Peptide cyclization and cyclodimerization by Cu(I)-mediated azide-alkyne cycloaddition

Access to Advanced Functional Materials through Postmodification of Biomimetic Assemblies via Click Chemistry

The design, synthesis, and fabrication of functional nanomaterials with specific properties remain a long-standing goal for many scientific fields. The self-assembly of sequence-defined biomimetic synthetic polymers presents a fundamental strategy to explore the chemical space beyond biological systems to create advanced nanomaterials. Moreover, subsequent chemical modification of existing nanostructures is a unique approach for accessing increasingly complex nanostructures and introducing functionalities. Of these modifications, covalent conjugation chemistries, such as the click reactions, have been the cornerstone for chemists and materials scientists. Herein, we highlight some recent advances that have successfully employed click chemistries for the postmodification of assembled one-dimensional (1D) and two-dimensional (2D) nanostructures to achieve applications in molecular recognition, mineralization, and optoelectronics. Specifically, biomimetic nanomaterials assembled from sequence-defined macromolecules such as peptides and peptoids are described.

Pyrrolysine-Inspired in Cellulo Synthesis of an Unnatural Amino Acid for Facile Macrocyclization of Proteins

Macrocyclization has been touted as an effective strategy to enhance the in vivo stability and efficacy of protein therapeutics. Herein, we describe a scalable and robust system based on the endogenous biosynthesis of a noncanonical amino acid coupled to the pyrrolysine translational machinery for the generation of lasso-grafted proteins. The in cellulo biosynthesis of the noncanonical amino acid d-Cys-ε-Lys was achieved by hijacking the pyrrolysine biosynthesis pathway, and then, its genetical incorporation into proteins was performed using an optimized PylRS/tRNA^Pyl pair and cell line. This system was then applied to the structurally inspired cyclization of a 23-mer therapeutic P16 peptide engrafted on a fusion protein, resulting in near-complete cyclization of the target cyclic subunit in under 3 h. The resulting cyclic P16 peptide fusion protein possessed much higher CDK4 binding affinity than its linear counterpart. Furthermore, a bifunctional bicyclic protein harboring a cyclic cancer cell targeting RGD motif on the one end and the cyclic P16 peptide on the other is produced and shown to be a potent cell cycle arrestor with improved serum stability.

Antifungal Activity, Structure-Activity Relationship and Molecular Docking Studies of 1,2,4-Triazole Schiff Base Derivatives

Fourteen novel Schiff base compounds (AS-1∼AS-14) containing 5-amino-1H-1,2,4-triazole-3-carboxylic acid and substituted benzaldehyde were successfully synthesized, and their structures were verified by melting point, elemental analysis (EA) and spectroscopic techniques (Fourier Transform Infra-Red (FT-IR) and Nuclear Magnetic Resonance (NMR)). In vitro hyphal measurements were used to investigate the antifungal activities of the synthesised compounds against Wheat gibberellic, Maize rough dwarf and Glomerella cingulate. The preliminary studies indicated that all compounds had good inhibitory effect on Wheat gibberellic and Maize rough dwarf, among which the compounds of AS-1 (7.44 mg/L, 7.27 mg/L), AS-4 (6.80 mg/L, 9.57 mg/L) and AS-14 (5.33 mg/L, 6.53 mg/L) showed better antifungal activity than that of the standard drug fluconazole (7.66 mg/L, 6.72 mg/L); while inhibitory effect against Glomerella cingulate was poor, only AS-14 (5.67 mg/L) was superior to that of fluconazole (6.27 mg/L). The research of structure-activity relationship exhibited that the introduction of halogen elements on the benzene ring and electron withdrawing groups at the 2,4,5 positions on the benzene ring was beneficial to the improvement of the activity against Wheat gibberellic, while the large steric hindrance was not conducive to the improvement of the activity. Additionally, except for AS-1, AS-3 and AS-10, the other compounds had one or several ratio systems to achieve synergistic effect after recombination with pyrimethamine, among which AS-7 had significant synergistic effect and was expected to be a combinated agent with application prospects. Finally, the molecular docking results of isocitrate lyase with Wheat gibberellic displayed that the presence of hydrogen bonds enabled stable binding of compounds to receptor proteins, and the residues of ARG A: 252, ASN A: 432, CYS A: 215, SER A: 436 and SER A: 434 were the key residues for their binding. Comparing the docking binding energy and biological activity results, it was revealed that the lower the docking binding energy was, the stronger the inhibitory ability of the Wheat gibberellic, when the same position on the benzene ring was substituted.

1,5-Benzodiazepines as a platform for the design of carbonic anhydrase inhibitors

A series of novel N-triazolo-benzene sulfonamides-1,5-benzodiazepines 9a-d and 10d were designed and prepared through the copper-catalyzed azide alkyne cycloaddition click chemistry procedure, reacting the N₁ -propargyl-1,5-benzodiazepine 2 and the N₁ ,N₅ -dipropargyl analog 6 with various benzene sulfonamide azides 8a-d. The synthesized compounds were found to show nanomolar affinity toward relevant isoforms of human carbonic anhydrase such as hCA I, II, IV, VII, IX, and XII. The divalent derivative 10d showed a particularly high inhibitory activity against all hCA isoforms when compared with acetazolamide, and showed potent multivalent effects, better than reported previously for divalent CA inhibitors.

Diels-Alder Cycloadditions for Peptide Macrocycle Formation

Macrocyclization can confer enhanced stability, target affinity, and membrane permeability to peptide scaffolds, all of which are desirable properties for chemical probes and therapeutics. A wide array of macrocyclization chemistries have been reported over the last few decades; however, these often have limited compatibility with each other and across chemical environments, thus restricting access to specific molecular properties. In an effort to address some of these limitations, we recently described the use of Diels-Alder [4 + 2] cycloadditions for peptide macrocyclization. Among the attributes of this chemistry, we demonstrated that Diels-Alder cyclization can template diverse peptide secondary structures, proceed in organic or aqueous environments, and endow improved pharmacologic properties on cyclized peptides. Here, we present synthetic processes and characterization methods for the synthesis of Diels-Alder cyclized peptides.

Macrocyclization strategies for cyclic peptides and peptidomimetics

Peptides are a growing therapeutic class due to their unique spatial characteristics that can target traditionally "undruggable" protein-protein interactions and surfaces. Despite their advantages, peptides must overcome several key shortcomings to be considered as drug leads, including their high conformational flexibility and susceptibility to proteolytic cleavage. As a general approach for overcoming these challenges, macrocyclization of a linear peptide can usually improve these characteristics. Their synthetic accessibility makes peptide macrocycles very attractive, though traditional synthetic methods for macrocyclization can be challenging for peptides, especially for head-to-tail cyclization. This review provides an updated summary of the available macrocyclization chemistries, such as traditional lactam formation, azide-alkyne cycloadditions, ring-closing metathesis as well as unconventional cyclization reactions, and it is structured according to the obtained functional groups. Keeping peptide chemistry and screening in mind, the focus is given to reactions applicable in solution, on solid supports, and compatible with contemporary screening methods.

Metabolic and conformational stabilization of a VEGF-mimetic beta-hairpin peptide by click-chemistry

HPLW is a Vascular Endothelial Growth Factor (VEGF)-mimicking beta-hairpin peptide endowed of proangiogenic effect and showing valuable biomedical application in the proangiogenic therapy. However, the translational potential of HPLW is limited by its low metabolic stability, which would shorten the in vivo efficacy of the molecule. Here, we developed a peptide analog of HPLW, named HPLW2, that retains the structural and biological properties of the original peptide but features an impressive resistance to degradation by human serum proteases. HPLW2 was obtained by covalently modifying the chemical structure of the peptide with molecular tools known to impart protease resistance. Notably, the peptide was cyclized by installing an interstrand triazole bridge through Cu(I)-catalyzed azide-alkyne 1,3-dipolar cycloaddition (CuAAC) reaction. HPLW2 appears as a novel and promising drug candidate with potential biomedical application in the proangiogenic therapy as a low molecular weight drug, alternative to the use of VEGF. Our work points out the utility of the interstrand triazole bridge as effective chemical platform for the conformational and metabolic stabilization of beta-hairpin bioactive peptides.

High-Throughput Quality Control Assay for the Solid-Phase Synthesis of DNA-Encoded Libraries of Macrocycles*

There is considerable interest in the development of libraries of scaffold-diverse macrocycles as a source of ligands for difficult targets, such as protein-protein interaction surfaces. A classic problem in the synthesis of high-quality macrocyclic libraries is that some linear precursors will cyclize efficiently while some will not, depending on their conformational preferences. We report here a powerful quality control method that can be employed to readily distinguish between scaffolds that do and do not cyclize efficiently during solid-phase synthesis of thioether macrocycles without the need for tedious liquid chromatography/mass spectrometry analysis. We demonstrate that this assay can be employed to identify linear impurities in a DNA-encoded library of macrocycles. We also use the method to establish a useful quality control protocol for re-synthesis of putative macrocyclic screening hits.

Copper-ligand clusters dictate size of cyclized peptide formed during alkyne-azide cycloaddition on solid support

Peptide and peptidomimetic cyclization by copper-catalyzed alkyne-azide cycloaddition (CuAAC) reaction have been used to mimic disulfide bonds, alpha helices, amide bonds, and for one-bead-one-compound (OBOC) library development. A limited number of solid-supported CuAAC cyclization methods resulting in monomeric cyclic peptide formation have been reported for specific peptide sequences, but there exists no general study on monocyclic peptide formation using CuAAC cyclization. Since several cyclic peptides identified from an OBOC CuAAC cyclized library has been shown to have important biological applications, we discuss here an efficient method of alkyne-azide 'click' catalyzed monomeric cyclic peptide formation on a solid support. The reason behind the efficiency of the method is explored. CuAAC cyclization of a peptide sequence with azidolysine and propargylglycine is performed under various reaction conditions, with different catalysts, in the presence or absence of an organic base. The results indicate that piperidine plays a critical role in the reaction yield and monomeric cycle formation by coordinating to Cu and forming Cu-ligand clusters. A previously synthesized copper compound containing piperidine, [Cu₄I₄(pip)₄], is found to catalyze the CuAAC cyclization of monomeric peptide effectively. The use of 1.5 equivalents of CuI and the use of DMF as solvent is found to give optimal CuAAC cyclized monomer yields. The effect of the peptide sequence and peptide length on monomer formation are also investigated by varying either parameter systemically. Peptide length is identified as the determining factor for whether the monomeric or dimeric cyclic peptide is the major product. For peptides with six, seven, or eight amino acids, the monomer is the major product from CuAAC cyclization. Longer and shorter peptides on cyclization show less monomer formation. CuAAC peptide cyclization of non-optimal peptide lengths such as pentamers is affected significantly by the amino acid sequence and give lower yields.

Structural Diversity of Peptoids: Tube-Like Structures of Macrocycles

Peptoids, or poly-N-substituted glycines, are characterised by broad structural diversity. Compared to peptides, they are less restricted in rotation and lack backbone-derived H bonding. Nevertheless, certain side chains force the peptoid backbone into distinct conformations. Designable secondary structures like helices or nanosheets arise from this knowledge. Herein, we report the copper-catalysed alkyne-azide cycloaddition (CuAAC) of macrocycles to form innovative tube-like tricyclic peptoids, giving access to host-guest chemistry or storage applications. Different linker systems make the single tubes tuneable in size and enable modifications within the gap. An azobenzene linker, which is reversibly switchable in conformation, was successfully incorporated and allowed for light-triggered changes of the entire tricyclic structure.

Synthesis and Biological Evaluation of Hapten-Clicked Analogues of The Antigenic Peptide Melan-A/MART-126(27L)-35

A click-chemistry-based approach was implemented to prepare peptidomimetics designed in silico and made from aromatic azides and a propargylated GIGI-mimicking platform derived from the altered Melan-A/MART-126(27L)-35 antigenic peptide ELAGIGILTV. The CuI -catalyzed Huisgen cycloaddition was carried out on solid support to generate rapidly a first series of peptidomimetics, which were evaluated for their capacity to dock at the interface between the major histocompatibility complex class-I (MHC-I) human leucocyte antigen (HLA)-A2 and T-cell receptors (TCRs). Despite being a weak HLA-A2 ligand, one of these 11 first synthetic compounds bearing a p-nitrobenzyl-triazole side chain was recognized by the receptor proteins of Melan-A/MART-1-specific T-cells. After modification of the N and C termini of this agonist, which was intended to enhance HLA-A2 binding, one of the resulting seven additional compounds triggered significant T-cell responses. Thus, these results highlight the capacity of naturally circulating human TCRs that are specific for the native Melan-A/MART-126-35 peptide to cross-react with peptidomimetics bearing organic motifs structurally different from the native central amino acids.

Effects of Cyclization on Activity and Stability of α-Conotoxin TxIB

α-Conotoxin TxIB specifically blocked α6/α3β2β3 acetylcholine receptors (nAChRs), and it could be a potential probe for studying addiction and other diseases related to α6/α3β2β3 nAChRs. However, as a peptide, TxIB may suffer from low stability, short half-life, and poor bioavailability. In this study, cyclization of TxIB was used to improve its stability. Four cyclic mutants of TxIB (cTxIB) were synthesized, and the inhibition of these analogues on α6/α3β2β3 nAChRs as well as their stability in human serum were measured. All cyclized analogues had similar activity compared to wild-type TxIB, which indicated that backbone cyclization of TxIB had no significant effect on its activity. Cyclization of TxIB with a seven-residue linker improved its stability significantly in human serum. Besides this, the results showed that cyclization maintained the activity of α-conotoxin TxIB, which is conducive to its future application.

In Vivo and In Vitro Analyses of Novel Peptidomimetic Disruptors for the Serotonin 5-HT2C Receptor Interaction With Phosphatase and Tensin Homolog

Hypofunction of the serotonin (5-HT) 5-HT_2C receptor (5-HT_2CR) has been implicated in a variety of disorders including substance use disorders. As such, approaches to enhance 5-HT_2CR signaling display therapeutic potential. In the present study, we show that disruption of the 5-HT_2CR interaction with the protein phosphatase and tensin homolog (PTEN) via peptidomimetics enhances 5-HT_2CR-mediating signaling in vitro and potentiates selective 5-HT_2CR agonists in behavioral rodent models. Overall, the present study provides further evidence that 5-HT_2CR activity can be modulated through an allosteric protein-protein interaction. This work provides the groundwork for the continued exploration of protein-protein interactions that can allosterically modulate this critical receptor and other important G protein-coupled receptors (GPCRs) for new therapeutic development through mechanisms that may display clinical utility.

Ligation Technologies for the Synthesis of Cyclic Peptides

Cyclic peptides have been attracting a lot of attention in recent decades, especially in the area of drug discovery, as more and more naturally occurring cyclic peptides with diverse biological activities have been discovered. Chemical synthesis of cyclic peptides is essential when studying their structure-activity relationships. Conventional peptide cyclization methods via direct coupling have inherent limitations, like the susceptibility to epimerization at the C-terminus, poor solubility of fully protected peptide precursors, and low yield caused by oligomerization. In this regard, chemoselective ligation-mediated cyclization methods have emerged as effective strategies for cyclic peptide synthesis. The toolbox for cyclic peptide synthesis has been expanded substantially in the past two decades, allowing more efficient synthesis of cyclic peptides with various scaffolds and modifications. This Review will explore different chemoselective ligation technologies used for cyclic peptide synthesis that generate both native and unnatural peptide linkages. The practical issues and limitations of different methods will be discussed. The advance in cyclic peptide synthesis will benefit the biological and medicinal study of cyclic peptides, an important class of macrocycles with potentials in numerous fields, notably in therapeutics.

Heteroaryl Rings in Peptide Macrocycles

This Review is devoted to the chemistry of macrocyclic peptides having heterocyclic fragments in their structure. These motifs are present in many natural products and synthetic macrocycles designed against a particular biochemical target. Thiazole and oxazole are particularly common constituents of naturally occurring macrocyclic peptide molecules. This frequency of occurrence is because the thiazole and oxazole rings originate from cysteine, serine, and threonine residues. Whereas other heteroaryl groups are found less frequently, they offer many insightful lessons that range from conformational control to receptor/ligand interactions. Many options to develop new and improved technologies to prepare natural products have appeared in recent years, and the synthetic community has been pursuing synthetic macrocycles that have no precedent in nature. This Review attempts to summarize progress in this area.

On-Resin Macrocyclization of Peptides Using Vinyl Sulfonamides as a Thiol-Michael "Click" Acceptor

Macrocyclization of linear peptides imparts improved stability to enzymatic degradation and increases potency of function. Many successful macrocyclization of peptides both in solution and on-resin have been achieved but are limited in scope as they lack selectivity, require long reaction times, or necessitate heat. To overcome these drawbacks a robust and facile strategy was developed employing thiol-Michael click chemistry via an N-methyl vinyl sulfonamide. We demonstrate its balance of reactivity and high stability through FTIR model kinetic studies, reaching 88% conversion over 30 min, and NMR stability studies, revealing no apparent degradation over an 8 day period in basic conditions. Using a commercially available reagent, 2-chloroethane sulfonyl chloride, the cell adhesion peptide, RGDS, was functionalized and macrocyclized on-resin with a relative efficiency of over 95%. The simplistic nature of this process demonstrates the effectiveness of vinyl sulfonamides as a thiol-Michael click acceptor and its applicability to many other bioconjugation applications.

Design, Synthesis, and Antifungal Activity of Novel Aryl-1,2,3-Triazole-β-Carboline Hybrids

The copper catalytic azide and terminal alkyne cycloaddition reaction, namely "click chemistry", gives a new and convenient way to create l,4-disubstitutd-l,2,3-triazoles. In this work, 2-pyrrolecarbaldiminato⁻Cu(II) complexes were established as efficient catalysts for the three-component 1,3-dipolar cycloaddition reaction of arylboronic acid and sodium azide (NaN₃) with terminal alkynes in ethanol at room temperature to 50 °C, 1,4-disubstituted 1,2,3-triazoles were synthesized. Following the optimized protocol, two series of new aryl-1,2,3-triazole-β-carboline hybrids have been designed and synthesized, and the chemical structures were characterized by ¹H NMR, 13C NMR, and high-resolution mass spectrometry (HRMS). All of the target compounds were evaluated in vitro for their antifungal activity against Rhizoctorzia solani, Fusarium oxysporum, Botrytis cinerea Pers., sunflower sclerotinia rot, and rape sclerotinia rot by mycelia growth inhibition assay at 50 μg/mL. The antifungal evaluation of the novel hybrids showed that, among the tested compounds, 5a, 5b, 5c, and 9b showed good antifungal activity against sunflower sclerotinia rot. Specifically, compound 9b also exhibited high broad-spectrum fungicidal against all the tested fungi with inhibition rates of 58.3%, 18.52%, 63.07%, 84.47%, and 81.23%. However, for F. oxysporum, all the target compounds showed no in vitro antifungal activities with an inhibition rate lower than 20%. These results provide an encouraging framework that could lead to the development of potent novel antifungal agents.

Conformational stabilization of a β-hairpin through a triazole–tryptophan interaction

Triazole and indole rings stabilize a β-hairpin conformation through an aromatic–aromatic interaction.

Click and Click-Inspired Chemistry for the Design of Sequence-Controlled Polymers

During the previous decade, many popular chemical reactions used in the area of "click" chemistry and similarly efficient "click-inspired" reactions have been applied for the design of sequence-defined and, more generally, sequence-controlled structures. This combination of topics has already made quite a significant impact on scientific research to date and has enabled the synthesis of highly functionalized and complex oligomeric and polymeric structures, which offer the prospect of many exciting further developments and applications in the near future. This minireview highlights the fruitful combination of these two topics for the preparation of sequence-controlled oligomeric and macromolecular structures and showcases the vast number of publications in this field within a relatively short span of time. It is divided into three sections according to the click-(inspired) reaction that has been applied: copper-catalyzed azide-alkyne cycloaddition, thiol-X, and related thiolactone-based reactions, and finally Diels-Alder-chemistry-based routes are outlined, respectively.

Optimized syntheses of Fmoc azido amino acids for the preparation of azidopeptides

The rise of CuI‐catalyzed click chemistry has initiated an increased demand for azido and alkyne derivatives of amino acid as precursors for the synthesis of clicked peptides. However, the use of azido and alkyne amino acids in peptide chemistry is complicated by their high cost. For this reason, we investigated the possibility of the in‐house preparation of a set of five Fmoc azido amino acids: β‐azido l‐alanine and d‐alanine, γ‐azido l‐homoalanine, δ‐azido l‐ornithine and ω‐azido l‐lysine. We investigated several reaction pathways described in the literature, suggested several improvements and proposed several alternative routes for the synthesis of these compounds in high purity. Here, we demonstrate that multigram quantities of these Fmoc azido amino acids can be prepared within a week or two and at user‐friendly costs. We also incorporated these azido amino acids into several model tripeptides, and we observed the formation of a new elimination product of the azido moiety upon conditions of prolonged couplings with 2‐(1H‐benzotriazol‐1‐yl)‐1,1,3,3‐tetramethyluronium hexafluorophosphate/DIPEA. We hope that our detailed synthetic protocols will inspire some peptide chemists to prepare these Fmoc azido acids in their laboratories and will assist them in avoiding the too extensive costs of azidopeptide syntheses.

Experimental procedures and/or analytical data for compounds 3–5, 20, 25, 26, 30 and 43–47 are provided in the supporting information. © 2017 The Authors Journal of Peptide Science published by European Peptide Society and John Wiley & Sons Ltd.

"Bis-Click" Ligation of DNA: Template-Controlled Assembly, Circularisation and Functionalisation with Bifunctional and Trifunctional Azides

Ligation and circularisation of oligonucleotides containing terminal triple bonds was performed with bifunctional or trifunctional azides. Both reactions are high yielding. Template-assisted bis-click ligation of two individual non-complementary oligonucleotide strands was accomplished to yield heterodimers exclusively. In this context, the template fulfils two functions: it accelerates the ligation reaction and controls product assembly (heterodimer vs. homodimer formation). Intermolecular bis-click circularisation of one oligonucleotide strand took place without template assistance. For construction of oligonucleotides with terminal triple bonds in the nucleobase side chain, 7- or 5-functionalised 7-deaza-dA and dU residues were used. These oligonucleotides are directly accessible by solid-phase synthesis. When trifunctional azides were employed instead of bifunctional linkers, functionalisation of the remaining azido group was performed with small molecules such as 1-ethynyl pyrene, biotin propargyl amide or with ethynylated oligonucleotides. By this means, branched DNA was constructed.

Novel pyrimidine-2,4-dione-1,2,3-triazole and furo[2,3-d]pyrimidine-2-one-1,2,3-triazole hybrids as potential anti-cancer agents: Synthesis, computational and X-ray analysis and biological evaluation

Regioselective 1,4-disubstituted 1,2,3-triazole tethered pyrimidine-2,4-dione derivatives (5-23) were successfully prepared by the copper(I)-catalyzed click chemistry. While known palladium/copper-cocatalyzed method based on Sonogashira cross-coupling followed by the intramolecular 5-endo-dig ring closure generated novel 6-alkylfuro[2,3-d]pyrimidine-2-one-1,2,3-triazole hybrids (24b-37b), a small library of their 5-alkylethynyl analogs (24a-37a) was synthesized and described for the first time by tandem terminal alkyne dimerization and subsequent 5-endo-trig cyclization, which was additionally corroborated with computational and X-ray crystal structure analyses. The nature of substituents on alkynes and thereof homocoupled 1,3-diynes predominantly influenced the ratio of the formed products in both pathways. In vitro antiproliferative activity of prepared compounds evaluated on five human cancer cell lines revealed that N,N-1,3-bis-(1,2,3-triazole)-5-bromouracil (5-7) and 5,6-disubstituted furo[2,3-d]pyrimidine-2-one-1,2,3-triazole 34a hybrids exhibited the most pronounced cytostatic acitivities against hepatocellular carcinoma (HepG2) and cervical carcinoma (HeLa) cells with higher potencies than the reference drug 5-fluorouracil. Cytostatic effect of pyrimidine-2,4-dione-1,2,3-triazole hybrid 7 in HepG2 cells could be attributed to the Wee-1 kinase inhibition and abolishment of sphingolipid signaling mediated by acid ceramidase and sphingosine kinase 1. Importantly, this compound proved to be a non-mitochondrial toxicant, which makes it a promising candidate for further lead optimization and development of a new and more efficient agent for the treatment of hepatocellular carcinoma.

Click-Based Libraries of SFTI-1 Peptides: New Methods Using Reversed-Phase Silica

Performing sequential reactions for the orthogonal derivatization of peptides in solution often requires intermediate handling and purification steps. To solve these problems, we have exploited the distinct adsorption kinetics of peptides toward particulate reversed-phase (RP) C18 silica material, enabling consecutive reactions to be performed without intermediate elution. To illustrate this approach, sequential CuAAC/click reactions were used to modify an analog of the bicyclic peptide sunflower trypsin inhibitor 1 (SFTI-1), a potent scaffold for trypsin and chymotrypsin-like enzyme inhibitors. The SFTI-1 scaffold was synthesized containing both β-azido alanine and propargyl glycine residues. Despite the mutual reactivity of these groups, site isolation on RP silica enabled consecutive click reactions and associated washing steps to be performed while the peptide remained immobilized. Importantly, this approach eliminated side products that could form between two peptides or within a single peptide. These studies suggest a broad utility for RP silica in solving both peptide handling problems and in improving synthetic workflows.

CuAAC: An Efficient Click Chemistry Reaction on Solid Phase

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

Grafting Techniques towards Production of Peptide-Tethered Hydrogels, a Novel Class of Materials with Biomedical Interest

In recent years, new highly functional polymeric biomaterials are being developed to increase the therapeutic efficacy in tissue regeneration approaches. Peptides regulate most physiological processes and display several other biological activities. Therefore, their importance in the field of biomedical research and drug development is rapidly increasing. However, the use of peptides as therapeutic agents is restricted by some of their physicochemical properties. The development of improved routes of delivery of peptide-based therapeutics is crucial and is crucial and its biomedical value is expected to increase in the near future. The unique properties of hydrogels triggered their spreading as localized drug depots. Several strategies, such as the carbodiimide chemistry, have been used to successfully immobilize bioactive peptide sequences into the hydrogels backbone. Peptide tethering through the so-called "click" chemistry reactions is also a highly promising, yet underexplored, approach to the synthesis of hydrogels with varying dimensions and patterns. The present review focus on the approaches that are being used for the establishment of chemical bonds between peptides and non-peptidic hydrogels throughout the last decade.

Azide-alkyne cycloaddition-mediated cyclization of phosphonopeptides and their evaluation as PTP1B binders and enrichment tools

Protein tyrosine phosphatases (PTPs) are important enzymes in health and disease, and chemical tools are crucial to understand and modulate their biological roles. PTP1B is involved in diabetes, obesity and cancer. One of the main challenges for the design of chemical tools for PTP1B is the homology to TCPTP, making tool selectivity a highly challenging task. Here, we aimed to study if azide-alkyne cycloaddition-mediated cyclization of a peptide inhibitor could increase its selectivity toward PTP1B over TCPTP, and if cyclic and linear peptide binders can be applied as enrichment tools of endogenous PTP1B. While the cyclization of the peptide binders did not improve the selectivity toward PTP1B over TCPTP, it enhanced strongly the efficiency to co-precipitate endogenous PTP1B out of cell lysates. Our results show that fine-tuning the molecular structure of peptidic pull-down baits can greatly enhance their efficiency compared to the parental peptide sequences.

Inhibiting EGFR dimerization using triazolyl-bridged dimerization arm mimics

The epidermal growth factor receptor (EGFR) is overexpressed in multiple carcinomas and is the focus of a variety of targeted therapies. Here we report the design of peptide-based compounds that mimic the EGFR dimerization arm and inhibit allosteric activation of EGFR. These peptides are modified to contain a triazolyl bridge between the peptide strands to constrain the EGFR dimerization arm β-loop. In this study, we demonstrate that these peptides have significantly improved proteolytic stability over the non-modified peptide sequence, and their inhibitory effects are dependent on the number of the methylene units and orientation of the introduced triazolyl bridge. We identified a peptide, EDA2, which downregulates receptor phosphorylation and dimerization and reduces cell viability. This is the first example of a biologically active triazolyl-bridged peptide targeting the EGFR dimerization interface that effectively downregulates EGFR activation.

Synthesis of a New Peptide-Coumarin Conjugate: A Potential Agent against Cryptococcosis

Antimicrobial peptides (AMPs) are currently being investigated as potential sources of novel therapeutics against an increasing number of microorganisms resistant to conventional antibiotics. The conjugation of an AMP to other bioactive compounds is an interesting approach for the development of new derivatives with increased antimicrobial efficiency and broader spectra of action. In this work, the synthesis of a new peptide-coumarin conjugate via copper(I)-catalyzed azide-alkyne cycloaddition is described. The conjugate was assayed for in vitro cytotoxicity and displayed antifungal activity against Cryptococcus gattii and Cryptococcus neoformans. Additionally, the conjugate exhibited increased antifungal efficacy when compared with the individual peptide, coumarin, or triazole moieties. Treatment of C. gattii with the peptide-coumarin conjugate enhanced the production of reactive oxygen species, suggesting that the oxidative burst plays an important role in the mechanism of action of the conjugate.

Screening of β-hairpin peptide-engrafted 1,2,3-triazoles to identify APEH enzyme inhibitors

Peptide-engrafted triazoles, obtained via click chemistry, drive the inhibition activity toward Acyl Peptide Hydrolase (APEH), a modulator of the proteasome activity.

Advancements in the mechanistic understanding of the copper-catalyzed azide-alkyne cycloaddition

The copper-catalyzed azide-alkyne cycloaddition (CuAAC) is one of the most broadly applicable and easy-to-handle reactions in the arsenal of organic chemistry. However, the mechanistic understanding of this reaction has lagged behind the plethora of its applications for a long time. As reagent mixtures of copper salts and additives are commonly used in CuAAC reactions, the structure of the catalytically active species itself has remained subject to speculation, which can be attributed to the multifaceted aggregation chemistry of copper(I) alkyne and acetylide complexes. Following an introductory section on common catalyst systems in CuAAC reactions, this review will highlight experimental and computational studies from early proposals to very recent and more sophisticated investigations, which deliver more detailed insights into the CuAAC's catalytic cycle and the species involved. As diverging mechanistic views are presented in articles, books and online resources, we intend to present the research efforts in this field during the past decade and finally give an up-to-date picture of the currently accepted dinuclear mechanism of CuAAC. Additionally, we hope to inspire research efforts on the development of molecularly defined copper(I) catalysts with defined structural characteristics, whose main advantage in contrast to the regularly used precatalyst reagent mixtures is twofold: on the one hand, the characteristics of molecularly defined, well soluble catalysts can be tuned according to the particular requirements of the experiment; on the other hand, the understanding of the CuAAC reaction mechanism can be further advanced by kinetic studies and the isolation and characterization of key intermediates.