β-Amino acid-containing hybrid peptides—new opportunities in peptidomimetics

Fine-tuning the tRNA anticodon arm for multiple/consecutive incorporations of β-amino acids and analogs

Ribosomal incorporation of β-amino acids into nascent peptides is much less efficient than that of the canonical α-amino acids. To overcome this, we have engineered a tRNA chimera bearing T-stem of tRNAGlu and D-arm of tRNAPro1, referred to as tRNAPro1E2, which efficiently recruits EF-Tu and EF-P. Using tRNAPro1E2 indeed improved β-amino acid incorporation. However, multiple/consecutive incorporations of β-amino acids are still detrimentally poor. Here, we attempted fine-tuning of the anticodon arm of tRNAPro1E2 aiming at further enhancement of β-amino acid incorporation. By screening various mutations introduced into tRNAPro1E2, C31G39/C28G42 mutation showed an approximately 3-fold enhancement of two consecutive incorporation of β-homophenylglycine (βPhg) at CCG codons. The use of this tRNA made it possible for the first time to elongate up to ten consecutive βPhg's. Since the enhancement effect of anticodon arm mutations differs depending on the codon used for β-amino acid incorporation, we optimized anticodon arm sequences for five codons (CCG, CAU, CAG, ACU and UGG). Combination of the five optimal tRNAs for these codons made it possible to introduce five different kinds of β-amino acids and analogs simultaneously into model peptides, including a macrocyclic scaffold. This strategy would enable ribosomal synthesis of libraries of macrocyclic peptides containing multiple β-amino acids.

Quest for New Generation Biocompatible Materials: Tailoring β-Peptide Structure and Interactions via Synergy of Experiments and Modelling

Peptide-based self-assembly has been used to produce a wide range of nanostructures. While most of these systems involve self-assembly of α-peptides, more recently β-peptides have also been shown to undergo supramolecular self-assembly, and have been used to produce materials for applications in tissue engineering, cell culture and drug delivery. In order to engineer new materials with specific structure and function, theoretical molecular modelling can provide significant insights into the collective balance of non-covalent interactions that drive the self-assembly and determine the structure of the resultant supramolecular materials under different conditions. However, this approach has only recently become feasible for peptide-based self-assembled nanomaterials, particularly those that incorporate non α-amino acids. This perspective provides an overview of the challenges associated with computational modelling of the self-assembly of β-peptides and the recent success using a combination of experimental and computational techniques to provide insights into the self-assembly mechanisms and fully atomistic models of these new biocompatible materials.

Interhelical E@g-N@a interactions modulate coiled coil stability within a de novo set of orthogonal peptide heterodimers

The designability of orthogonal coiled coil (CC) dimers, which draw on well-established design rules, plays a pivotal role in fueling the development of CCs as synthetically versatile assembly-directing motifs for the fabrication of bionanomaterials. Here, we aim to expand the synthetic CC toolkit through establishing a "minimalistic" set of orthogonal, de novo CC peptides that comprise 3.5 heptads in length and a single buried Asn to prescribe dimer formation. The designed sequences display excellent partner fidelity, confirmed via circular dichroism (CD) spectroscopy and Ni-NTA binding assays, and are corroborated in silico using molecular dynamics (MD) simulation. Detailed analysis of the MD conformational data highlights the importance of interhelical E@g-N@a interactions in coordinating an extensive 6-residue hydrogen bonding network that "locks" the interchain Asn-Asn' contact in place. The enhanced stability imparted to the Asn-Asn' bond elicits an increase in thermal stability of CCs up to ~15°C and accounts for significant differences in stability within the collection of similarly designed orthogonal CC pairs. The presented work underlines the utility of MD simulation as a tool for constructing de novo, orthogonal CCs, and presents an alternative handle for modulating the stability of orthogonal CCs via tuning the number of interhelical E@g-N@a contacts. Expansion of CC design rules is a key ingredient for guiding the design and assembly of more complex, intricate CC-based architectures for tackling a variety of challenges within the fields of nanomedicine and bionanotechnology.

Lewis-Base-Catalyzed N-Allylation of Silyl Carbamate Latent Pronucleophiles with Allylic Fluorides

Silyl carbamates, latent pronucleophile surrogates of carbamates, undergo allylation using allylic fluorides in the presence of common Lewis base catalysts. The reactions are rendered enantioselective in the presence of chiral Lewis base catalysts and produce suitably protected derivatives of enantioenriched chiral β-amino acids. The design of the latent pronucleophile featuring both a silyl group and an electron-deficient carbamate is instrumental in lowering the nucleophilicity of nitrogen and enabling enantioselective allylation in the presence of chiral cinchona alkaloid-based catalysts.

Stereoselective Synthesis of α-Disubstituted β-Homoprolines

An efficient enantioselective synthesis of chiral α-disubstituted β-homoprolines was developed, starting with the stereodivergent allylation of chiral N-tert-butanesulfinyl imines derived from 4-bromobutanal with indium or zinc and using well-established and reliable synthetic transformations. This methodology allows the easy introduction of different substituents at the α-position of the pyrrolidine scaffold and is characterized by the possibility of switching the absolute configuration of the newly formed stereocenter either by changing the configuration of the tert-butanesufinamide chiral auxiliary or by using a different stereodivergent allylation protocol with the same auxiliary.

Modular enantioselective access to β-amino amides by Brønsted acid-catalysed multicomponent reactions

β-Amino acids are structural motifs widely found in therapeutic natural products, novel biomimetic polymers and peptidomimetics. As a convergent method, the synthesis of stereoenriched β-amino amides through the asymmetric Mannich reaction requires specialized amide substrates or a metal catalyst for enolate formation. By a redesign of the Ugi reaction, a conceptually different solution to prepare chiral β-amino amides was established using ambiphilic ynamides as two-carbon synthons. The modulation of ynamides or oxygen nucleophiles concisely furnished three classes of β-amino amides with generally good efficiency as well as excellent chemo- and stereo-control. The utility is verified in the preparation of over 100 desired products that bear one or two contiguous carbon stereocentres, including those that directly incorporate drug molecules. This advance also provides a synthetic shortcut to other valuable structures. The amino amides could be elaborated into β-amino acids, anti-vicinal diamines, γ-amino alcohols and β-lactams or undergo transamidation with amino acids and amine-containing pharmaceuticals.

Synthesis, conformation and cytotoxic activity of short hybrid peptides containing conformationally constrained 1-(aminomethyl)cyclohexanecarboxylic acid and gabapentin

The present work describes the synthesis,conformation and cytotoxic activities of short β/γ hybrid peptides, Boc-β^2,2-Ac₆c-Gpn-NHMe, BG1; Boc-(β^2,2-Ac₆c-Gpn)₂-OMe, BG2; Boc-(β^2,2-Ac₆c-Gpn)₃-OMe, BG3; H-β^2,2-Ac₆c-Gpn-NHMe, BG4; H-(β^2,2-Ac₆c-Gpn)₂-OMe, BG5; H-(β^2,2-Ac₆c-Gpn)₃-OMe, BG6, Boc-β^2,2-Ac₆c-Gpn-OMe, BG7 and H-β^2,2-Ac₆c-Gpn-OMe, BG8. Mixed C₆/C₇ conformations were observed for β/γ hybrid peptides. Further, BG1-BG8 were screened against MCF-7 (Breast cancer), A549 (Lung Cancer), PC-3 (Prostate cancer), HCT-116 (Colon cancer), and MDA-MB-231 (Breast cancer) cell lines. Among all, BG6 exhibited potent cytotoxicity against all cancer cell lines with IC₅₀ ranging from 1.6 μM to 6.3 μM with relatively low cytotoxicity against normal epithelial breast cell line fR-2 and human embryonic kidney cell line HEK-293. Minimal hemolytic activity was observed for BG6 against human erythrocytes. Peptide BG6 displayed anti-migratory and anti-invasive potentials showing strong interactions with intrinsic apoptotic markers Bcl-2, Bax, and cleaved-PARP, as well as the induction of the mitochondria maladjustment mediated apoptosis.

In Vitro Selection of Macrocyclic α/β3-Peptides against Human EGFR

Here, we report ribosomal construction of thioether-macrocyclic α/β3-peptide libraries in which β-homoglycine, β-homoalanine, β-homophenylglycine, and β-homoglutamine are introduced by genetic code reprogramming. The libraries were applied to the RaPID (Random nonstandard Peptides Integrated Discovery) selection against human EGFR to obtain PPI (protein-protein interaction) inhibitors. The resulting peptides contained up to five β3-amino acid (β3AA) residues and exhibited outstanding binding affinity, PPI inhibitory activity, and proteolytic stability, which were attributed to the β3AAs included in the peptides. This showcase work has demonstrated that the use of such β3AAs enhances the drug-like properties of peptides, providing a unique platform for the discovery of de novo macrocycles against a protein of interest.

5-Amino-3-methyl-Isoxazole-4-carboxylic Acid as a Novel Unnatural Amino Acid in the Solid Phase Synthesis of α/β-Mixed Peptides

The hybrid peptides consisting of α and β-amino acids show great promise as peptidomimetics that can be used as therapeutic agents. Therefore, the development of new unnatural amino acids and the methods of their incorporation into the peptide chain is an important task. Here, we described our investigation of the possibility of 5-amino-3-methyl-isoxazole-4-carboxylic acid (AMIA) application in the solid phase peptide synthesis. This new unnatural β-amino acid, presenting various biological activities, was successfully coupled to a resin-bound peptide using different reaction conditions, including classical and ultrasonic agitated solid-phase synthesis. All the synthesized compounds were characterized by tandem mass spectrometry. The obtained results present the possibility of the application of this β-amino acid in the synthesis of a new class of bioactive peptides.

“Cut and Paste” Processes in the Search of Bioactive Products: One-Pot, Metal-free O-Radical Scission-Oxidation-Addition of C, N or P-Nucleophiles

Hypervalent iodine reagents have been applied in many metal-free, efficient synthesis of natural products and other bioactive compounds. In particular, treatment of alcohols, acetals and acids with hypervalent iodine reagents and iodine results in O-radicals that can undergo a β-scission reaction. Under these oxidative conditions, derivatives of amino acids, peptides or carbohydrates are converted into cationic intermediates, which can subsequently undergo inter- or intramolecular addition of nucleophiles. Most reported papers describe the addition of oxygen nucleophiles, but this review is focused on the addition of carbon, nitrogen and phosphorous nucleophiles. The resulting products (nucleoside and alkaloid analogs, unnatural amino acids, site-selectively modified peptides) are valuable intermediates or analogs of bioactive compounds.

Angiotensin Type-2 Receptors: Transducers of Natriuresis in the Renal Proximal Tubule

Angiotensin II (Ang II) type-2 receptors (AT₂R) are expressed in the adult kidney, prominently in renal proximal tubule cells (RPTCs), and play an important role in opposing renal sodium (Na⁺) retention induced by Ang II stimulation of Ang II type-1 receptor (AT₁R). Natriuresis induced by AT₁R blockade is due at least in part to AT₂R activation and whole body deletion of AT₂Rs reduces the natriuretic response to increased blood pressure (BP). The major endogenous AT₂R agonist mediating the natriuretic response is Ang III, the Ang II heptapeptide metabolite generated by aminopeptidase A, and the principal nephron site mediating inhibition of Na⁺ reabsorption by the AT₂R is the renal proximal tubule (RPT). AT₂Rs induce natriuresis via a bradykinin, nitric oxide and cyclic GMP (cGMP) signaling cascade. Recent studies demonstrated a key role for protein phosphatase 2A (PP2A) in the AT₂R-mediated natriuretic response upstream of cGMP. By inducing natriuresis, AT₂Rs lower BP in the Ang II-infusion model of hypertension. PP2A activation and the natriuretic response to AT₂R stimulation are defective in spontaneously hypertensive rats, a model of primary hypertension in humans. AT₂R agonists are candidates for proximal tubule natriuretic agents in Na⁺ and fluid retention disorders.

Polyhydroxylated Cyclopentane β-Amino Acids Derived from d-Mannose and d-Galactose: Synthesis and Protocol for Incorporation into Peptides

A stereoselective synthesis of polyhydroxylated cyclopentane β-amino acids from hexoses is reported. The reaction sequence comprises, as key steps, ring-closing metathesis of a polysubstituted diene intermediate followed by the stereoselective aza-Michael functionalization of the resulting cyclopent-1-ene-1-carboxylic acid ester. Examples of synthesis of polysubstituted 2-aminocyclopentanecarboxylic acid derivatives starting from protected d-mannose and d-galactose are presented. A general protocol for the incorporation of these highly functionalized alicyclic β-amino acids into peptides is also reported.

Enantioseparation of ß2-amino acids by liquid chromatography using core-shell chiral stationary phases based on teicoplanin and teicoplanin aglycone

Enantioseparation of nineteen ß²-amino acids has been performed by liquid chromatography on chiral stationary phases based on native teicoplanin and teicoplanin aglycone covalently bonded to 2.7 µm superficially porous silica particles. Separations were carried out in unbuffered (water/methanol), buffered [aqueous triethylammonium acetate (TEAA)/methanol] reversed-phase (RP) mode, and in polar-ionic (TEAA containing acetonitrile/methanol) mobile phases. Effects of pH in the RP mode, acid and salt additives, as well as counter-ion concentrations on chromatographic parameters have been studied. The structure of selectands (ß²-amino acids possessing aliphatic or aromatic side chains) and selectors (native teicoplanin or teicoplanin aglycone) was found to have a considerable influence on separation performance. Analysis of van Deemter plots and determination of thermodynamic parameters were performed to further explore details of the separation performance.

Ribosomal Synthesis of Macrocyclic Peptides with β2- and β2,3-Homo-Amino Acids for the Development of Natural Product-Like Combinatorial Libraries

The development of large, natural-product-like, combinatorial macrocyclic peptide libraries is essential in the quest to develop therapeutics for "undruggable" cellular targets. Herein we report the ribosomal synthesis of macrocyclic peptides containing one or more β²-homo-amino acids (β²haa) to enable their incorporation into mRNA display-based selection libraries. We confirmed the compatibility of 14 β²-homo-amino acids, (S)- and (R)-stereochemistry, for single incorporation into a macrocyclic peptide with low to high translation efficiency. Interestingly, N-methylation of the backbone amide of β²haa prevented the incorporation of this amino acid subclass by the ribosome. Additionally, we designed and incorporated several α,β-disubstituted β^2,3-homo-amino acids (β^2,3haa) with different R-groups on the α- and β-carbons of the same amino acid. Incorporation of these β^2,3haa enables increased diversity in a single position of a macrocyclic peptide without significantly increasing the overall molecular weight, which is an important consideration for passive cell permeability. We also successfully incorporated multiple (S)-β²hAla into a single macrocycle with other non-proteinogenic amino acids, confirming that this class of β-amino acid is suitable for development of large scale macrocyclic peptide libraries.

Catalytic Asymmetric Synthesis of Unprotected β2-Amino Acids

We report here a scalable, catalytic one-pot approach to enantiopure and unmodified β²-amino acids. A newly developed confined imidodiphosphorimidate (IDPi) catalyzes a broadly applicable reaction of diverse bis-silyl ketene acetals with a silylated aminomethyl ether, followed by hydrolytic workup, to give free β²-amino acids in high yields, purity, and enantioselectivity. Importantly, both aromatic and aliphatic β²-amino acids can be obtained using this method. Mechanistic studies are consistent with the aminomethylation to proceed via silylium-based asymmetric counteranion-directed catalysis (Si-ACDC) and a transition state to explain the enantioselectivity is suggested on the basis of density functional theory calculation.

"Doubly Customizable" Unit for the Generation of Structural Diversity: From Pure Enantiomeric Amines to Peptide Derivatives

Readily available, low-cost 4R-hydroxy-l-proline (Hyp) is introduced as a "doubly customizable" unit for the generation of libraries of structurally diverse compounds. Hyp can be cleaved at two points, followed by the introduction of new functionalities. In the first cycle, the removal and replacement of the carboxylic group are carried out, followed (second cycle) by the scission of the 4,5-position and manipulation of the resulting chains. In this way, three new chains are generated and can be transformed independently to afford a diversity of products with tailored substituents, such as β-amino aldehydes, diamines, β-amino acid derivatives, including N-alkylated ones, or modified peptides. Many of these products are high-profit compounds but, in spite of their commercial value, are still scarce. Moreover, the process takes place with stereochemical control, and either pure R or S isomers can be obtained with small variations of the synthetic route.

Site-selective modification of peptide backbones

Exciting developments in the site-selective modification of peptide backbones are allowing an outstanding fine-tuning of peptide conformation, folding ability, and physico-chemical and biological properties.

Forming All-Carbon Quaternary Stereocenters by Organocatalytic Aminomethylation: Concise Access to β2,2 -Amino Acids

The asymmetric synthesis of β^2,2 -amino acids remains a formidable challenge in organic synthesis. Here a novel organocatalytic enantioselective aminomethylation of ketenes with stable and readily available N,O-acetals is reported, providing β^2,2 -amino esters bearing an all-carbon quaternary stereogenic center in high enantiomeric ratios with a catalytic amount of chiral phosphoric acid. Typically, this transformation probably proceeds through an asymmetric counter-anion-directed catalysis. As a result, a concise, practical, and atom-economic protocol toward rapidly access to β^2,2 -amino acids has been developed.

Renal functional effects of the highly selective AT2R agonist, β-Pro7 Ang III, in normotensive rats

Recently, we designed a group of peptides by sequential substitution of the naturally occurring α-amino acid throughout the Ang III peptide sequence with the corresponding β-amino acid. β-Amino acid substitution at the proline residue of Ang III (β-Pro⁷-Ang III) resulted in a highly selective AT₂R ligand, demonstrating remarkable selectivity for the AT₂R in both binding and functional studies. To provide additional functional evidence for the suitability of β-Pro⁷ Ang III as a novel AT₂R agonist, we tested effects of acute systemic administration of β-Pro⁷-Ang III on renal hemodynamic and excretory function in anesthetized normotensive male and female rats. We also compared the natriuretic effects of acute intrarenal administration of native Ang III and β-Pro⁷-Ang III in the presence of systemic AT₁R blockade in anesthetized female rats to allow for the differentiation of systemic versus direct intrarenal natriuretic actions of β-Pro⁷-Ang III. In both male and female rats, acute systemic administration of β-Pro⁷-Ang III elicited renal vasodilatation and natriuresis. Notably, greater renal vasodilatory effects were observed in female versus male rats at the highest dose of β-Pro⁷-Ang III administered. Moreover, intra-renal administration of β-Pro⁷-Ang III produced significant natriuretic effects in female rats and, like Ang III, evoked AT₂R translocation to the apical plasma membrane in renal proximal tubular cells. Taken together, our findings support the use of β-Pro⁷-Ang III as a novel AT₂R agonist and experimental tool for exploring AT₂R function and its potential as a therapeutic target. Furthermore, our findings provide further evidence of a sex-specific influence of AT₂R stimulation on renal function.

Organocatalytic Multicomponent Synthesis of α/β-Dipeptide Derivatives

A straightforward multicomponent Knoevenagel-aza-Michael-cyclocondensation reaction involving readily available hydroxamic acid-derived from naturally occurring α-amino acids allows a diversity-oriented synthesis of novel isoxazolidin-5-ones possessing an N-protected α-amino acid pendant with good to high diastereoselectivities thanks to a match effect with a chiral organocatalyst. These diversely substituted heterocycles, easily isolated as a single diastereoisomer, proved to be versatile platforms for the formation of an array of α/β-dipeptide fragments.

New Morphiceptin Peptidomimetic Incorporating (1S,2R,3S,4S,5R)-2-Amino-3,4,5-trihydroxycyclopen-tane-1-carboxylic acid: Synthesis and Structural Study

We present the synthesis and structural study of a new peptidomimetic of morphiceptin, which can formally be considered as the result of the replacement of the central proline residue of this natural analgesic drug with a subunit of (1S,2R,3S,4S,5R)-2-amino-3,4,5-trihydroxycyclopentane-1-carboxylic acid, previously obtained from L-idose. An optimized synthesis of this trihydroxylated cispentacin derivative is also reported. Molecular docking calculations on the target receptor support a favorable role of the hydroxy substituents of the non-natural β-amino acid incorporated into the peptidomimetic.

Local versus Global Control of Helical Folding in β-Peptide Segments Using Hydrazino Turns

Rational control of the self-organization of β-peptides sequences to adopt regular secondary structures is an important challenge in peptidomimetic foldamer science. By replacing the N- and C-terminal residues of homooligomers of trans-2-aminocyclobutanecarboxylic acid (tACBC)_n with N-aminoazetidine-2-carboxylic acid, an 8-helical topology is shown to dominate for sequences up to n = 7. This constitutes an atomic-level tool to override locally the preferred global 12-helix secondary structure of the corresponding tACBC homooligomers of the same length.

Enantioselective catalytic synthesis of α-aryl-α-SCF3-β2,2-amino acids

We herein report a novel entry towards chiral α-SCF3-β2,2-amino acids by carrying out the ammonium salt-catalyzed α-trifluoromethylthiolation of isoxazolidin-5-ones. This approach allowed for high enantioselectivities and high yields and the obtained heterocycles proved to be versatile platforms to access other targets of potential interest.

Peptidomimetic Modulators of BACE1

The β-site APP Cleaving enzyme 1 (BACE1) is a membrane-associated aspartyl protease which mediates the production of amyloid-β (Aβ), a major component of amyloid plaques in the Alzheimer’s disease brain. We have synthesised and characterised a series of peptidomimetic analogues of BACE substrates that incorporate two distinct stabilising structures. To demonstrate the potential activity of these compounds, a variety of assaying strategies were used to investigate cleavage susceptibility and inhibition potency under competitive and non-competitive conditions. β-Amino acids and scissile site N-methylation were incorporated into peptide substrate templates as transition state isostere (TSI) substitutes by positional scanning to generate series of non-TSI β-peptidomimetics. The amino acid sequences flanking the β-cleavage site within APP carrying the Swedish double mutation (APPSW), Neuregulin, the synthetic hydroxyethylene-based TSI peptide inhibitor OM99-2, and the high affinity peptide sequence SEISYEVEFR, served as the four substrate templates from which over 60 peptides were designed and synthesised by solid phase peptide synthesis. A quenched fluorescent substrate BACE1 assay in conjunction with liquid chromatography–mass spectrometry (LC-MS) analysis was established to investigate cleavage susceptibility and inhibition potency under competitive and non-competitive conditions. It was determined that β-amino acids substituted at the P1 scissile site position within known peptide substrates were resistant to proteolysis, and particular substitutions induced a concentration-dependent stimulation of BACE1, indicating a possible modulatory role of native BACE1 substrates.

Quaternary β2,2-amino acid derivatives by asymmetric addition of isoxazolidin-5-ones to para-quinone methides

The highly enantioselective (>99.5% ee) synthesis of a new class of densely functionalized β^2,2-amino acid derivatives by reacting isoxazolidin-5-ones with para-quinone methides in the presence of chiral ammonium salt phase-transfer catalysts was developed. The reaction proceeds with exceptionally low catalyst loadings down to 20 ppm on gram scale and the utilization of the primary addition products towards further manipulations was demonstrated for selected examples.

Organocatalytic Enantioselective Mukaiyama-Mannich Reaction of Isatin-Derived Ketimines for the Synthesis of Oxindolyl-β3, 3 -Amino Acid Esters

Mukaiyama-Mannich reactions of ester enolate equivalents with aldimines have been elegantly used for the asymmetric synthesis of β-amino acids; nevertheless, the corresponding asymmetric reaction employing ketimines are unexplored. Herein, the first organocatalytic enantioselective Mukaiyama-Mannich reaction employing isatin-derived ketimines with unsubstituted silyl ketene acetals is disclosed towards the scalable synthesis of 2-oxoindolinyl-β^3, 3 -amino acid esters at room temperature with excellent enantioselectivities (ee >99.5 %). Ultra-low catalyst loadings (as low as 250 ppm) could be used for the quantitative product formation with high enantiopurity. The synthetic utility of this protocol has been showcased in the short formal synthesis of pharmaceutically demanded (+)-AG-041R, a potent gastrin/CCK-B receptor antagonist.

Migration and Differentiation of Neural Stem Cells Diverted From the Subventricular Zone by an Injectable Self-Assembling β-Peptide Hydrogel

Neural stem cells, which are confined in localised niches are unable to repair large brain lesions because of an inability to migrate long distances and engraft. To overcome these problems, previous research has demonstrated the use of biomaterial implants to redirect increased numbers of endogenous neural stem cell populations. However, the fate of the diverted neural stem cells and their progeny remains unknown. Here we show that neural stem cells originating from the subventricular zone can migrate to the cortex with the aid of a long-lasting injectable hydrogel within a mouse brain. Specifically, large numbers of neuroblasts were diverted to the cortex through a self-assembling β-peptide hydrogel that acted as a tract from the subventricular zone to the cortex of transgenic mice (NestinCreER^T2:R26eYFP) in which neuroblasts and their progeny are permanently fluorescently labelled. Moreover, neuroblasts differentiated into neurons and astrocytes 35 days post implantation, and the neuroblast-derived neurons were Syn1 positive suggesting integration into existing neural circuitry. In addition, astrocytes co-localised with neuroblasts along the hydrogel tract, suggesting that they assisted migration and simulated pathways similar to the native rostral migratory stream. Lower levels of astrocytes were found at the boundary of hydrogels with encapsulated brain-derived neurotrophic factor, comparing with hydrogel implants alone.

β-Aminopeptidases: Insight into Enzymes without a Known Natural Substrate

β-Aminopeptidases have the unique capability to hydrolyze N-terminal β-amino acids, with varied preferences for the nature of β-amino acid side chains. This unique capability makes them useful as biocatalysts for synthesis of β-peptides and to kinetically resolve β-peptides and amides for the production of enantiopure β-amino acids. To date, six β-aminopeptidases have been discovered and functionally characterized, five from Gram-negative bacteria and one from a fungus, Aspergillus Here we report on the purification and characterization of an additional four β-aminopeptidases, one from a Gram-positive bacterium, Mycolicibacterium smegmatis (BapAMs), one from a yeast, Yarrowia lipolytica (BapAYlip), and two from Gram-negative bacteria isolated from activated sludge identified as Burkholderia spp. (BapABcA5 and BapABcC1). The genes encoding β-aminopeptidases were cloned, expressed in Escherichia coli, and purified. The β-aminopeptidases were produced as inactive preproteins that underwent self-cleavage to form active enzymes comprised of two different subunits. The subunits, designated α and β, appeared to be tightly associated, as the active enzyme was recovered after immobilized-metal affinity chromatography (IMAC) purification, even though only the α-subunit was 6-histidine tagged. The enzymes were shown to hydrolyze chromogenic substrates with the N-terminal l-configurations β-homo-Gly (βhGly) and β3-homo-Leu (β3hLeu) with high activities. These enzymes displayed higher activity with H-βhGly-p-nitroanilide (H-βhGly-pNA) than previously characterized enzymes from other microorganisms. These data indicate that the new β-aminopeptidases are fully functional, adding to the toolbox of enzymes that could be used to produce β-peptides. Overexpression studies in Pseudomonas aeruginosa also showed that the β-aminopeptidases may play a role in some cellular functions.IMPORTANCE β-Aminopeptidases are unique enzymes found in a diverse range of microorganisms that can utilize synthetic β-peptides as a sole carbon source. Six β-aminopeptidases have been previously characterized with preferences for different β-amino acid substrates and have demonstrated the capability to catalyze not only the degradation of synthetic β-peptides but also the synthesis of short β-peptides. Identification of other β-aminopeptidases adds to this toolbox of enzymes with differing β-amino acid substrate preferences and kinetics. These enzymes have the potential to be utilized in the sustainable manufacture of β-amino acid derivatives and β-peptides for use in biomedical and biomaterial applications. This is important, because β-amino acids and β-peptides confer increased proteolytic resistance to bioactive compounds and form novel structures as well as structures similar to α-peptides. The discovery of new enzymes will also provide insight into the biological importance of these enzymes in nature.

Novel Materials From the Supramolecular Self-Assembly of Short Helical β3-Peptide Foldamers

Self-assembly is the spontaneous organization of small components into higher-order structures facilitated by the collective balance of non-covalent interactions. Peptide-based self-assembly systems exploit the ability of peptides to adopt distinct secondary structures and have been used to produce a range of well-defined nanostructures, such as nanotubes, nanofibres, nanoribbons, nanospheres, nanotapes, and nanorods. While most of these systems involve self-assembly of α-peptides, more recently β-peptides have also been reported to undergo supramolecular self-assembly, and have been used to produce materials-such as hydrogels-that are tailored for applications in tissue engineering, cell culture and drug delivery. This review provides an overview of self-assembled peptide nanostructures obtained via the supramolecular self-assembly of short β-peptide foldamers with a specific focus on N-acetyl-β3-peptides and their applications as bio- and nanomaterials.

The inverted ketene synthon: a double umpolung approach to enantioselective β2,3-amino amide synthesis

A stereocontrolled synthesis of β2,3-amino amides is reported. Innovation is encapsulated by the first use of nitroalkenes to achieve double umpolung in enantioselective β-amino amide synthesis. Step economy is also fulfilled by the use of Umpolung Amide Synthesis (UmAS) in the second step, delivering the amide product without intermediacy of a carboxylic acid or activated derivative. Molybdenum oxide-mediated hydride reduction provides the anti-β2,3-amino amide with high selectivity.

Evaluation of β-Amino Acid Replacements in Protein Loops: Effects on Conformational Stability and Structure

β-Amino acids have a backbone that is expanded by one carbon atom relative to α-amino acids, and β residues have been investigated as subunits in protein-like molecules that adopt discrete and predictable conformations. Two classes of β residue have been widely explored in the context of generating α-helix-like conformations: β³ -amino acids, which are homologous to α-amino acids and bear a side chain on the backbone carbon adjacent to nitrogen, and residues constrained by a five-membered ring, such the one derived from trans-2-aminocyclopentanecarboxylic acid (ACPC). Substitution of α residues with their β³  homologues within an α-helix-forming sequence generally causes a decrease in conformational stability. Use of a ring-constrained β residue, however, can offset the destabilizing effect of α→β substitution. Here we extend the study of α→β substitutions, involving both β³ and ACPC residues, to short loops within a small tertiary motif. We start from previously reported variants of the Pin1 WW domain that contain a two-, three-, or four-residue β-hairpin loop, and we evaluate α→β replacements at each loop position for each variant. By referral to the ϕ,ψ angles of the native structure, one can choose a stereochemically appropriate ACPC residue. Use of such logically chosen ACPC residues enhances conformational stability in several cases. Crystal structures of three β-containing Pin1 WW domain variants show that a native-like tertiary structure is maintained in each case.

Synthesis and structural investigation of 2-aminomethyl-3-(4-methoxy-phenyl)-propionic acid containing a peptide analogue of the amyloidogenic AS(6–7) sequence: inhibition of fibril formation

The incorporation of a β-amino acid, namely 2-aminomethyl-3-(4-methoxy-phenyl)-propionic acid, inhibits amyloid-like fibril formation.

Effects of Single α-to-β Residue Replacements on Structure and Stability in a Small Protein: Insights from Quasiracemic Crystallization

Synthetic peptides that contain backbone modifications but nevertheless adopt folded structures similar to those of natural polypeptides are of fundamental interest and may provide a basis for biomedical applications. Such molecules can, for example, mimic the ability of natural prototypes to bind to specific target macromolecules but resist degradation by proteases. We have previously shown that oligomers containing mixtures of α- and β-amino acid residues ("α/β-peptides") can mimic the α-helix secondary structure, and that properly designed α/β-peptides can bind to proteins that evolved to bind to α-helical partners. Here we report fundamental studies that support the long-range goal of extending the α/β approach to tertiary structures. We have evaluated the impact of single α → β modifications on the structure and stability of the small and well-studied villin headpiece subdomain (VHP). The native state of this 35-residue polypeptide contains several α-helical segments packed around a small hydrophobic core. We examined α → β substitution at four solvent-exposed positions, Asn19, Trp23, Gln26 and Lys30. In each case, both the β(3) homologue of the natural α residue and a cyclic β residue were evaluated. All α → β(3) substitutions caused significant destabilization of the tertiary structure as measured by variable-temperature circular dichroism, although at some of these positions, replacing the β(3) residue with a cyclic β residue led to improved stability. Atomic-resolution structures of four VHP analogues were obtained via quasiracemic crystallization. These findings contribute to a fundamental α/β-peptide knowledge-base by confirming that β(3)-amino acid residues can serve as effective structural mimics of homologous α-amino acid residues within a natural tertiary fold, which should support rational design of functional α/β analogues of natural poly-α-peptides.

A self-assembling β-peptide hydrogel for neural tissue engineering

We report a new class of β-peptide based hydrogel for neural tissue engineering. Our β-peptide forms a network of nanofibres in aqueous solution, resulting in a stable hydrogel at physiological conditions. The hydrogel shows excellent compatibility with neural cells and provides a suitable environment for cells to adhere and proliferate.

The Bacterial Ammonia Lyase EncP: A Tunable Biocatalyst for the Synthesis of Unnatural Amino Acids

Enzymes of the class I lyase-like family catalyze the asymmetric addition of ammonia to arylacrylates, yielding high value amino acids as products. Recent examples include the use of phenylalanine ammonia lyases (PALs), either alone or as a gateway to deracemization cascades (giving (S)- or (R)-α-phenylalanine derivatives, respectively), and also eukaryotic phenylalanine aminomutases (PAMs) for the synthesis of the (R)-β-products. Herein, we present the investigation of another family member, EncP from Streptomyces maritimus, thereby expanding the biocatalytic toolbox and enabling the production of the missing (S)-β-isomer. EncP was found to convert a range of arylacrylates to a mixture of (S)-α- and (S)-β-arylalanines, with regioselectivity correlating to the strength of electron-withdrawing/-donating groups on the ring of each substrate. The low regioselectivity of the wild-type enzyme was addressed via structure-based rational design to generate three variants with altered preference for either α- or β-products. By examining various biocatalyst/substrate combinations, it was demonstrated that the amination pattern of the reaction could be tuned to achieve selectivities between 99:1 and 1:99 for β:α-product ratios as desired.