we demonstrate that the homooligomers of exoCDA adopt 6-strand conformation, which switches to a robust 10/12-helix simply by inserting flexible β-hGly spacer at alternate positions (1 : 1 β-hGly/exoCDA heterooligomers), as evident by DFT-calculations, solution-state NMR spectroscopy and X-ray crystallography. To the best of our knowledge, this is the first example of crystalline-state structure of left-handed 10/12-mixed helix, that is free from the conventional approach of employing β-amino acids of either alternate chirality or alternate β2/β3 substitutions, to access the 10/12-helix. The results also show that the homooligomers of heterochiral exoCDA don't adopt helical fold, instead exhibit banana-shaped strands, whereas the homodimers of the other diastereomer endoCDA, nucleate 8-membered turns. Furthermore, the homo-exoCDA and hetero-[β-hGly-exoCDA] oligomers are found to exhibit self-association properties with distinct morphological features. Overall, the results offer new possibilties of constructing discrete stable secondary and tertiary structures based on CDAs, which can accommodate flexible residues with desired side-chain substitutions.

Exploring helix structures of γ-peptides based on 2-(aminomethyl)cyclopentanecarboxylic acid

Conformational search and density functional theory calculations were performed to explore the preferences of helical structures for chiro-specific oligo-γ-peptides of 2-(aminomethyl)cyclopentanecarboxylic acid (γAmc5) with a cyclopentyl constraint on the Cα-Cβ bond in solution. The dimer and tetramer of γAmc5 (1) with homochiral (1S, 2S) configurations exhibited a strong preference for the 9-membered helix foldamer in solution, except for the tetramer in water. However, the oligomers of γAmc5 (1) longer than tetramer preferentially adopted a right-handed (P)-2.614-helix (H1-14) as the peptide sequence becomes longer and as solvent polarity increases. The high stabilities for H1-14 foldamers of γAmc5 (1) in solution were ascribed to the favored solvation free energies. The calculated mean backbone torsion angles for H1-14 helix foldamers of γAmc5 (1) were similar to those calculated for oligomers of other γ-residues with cyclopentane or cyclohexane rings. However, the substitution of cyclopentane constraints on the Cα-Cβ bond of the γAmc5 (1) residue resulted in different conformational preferences and/or handedness of helix foldamers. In particular, the pyrrolidine-substituted analogs of the H1-14 foldamers of γAmc5 (1) with adjacent amine diads substituted at a proximal distance are expected to be potential catalysts for the crossed aldol condensation in nonpolar and polar solvents.

β-Peptides incorporating polyhydroxylated cyclohexane β-amino acid: synthesis and conformational study

We describe the synthesis of trihydroxylated cyclohexane β-amino acids from (-)-shikimic acid, in their cis and trans configuration, and the incorporation of the trans isomer into a trans-2-aminocyclohexanecarboxylic acid peptide chain. Subsequently, the hydroxyl groups were partially or totally deprotected. The structural study of the new peptides by FTIR, CD, solution NMR and DFT calculations revealed that they all fold into a 14-helix secondary structure, similarly to the homooligomer of trans-2-aminocyclohexanecarboxylic acid. This means that the high degree of substitution of the cyclohexane ring of the new residue is compatible with the adoption of a stable helical secondary structure and opens opportunities for the design of more elaborate peptidic foldamers with oriented polar substituents at selected positions of the cycloalkane residues.

Exploring a β-Amino Acid with a Seven-Membered Ring Constraint as a Foldamer Building Block for Nontraditional Helices

We explored trans- and cis-2-aminocycloheptanecarboxylic acid (ACHpC) as potential building blocks for helical foldamers. trans-ACHpC does not show sufficient folding propensity in unnatural peptides. cis-ACHpC promotes nontraditional helices of two unnatural peptide backbones: the 11/9-helix for 1:1 α/β-peptides and the 12/10-helix for β-peptides with interconvertible handedness. The two opposite-handed 12/10-helices rapidly interconvert in solution by pseudorotation of the two twist chair forms of the cycloheptane moiety in each cis-ACHpC residue.

A switch in N-terminal capping of β-peptides creates novel self-assembled nanoparticles

Small tripeptides composed entirely of β3-amino acids have been shown to self-assemble into fibres following acylation of the N-terminus. Given the use of Fmoc as a strategy to initiate self-assembly in α-peptides, we hypothesized that the acyl cap can be replaced by an Fmoc without perturbation to the self-assembly and enable simpler synthetic protocols. We therefore replaced the N-acyl cap for an Fmoc group and herein we show that these Fmoc-protected β3-peptides produce regular spherical particles, rather than fibrous structures, that are stable and capable of encapsulating cargo. We then demonstrated that these particles were able to deliver cargo to cells without any obvious signs of cytotoxicity. This is the first description of such regular nanoparticles derived from Fmoc-protected β3-peptides.

Advances in hybrid peptide-based self-assembly systems and their applications

Self-assembly of peptides demonstrates a great potential for designing highly ordered, finely tailored supramolecular arrangements enriched with high specificity, improved efficacy and biological activity. Along with natural peptides, hybrid peptide systems composed of natural and chemically diverse unnatural amino acids have been used in various fields, including drug delivery, wound healing, potent inhibition of diseases, and prevention of biomaterial related diseases to name a few. In this review, we provide a brief outline of various methods that have been utilized for obtaining fascinating structures that create an avenue to reproduce a range of functions resulting from these folds. An overview of different self-assembled structures as well as their applications will also be provided. We believe that this review is very relevant to the current scenario and will cover conformations of hybrid peptides and resulting self-assemblies from the late 20^th century through 2022. This review aims to be a comprehensive and reliable account of the hybrid peptide-based self-assembly owing to its enormous influence in understanding and mimicking biological processes.

Effect of a cis-4-aminopiperidine-3-carboxylic acid (cis-APiC) residue on mixed-helical folding of unnatural peptides

The α/β-peptide 11/9-helix and the β-peptide 12/10-helix belong to "mixed" helices, in which two types of hydrogen bonds with opposite directionality alternate along the helical axis. cis-2-Aminocyclohexanecarboxylic acid (cis-ACHC) is known to promote these mixed helices and stabilize the helical propensity more than other acyclic β-residues. Application of a mixed-helical backbone still requires sufficient solubility in aqueous solution. In this regard, we chose cis-4-aminopiperidine-3-carboxylic acid (cis-APiC) as a foldamer building block that can provide both sufficient aqueous solubility and mixed-helical propensity. Conformational analyses of α/β- and β-peptides containing a cis-APiC residue by circular dichroism spectroscopy and single-crystal X-ray crystallography suggest that the incorporation of cis-APiC instead of cis-ACHC can enhance the aqueous solubility of the mixed-helical peptides without any adverse effect on helical folding. In addition, the ratio between right- and left-handed 12/10-helices of β-peptides can be rationalized by relative energies between the local conformations of the cis-APiC residue.

Peptide foldamer-based self-assembled nanostructures containing cyclic beta-amino acids

Peptide soft materials belong to an emerging branch of materials sciences due to their growing importance as responsive materials in diagnostics, therapeutics, and biomedical applications. The diversity provided by easily modifiable peptide sequences can be further increased by introducing nonnatural amino acids such as cyclic β-amino acids, leading to the formation of foldamers. Moreover, it is possible to combine peptide chains with other polymers, aromatic compounds, etc. to create hybrids with completely new properties and applications. In this review, we focus on the cis/trans enantiomers of three cyclic β-amino acids: 2-aminocyclobutane-1-carboxylic acid (ACBC), 2-aminocyclopentane-1-carboxylic acid (ACPC) and 2-aminocyclohexane-1-carboxylic acid (ACHC). The peptides discussed here either contain exclusively β-amino acids or are α,β-peptides, and they undergo self-assembly by forming different interactions that lead to the creation of well-defined nanostructures.

The Diverse World of Foldamers: Endless Possibilities of Self-Assembly

Different classes of foldamers, which are synthetic oligomers that adopt well-defined conformations in solution, have been the subject of extensive studies devoted to the elucidation of the forces driving their secondary structures and their potential as bioactive molecules. Regardless of the backbone type (peptidic or abiotic), the most important features of foldamers are the high stability, easy predictability and tunability of their folding, as well as the possibility to endow them with enhanced biological functions, with respect to their natural counterparts, by the correct choice of monomers. Foldamers have also recently started playing a starring role in the self-assembly of higher-order structures. In this review, selected articles will be analyzed to show the striking number of self-assemblies obtained for foldamers with different backbones, which will be analyzed in order of increasing complexity. Starting from the simplest self-associations in solution (e.g., dimers of β-strands or helices, bundles, interpenetrating double and multiple helices), the formation of monolayers, vesicles, fibers, and eventually nanostructured solid tridimensional morphologies will be subsequently described. The experimental techniques used in the structural investigation, and in the determination of the driving forces and mechanisms underlying the self-assemblies, will be systematically reported. Where applicable, examples of biomimetic self-assembled foldamers and their interactions with biological components will be described.

Coexistence of Left- and Right-Handed 12/10-Mixed Helices in Cyclically Constrained β-Peptides and Directed Formation of Single-Handed Helices upon Site-Specific Methylation

The inherent conformational preferences of the neutral β-peptide foldamer series, Ac-(ACHC)_n-NHBn, n = 2-4, are studied in the gas phase using conformation-specific IR-UV double resonance methods. The cyclically constrained chiral β-amino acid cis-2-aminocyclohexane carboxylic acid (ACHC) is designed to bring both right- and left-handed helices into close energetic proximity. Comparison of the infrared spectra in the NH stretch and amide I/II regions with the predictions of DFT calculations lead to the unambiguous assignment of four out of the six observed conformations of the molecules in this series, while corroborating computational and spectral evidence, affords tentative assignments of the remaining two conformers for which IR data were not recorded. The observed structures fall into one of two conformational families: a right-handed 12/10-mixed helix or its "cap-disrupted" left-handed helical analogue, which coexist with significant populations. Site-specific and stereospecific methylation on the cyclohexane backbone at the dipeptide (n = 2) level is also tested as a means to sterically lock in a predetermined cyclohexane chair conformation. These substitutions are proven to be a means of selectively driving formation of one helical screw sense or the other. Calculated relative energies and free energies of all possible structures for the molecules provide strong supporting evidence that the rigid nature of the ACHC residue confers unusual stability to the 12/10-mixed helix conformation, regardless of local environment, temperature, or C-terminal capping unit. The simultaneous presence of both handed helices offers unique opportunities for future studies of their interconversion.

Structural insight into hybrid peptide ε-helices

Unique ε-helical organizations (11-helices) from β,γ-hybrid peptides composed of chiral β³-amino acids along with achiral 3,3- or 4,4-dimethyl substituted γ-amino acids are disclosed.

FoldamerDB: a database of peptidic foldamers

Foldamers are non-natural oligomers that mimic the structural behaviour of natural peptides, proteins and nucleotides by folding into a well-defined 3D conformation in solution. Since their first description about two decades ago, numerous studies have been undertaken dealing with the design, synthesis, characterization and application of foldamers. They have huge application potential as antimicrobial, anticancer and anti-HIV agents and in materials science. Despite their importance, there is no publicly available web resource providing comprehensive information on these compounds. Here we describe FoldamerDB, an open-source, fully annotated and manually curated database of peptidic foldamers. FoldamerDB holds the information about the sequence, structure and biological activities of the foldamer entries. It contains the information on over 1319 species and 1018 activities, collected from more than 160 research papers. The web-interface is designed to be clutter-free, user-friendly and it is compatible with devices of different screen sizes. The interface allows the user to search the database, browse and filter the foldamers using multiple criteria. It also offers a detailed help page to assist new users. FoldamerDB is hoped to bridge the gap in the freely available web-based resources on foldamers and will be of interest to diverse groups of scientists from chemists to biologists. The database can be accessed at http://foldamerdb.ttk.hu/.

Self-assembly of Functional Nanostructures by Short Helical Peptide Building Blocks

The self-assembly of short peptide building blocks into well-ordered nanostructures is a key direction in bionanotechnology. The formation of β -sheet organizations by short peptides is well explored, leading to the development of a wide range of functional assemblies. Likewise, many natural proteinaceous materials, such as silk and amyloid fibrils, are based on β-sheet structures. In contrast, collagen, the most abundant protein in mammals, is based on helical arrangement. Similar to β-sheet structures, short helical peptides have been recently discovered to possess a diverse set of functionalities with the potential to fabricate artificial self-assembling materials. Here, we outline the functional roles of self-assembled nanostructures formed by short helical peptides and their potential as artificial materials. We focus on the association between self-assembled mesoscale structures and their material function and demonstrate the way by which this class of building blocks bears the potential for diverse applications, such as the future fabrication of smart devices.

Flow-chemistry enabled efficient synthesis of β-peptides: backbone topology vs. helix formation

Enantiodiscriminative helix formation was observed for β-peptide H14 helices when enantiomers of bridged bicyclic residues were introduced.

Modulating the Structural Properties of α,γ-Hybrid Peptides by α-Amino Acid Residues: Uniform 12-Helix Versus "Mixed" 12/10-Helix

The most important natural α- and 3₁₀ -helices are stabilized by unidirectional intramolecular hydrogen bonds along the helical cylinder. In contrast, we report here on 12/10-helical conformations with alternately changing hydrogen-bond directionality in sequences of α,γ-hybrid peptides P1-P5 [P1: Boc-Ala-Aic-Ala-Aic-COOH; P2: Boc-Leu-Aic-Leu-Aic-OEt; P3: Boc-Leu-Aic-Leu-Aic-Leu-Aic-Aib-OMe; P4: Boc-Ala-Aic-Ala-Aic-Ala-Aic-Ala-OMe; P5: Boc-Leu-Aic-Leu-Aic-Leu-Aic-Leu-Aic-Aib-OMe; Aic=4-aminoisocaproic acid, Aib=2-aminoisobutyric acid] composed of natural α-amino acids and the achiral γ^4,4 -dimethyl substituted γ-amino acid Aic in solution and in single crystals. The helical conformations are stabilized by alternating i→i+3 and i→i-1 intramolecular hydrogen bonds. The experimental data are supported by ab initio MO calculations. Surprisingly, replacing the natural α-amino acids of the sequence by the achiral dialkyl amino acid Ac₆ c [P6: Boc-Ac₆ c-Aic-Ac₆ c-Aic-Ac₆ c-Aic-Ac₆ c-Aic-Ac₆ c-CONHMe; Ac₆ c = 1-aminocyclohexane-1-carboxylic acid] led to a 12-helix with unidirectional hydrogen bonds showing an entirely different backbone conformation. The results presented here emphasize the influence of the structure of the α-amino acid residues in dictating the helix types in α,γ-hybrid peptide foldamers and demonstrate the consequences for folding of small structural variations in the monomers.

Unique Functional Materials Derived from β-Amino Acid Oligomers

The unique structures formed by β-amino acid oligomers, or β-peptide foldamers, have been studied for almost two decades, which has led to the discovery of several distinctive structures and bioactive molecules. Recently, this area of research has expanded from conventional peptide drug design to the formation of assemblies and nanomaterials by peptide self-assembly. The unique structures formed by β-peptides give rise to a set of new materials with altered properties that differ from conventional peptide-based materials; such new materials may be useful in several bio- and nanomaterial applications.

Therapeutic Potential of Foldamers: From Chemical Biology Tools To Drug Candidates?

Over the past decade, foldamers have progressively emerged as useful architectures to mimic secondary structures of proteins. Peptidic foldamers, consisting of various amino acid based backbones, have been the most studied from a therapeutic perspective, while polyaromatic foldamers have barely evolved from their nascency and remain perplexing for medicinal chemists due to their poor drug-like nature. Despite these limitations, this compound class may still offer opportunities to study challenging targets or provide chemical biology tools. The potential of foldamer drug candidates reaching the clinic is still a stretch. Nevertheless, advances in the field have demonstrated their potential for the discovery of next generation therapeutics. In this perspective, the current knowledge of foldamers is reviewed in a drug discovery context. Recent advances in the early phases of drug discovery including hit finding, target validation, and optimization and molecular modeling are discussed. In addition, challenges and focus areas are debated and gaps highlighted.

An overview of peptide and peptoid foldamers in medicinal chemistry

INTRODUCTION

Foldamers are artificial self-organizing systems with various critical properties: i) a stable and designable secondary structure; ii) a larger molecular surface as compared with ordinary organic drug molecules; iii) appropriate control of the orientation of the side-chain functional groups; iv) resistance against proteolytic degradation, which leads to potentially increased oral bioavailability and a longer serum half-life relative to ordinary α-peptides; and v) the lower conformational freedom may result in increased receptor binding in comparison with the natural analogs.

AREAS COVERED

This article covers the general properties and types of foldamers. This includes highlighted examples of medicinal chemical applications, including antibacterial and cargo molecules, anti-Alzheimer compounds and protein-protein interaction modifiers.

EXPERT OPINION

Various new foldamers have been created with a range of structures and biological applications. Membrane-acting antibacterial foldamers have been introduced. A general property of these structures is their amphiphilic nature. The amphiphilicity can be stationary or induced by the membrane binding. Cell-penetrating foldamers have been described which serve as cargo molecules, and foldamers have been used as autophagy inducers. Anti-Alzheimer compounds too have been created and the greatest breakthrough was attained via the modification of protein-protein interactions. This can serve as the chemical and pharmaceutical basis for the relevance of foldamers in the future.

Conformational properties of 1,4- and 1,5-substituted 1,2,3-triazole amino acids – building units for peptidic foldamers

Peptidic foldamers have recently emerged as a novel class of artificial oligomers with properties and structural diversity similar to that of natural peptides, but possessing additional interesting features granting them great potential for applications in fields from nanotechnology to pharmaceuticals. Among these, foldamers containing 1,4- and 1,5-substitued triazole amino acids are easily prepared via the Cu- and Ru-catalyzed click reactions and may offer increased side chain variation, but their structural capabilities have not yet been widely explored. We here describe a systematic analysis of the conformational space of the two most important basic units, the 1,4-substitued (4Tzl) and the 1,5-substitued (5Tzl) 1,2,3-triazole amino acids, using quantum chemical calculations and NMR spectroscopy. Possible conformations of the two triazoles were scanned and their potential minima were located using several theoretical approaches (B3LYP/6-311++G(2d,2p), ωB97X-D/6-311++G(2d,2p), M06-2X/6-311++G(2d,2p) and MP2/6-311++G(2d,2p)) in different solvents. BOC-protected versions of 4Tzl and 5Tzl were also prepared via one step transformations and analyzed by 2D NOESY NMR. Theoretical results show 9 conformers for 5Tzl derivatives with relative energies lying close to each other, which may lead to a great structural diversity. NMR analysis also indicates that conformers preferring turn, helix and zig-zag secondary structures may coexist in solution. In contrast, 4Tzl has a much lower number of conformers, only 4, and these lack strong intraresidual interactions. This is again supported by NMR suggesting the presence of both extended and bent conformers. The structural information provided on these building units could be employed in future design of triazole foldamers.

Induced folding of protein-sized foldameric β-sandwich models with core β-amino acid residues

The mimicry of protein-sized β-sheet structures with unnatural peptidic sequences (foldamers) is a considerable challenge. In this work, the de novo designed betabellin-14 β-sheet has been used as a template, and α→β residue mutations were carried out in the hydrophobic core (positions 12 and 19). β-Residues with diverse structural properties were utilized: Homologous β(3) -amino acids, (1R,2S)-2-aminocyclopentanecarboxylic acid (ACPC), (1R,2S)-2-aminocyclohexanecarboxylic acid (ACHC), (1R,2S)-2-aminocyclohex-3-enecarboxylic acid (ACEC), and (1S,2S,3R,5S)-2-amino-6,6-dimethylbicyclo[3.1.1]heptane-3-carboxylic acid (ABHC). Six α/β-peptidic chains were constructed in both monomeric and disulfide-linked dimeric forms. Structural studies based on circular dichroism spectroscopy, the analysis of NMR chemical shifts, and molecular dynamics simulations revealed that dimerization induced β-sheet formation in the 64-residue foldameric systems. Core replacement with (1R,2S)-ACHC was found to be unique among the β-amino acid building blocks studied because it was simultaneously able to maintain the interstrand hydrogen-bonding network and to fit sterically into the hydrophobic interior of the β-sandwich. The novel β-sandwich model containing 25 % unnatural building blocks afforded protein-like thermal denaturation behavior.

Solvent assisted structural diversity: supramolecular sheet and double helix of a short aromatic γ-peptide

Solvent interaction has a significant effect on the molecular motions, folding and self-assembly of short aromatic γ-peptides that leads to a change in initial helical conformation.

cis-2-Aminocyclohex-4-enecarboxylic acid as a new building block of helical foldamers

cis-2-Aminocyclohex-4-enecarboxylic acid can promote the α/β-peptide 11/9-helix in solution and in the crystal state.

Continuous-flow solid-phase peptide synthesis: a revolutionary reduction of the amino acid excess

A highly efficient continuous-flow technique for the synthesis of peptides was developed. The method allows the application of only 1.5 equivalents of amino acids during coupling, while yielding virtually quantitative conversions. A mesoscale reactor was constructed which permits the use of high temperature and pressure during the synthesis. A complete reaction parameter optimization was carried out. Under the optimum conditions, the couplings of all 20 proteinogenic amino acids were achieved with 1.5 amino acid equivalents with quantitative conversions. As a demonstration of the efficiency of the methodology, difficult sequences and β-peptide foldamers with alicyclic side-chains were synthetized in excellent yields and with lower costs thanks to the lower amounts of amino acid and solvent used. By this the synthesis is highly economic and sustainable. Importantly, exotic and expensive artificial amino acids were incorporated into peptidic sequences by the utilization of a reasonable number of amino acid equivalents. The synthesis can be performed in quantities of microgram to gram in an automated way.

Carboxylated glucuronic poly-amido-saccharides as protein stabilizing agents

The synthesis of novel carbohydrate-based polymers allows the structure to be tailored at the monomer level for a specific property and expands the range of available structures beyond those found in nature. Using a controlled anionic polymerization, a new type of carbohydrate polymer is synthesized in which glucose-derived monomers are joined by an α-1,2 amide linkage to give enantiopure poly-amido-saccharides (PASs). To investigate the effect of adding ionizable carboxylic acid groups, such as those found in natural polysaccharides containing glucuronic acid, the oxidation of the primary alcohol at the C6-position of the repeat unit to a carboxylic acid is reported. TEMPO-mediated oxidation provides control over the degree of oxidation in excellent yield. Based on circular dichroism, the oxidized polymers possess an ordered helical secondary structure in aqueous solution. Finally, oxidized PASs stabilize lysozyme toward dehydration and freezing stresses better than a current, widely used protein stabilizing agent, trehalose.

Exploiting aromatic interactions for β-peptide foldamer helix stabilization: a significant design element

Tetrameric H10/12 helix stabilization was achieved by the application of aromatic side-chains in β-peptide oligomers by intramolecular backbone-side chain CH-π interactions. Because of the enlarged hydrophobic surface of the oligomers, a further aim was the investigation of the self-assembly in a polar medium for the β-peptide H10/12 helices. NMR, ECD, and molecular modeling results indicated that the oligomers formed by cis-[1S,2S]- or cis-[1R,2R]-1-amino-1,2,3,4-tetrahydronaphthalene-2-carboxylic acid (ATENAC) and cis-[1R,2S]- or cis-[1S,2R]-2-aminocyclohex-3-enecarboxylic acid (ACHEC) residues promote stable H10/12 helix formation with an alternating backbone configuration even at the tetrameric chain length. These results support the view that aromatic side-chains can be applied for helical structure stabilization. Importantly, this is the first observation of a stable H10/12 helix with tetrameric chain-length. The hydrophobically driven self-assembly was achieved for the helix-forming oligomers, seen as vesicles in transmission electron microscopy images. The self-association phenomenon, which supports the helical secondary structure of these oligomers, depends on the hydrophobic surface area, because a higher number of aromatic side-chains yielded larger vesicles. These results serve as an essential element for the design of helices relating to the H10/12 helix. Moreover, they open up a novel area for bioactive foldamer construction, while the hydrophobic area gained through the aromatic side-chains may yield important receptor-ligand interaction surfaces, which can provide amplified binding strength.

Evaluation of a cyclopentane-based γ-amino acid for the ability to promote α/γ-peptide secondary structure

We report the asymmetric synthesis of the γ-amino acid (1R,2R)-2-aminomethyl-1-cyclopentane carboxylic acid (AMCP) and an evaluation of this residue's potential to promote secondary structure in α/γ-peptides. Simulated annealing calculations using NMR-derived distance restraints obtained for α/γ-peptides in chloroform reveal that AMCP-containing oligomers are conformationally flexible. However, additional evidence suggests that an internally hydrogen-bonded helical conformation is partially populated in solution. From these data, we propose characteristic NOE patterns for the formation of the α/γ-peptide 12/10-helix and discuss the apparent conformational frustration of AMCP-containing oligomers.