1,2,3-triazole as a peptide surrogate in the rapid synthesis of HIV-1 protease inhibitors

Recent advances in triazole synthesis via click chemistry and their pharmacological applications: A review

Click chemistry is a flexible method featuring only the most feasible and efficient chemical reactions. The synthesis of 1,2,3-triazole from azides and terminal acetylenes using copper(I) as a catalyst is an extremely powerful reaction due to the extreme dependability, good selectivity, and biocompatibility of the starting materials. Triazole molecules are more than simple passive linkers; through hydrogen bonding and dipole interactions, they rapidly bind with biological targets. Its applications in drug development are expanding, ranging from target-oriented in situ chemistry and combinatorial mechanisms for lead generation to bioconjugation methods to study proteins and DNA. The click chemistry has frequently been used to speed up drug discovery and optimization processes in the past few years. The click chemistry reaction based on copper-catalyzed azide-alkyne cycloaddition (CuAAC) is a biochemical process with applications in medicinal chemistry and chemical biology. Thus, click reactions are an essential component of the toolkit for medicinal chemistry and help medicinal chemists overcome the barriers in chemical reactions, increase throughput, and improve the standards of compound libraries. The review highlights the recent advancements in the copper-catalyzed azide-alkyne cycloaddition (CuAAC) click chemistry approach for synthesizing biologically important triazole moieties with a greater emphasis on synthesis methodologies and pharmacological applications. Additionally, the triazole-based FDA-approved drugs are also discussed with their mode of action to highlight the importance of the click chemistry approach in synthesizing the bioactive triazole compounds.

Protein-templated ligand discovery via the selection of DNA-encoded dynamic libraries

DNA-encoded chemical libraries (DELs) have become a powerful technology platform in drug discovery. Dual-pharmacophore DELs display two sets of small molecules at the termini of DNA duplexes, thereby enabling the identification of synergistic binders against biological targets, and have been successfully applied in fragment-based ligand discovery and affinity maturation of known ligands. However, dual-pharmacophore DELs identify separate binders that require subsequent linking to obtain the full ligands, which is often challenging. Here we report a protein-templated DEL selection approach that can identify full ligand/inhibitor structures from DNA-encoded dynamic libraries (DEDLs) without the need for subsequent fragment linking. Our approach is based on dynamic DNA hybridization and target-templated in situ ligand synthesis, and it incorporates and encodes the linker structures in the library, along with the building blocks, to be sampled by the target protein. To demonstrate the performance of this method, 4.35-million- and 3.00-million-member DEDLs with different library architectures were prepared, and hit selection was achieved against four therapeutically relevant target proteins.

Average Electron Density: A Quantitative Tool for Evaluating Non-Classical Bioisosteres of Amides

Bioisosterism is strategically used in drug design to enhance the pharmacokinetic and pharmacodynamic properties of therapeutic molecules. The average electron density (AED) tool has been used in several studies to quantify similarities among nonclassical bioisosteres of carboxylic acid. In this study, the AED tool is used to quantify the similarities among nonclassical bioisosteres of an amide group. In particular, amide-to-1,2,3-triazole bioisosterism is considered. To evaluate the AED differences exhibited by isomers of nonclassical bioisosteres, both isomers of amide (cis and trans) and 1,2,3-triazole (1,4 and 1,5 disubstituted moieity) were considered. The amide and 1,2,3-triazole bioisosteric moieties were capped with various R groups (R= methyl, hydrogen, and chloro) to account for changes in their environment. Amide-to-triazole bioisosteric substitutions were then explored in a more realistic environment, that is, within the FDA-approved anticancer imatinib drug. The AED tool effectively identified similarities between substantially different moieties, 1,2,3-triazole and amide, showing AED differences of no more than 4%. The AED tool was also proven to be useful in evaluating the contribution of various factors affecting triazole-amide bioisosterism including isomerism and changes in their environment. The AED values of each bioisostere were transferable within a maximum difference of 2.6%, irrespective of the change in environment. The 1,4- and 1,5-disubstituted isomers of 1,2,3-triazole have AED values that differ by less than unity, 0.52%. Similarly, the AED values of the cis- and trans-amide isomers differ by only 1.31%. Overall, the AED quantitative tool not only replicated experimental observations regarding similarities in bioisosteres, but also explained and quantified each contributing factor. This demonstrates the extended utility of the AED tool from nonclassical carboxylic acid bioisosteres to amide equivalents.On the contrary, electrostatic potential maps, usually used in the literature to qualitatively evaluate bioisosterism, were not similar for the 1,2,3-triazole and amide bioisosteres, under different environments. Overall, the AED tool proves to be powerful in quantitatively evaluating and predicting bioisosterism across diverse moieties considering structural and environmental variations, making it valuable in drug design.

Synthesis, antimicrobial and antioxidant activity of triazole, pyrazole containing thiazole derivatives and molecular docking studies on COVID-19

New series of biologically active triazole and pyrazole compounds containing 2, 4-disubstituted thiazole analogues (12a-l) were synthesized from p-hydroxy benzaldehyde and phenyl hydrazine in excellent yields and purity. All the synthesized compounds were unambiguously identified based on their spectral data analyses (IR, ¹H-NMR, ¹³C-NMR spectra, and HRMS). The final derivatives were evaluated for their in vitro anti-microbial activity after thorough purification. Among all the tested compounds, the compound 12e, 12f and 12 k possess the highest growth inhibitory activity at MIC values of 4.8, 5.1 and 4.0 μg/ml respectively. The antioxidant properties of these compounds demonstrated and revealed remarkable activity compared to the standard antioxidant by using the DPPH free radical-scavenging assay. Moreover, molecular docking studies to evaluate the probable interactions with the catalytic domain of the gram-positive S. aureus topoisomerase IV enzyme may provide new insights for developing these new hybrids as potential antimicrobial agents. The binding affinities of compounds 12a-l were ranging from - 10.0 to - 11.0 kcal/mol with topoisomerase IV enzyme and with COVID-19 main protease binding affinities are ranging from - 8.2 to - 9.3 kcal/mol. These docking studies reveal that the compounds 12a-l could be the best inhibitors for the novel SARS Cov-2 virus and have more future in discovery of potent drug candidates.

Synthesis of Dense 1,2,3-Triazole Oligomers Consisting Preferentially of 1,5-Disubstituted Units via Ruthenium(II)-Catalyzed Azide-Alkyne Cycloaddition

Ruthenium(II)-catalyzed azide-alkyne cycloaddition (RuAAC) polymerization of t-butyl 4-azido-5-hexynoate (tBuAH), i.e., a heterobifunctional monomer carrying azide and alkyne moieties, was investigated in this study. RuAAC of the monofunctional precursors of tBuAH yielded a dimer possessing a 1,5-disubstituted 1,2,3-triazole moiety. ¹H NMR data showed that the dimer was a mixture of diastereomers. Polymerization of tBuAH using ruthenium(II) (Ru(II)) catalysts produced oligomers of M_w ≈ (2.7-3.6) × 10³ consisting of 1,5-disubstituted 1,2,3-triazole units (1,5-units) as well as 1,4-disubstituted 1,2,3-triazole units (1,4-units). The fractions of 1,5-unit (f_1,5) were roughly estimated to be ca. 0.8 by comparison of signals of the methine and triazole protons in ¹H NMR spectra, indicating that RuAAC proceeded preferentially and thermal Huisgen cycloaddition (HC) somehow took place during the polymerization. The oligomer samples obtained were also characterized by solubility test, size exclusion chromatography (SEC), ultraviolet-visible (UV-Vis) absorption spectroscopy, and thermogravimetric analysis (TGA). The UV-Vis and TGA data indicated that the oligomer samples contained a substantial amount of Ru(II) catalysts. To the best of our knowledge, this is the first report on dense 1,2,3-triazole oligomers consisting of 1,5-units linked via a carbon atom.

Design, synthesis, and biological evaluation of trizole-based heteroaromatic derivatives as Bcr-Abl kinase inhibitors

Bcr-Abl is a key driver in the pathophysiology of CML. Broadening the chemical diversity of Bcr-Abl kinase inhibitors to overcome drug resistance is a current medical demand for CML treatment. As a continuation to our research, a series of compounds with heteroaromatics-trizole scaffold as hinge binding moiety (HBM) were developed as Bcr-Abl inhibitors based on in silico modeling analysis. Biological results indicated that these compounds exhibited a significantly enhanced inhibition against Bcr-Abl^WT and Bcr-Abl^T315I in kinases assays, along with improved anti-proliferative activities in leukemia cell assays, compared with previous disclosed compounds. In particular, compounds 9f, 28c, 31, and 44c displayed comparable even better potency with that of Imatinib in enzymatic assay and cell assays including K562 cells and adriamycin-resistant K562/A cells. Moreover, compounds 9f, 28c, and 44c exhibited potent inhibition activities against K562R cells bearing T315I mutant with IC₅₀ of 13.35 μM, 40.14 μM, and 1.91 μM, respectively, outperforming that of Imatinib. Meanwhile, the inhibition of Bcr-Abl activity in Ba/F3 cells demonstrated that these compounds exerted effects mainly by acting on Bcr-Abl. Additionally, compounds 9f, 28c, and 44c effectively induced apoptosis, arrest the cell cycle at S or G₂/M phase, and inhibited phosphorylation of Bcr-Abl and STAT5 in a dose-dependent manner. Docking studies indicated that trizole indeed retained the hydrophobic interaction of aromatic heterocycles with hinge region, and ADME prediction suggested that tested compounds had a favorable safety profile. Therefore, aromatic heterocycles incorporated with trizole could serve as a promising HBM for Bcr-Abl inhibitors with proline as fexibile linker, and compounds 9f, 28c, especially 44c could be served as a starting point for further optimization.

Structure-Based Bioisosterism Design, Synthesis, Biological Evaluation and In Silico Studies of Benzamide Analogs as Potential Anthelmintics

A recent screen of 67,012 compounds identified a new family of compounds with excellent nematicidal activity: the ortho-substituted benzamide families Wact-11 and Wact-12. These compounds are active against Caenorhabditis elegans and parasitic nematodes by selectively inhibiting nematode complex II, and they display low toxicity in mammalian cells and vertebrate organisms. Although a big number of benzamides were tested against C. elegans in high-throughput screens, bioisosteres of the amide moiety were not represented in the chemical space examined. We thus identified an opportunity for the design, synthesis and evaluation of novel compounds, using bioisosteric replacements of the amide group present in benzamides. The compound Wact-11 was used as the reference scaffold to prepare a set of bioisosteres to be evaluated against C. elegans. Eight types of amide replacement were selected, including ester, thioamide, selenoamide, sulfonamide, alkyl thio- and oxo-amides, urea and triazole. The results allowed us to perform a structure-activity relationship, highlighting the relevance of the amide group for nematicide activity. Experimental evidence was complemented with in silico structural studies over a C. elegans complex II model as a molecular target of benzamides. Importantly, compound Wact-11 was active against the flatworm Echinococcus granulosus, suggesting a previously unreported pan-anthelmintic potential for benzamides.

Small molecule peptidomimetic trypsin inhibitors: validation of an EKO binding mode, but with a twist

Examination of a series of naturally-occurring trypsin inhibitor proteins, led to identification of a set of three residues (which we call the "interface triplet") to be determinant of trypsin binding affinity, hence excellent templates for small molecule mimicry. Consequently, we attempted to use the Exploring Key Orientation (EKO) strategy developed in our lab to evaluate small molecules that mimic the interface triplet regions of natural trypsin inhibitors, and hence potentially might bind and inhibit the catalytic activity of trypsin. A bis-triazole scaffold ("TT-mer") was the most promising of the molecules evaluated in silico. Twelve such compounds were synthesized and assayed against trypsin, among which the best showed a Kd of 2.1 μM. X-ray crystallography revealed a high degree of matching between an illustrative TT-mer's actual binding mode and that of the mimics that overlaid the interface triplet in the crystal structure. Deviation of the third side chain from the PPI structure seems to be due to alleviation of an unfavorable dipole-dipole interaction in the small molecule's actual bound conformation.

Design of new bis-triazolyl structure for identification of inhibitory activity on COVID-19 main protease by molecular docking approach

In the rapidly growing COVID-19 pandemic, designing of new drugs and evaluating their inhibitory action against main targets of corona virus could be an effective strategy to accelerate the drug discovery process and their efficacy towards corona virus disease. Herein, we design new bis-triazolyl probe for an investigation of inhibitory activity towards COVID-19 main protease by Molecular docking approach. The formulated compound has been thoroughly characterized by elemental analysis, NMR (¹H and ¹³C) and complete structure elucidation was achieved via X-ray crystallography. Docking study reveals that newly synthesized compound confers good inhibitory response to COVID-19 main protease as supported by calculated docking score and binding energy. Strong hydrogen bonding and hydrophobic interactions of the newly synthesized compound with several important amino acids of the main protease also helps to explain the potency of the compound to inhibit the main protease. We hope that the present study would help the researcher in the field of Medicinal chemistry and to develop potential drug against the novel corona virus.

Bio-click chemistry: a bridge between biocatalysis and click chemistry

The fields of click chemistry and biocatalysis have rapidly grown over the last two decades. The development of robust and active biocatalysts and the widespread use of straightforward click reactions led to significant interactions between these two fields. Therefore the name bio-click chemistry seems to be an accurate definition of chemoenzymatic reactions cooperating with click transformations. Bio-click chemistry can be understood as the approach towards molecules of high-value using a green and sustainable approach by exploiting the potential of biocatalytic enzyme activity combined with the reliable nature of click reactions. This review summarizes the principal bio-click chemistry reactions reported over the last two decades, with a special emphasis on small molecules. Contributions to the field of bio-click chemistry are manifold, but the synthesis of chiral molecules with applications in medicinal chemistry and sustainable syntheses will be especially highlighted.

Green by Design: Convergent Synthesis, Computational Analyses, and Activity Evaluation of New FXa Inhibitors Bearing Peptide Triazole Linking Units

Green chemistry implementation has led to promising results in waste reduction in the pharmaceutical industry. However, the early sustainable development of pharmaceutically active compounds and ingredients remains a considerable challenge. Herein, we wish to report a green synthesis of new pharmaceutically active peptide triazoles as potent factor Xa inhibitors, an important drug target associated with the treatment of diverse cardiovascular diseases. The new inhibitors were synthesized in three steps, featuring cycloaddition reactions (high atom economy), microwave-assisted organic synthesis (energy efficiency), and copper nanoparticle catalysis, thus featuring Earth-abundant metals. The molecules obtained showed FXa inhibition, with IC50-values as low as 17.2 μM and no associated cytotoxicity in HEK293 and HeLa cells. These results showcase the environmental potential and chemical implications of the applied methodologies for the development of new molecules with pharmacological potential.

Novel 3-chloro-6-nitro-1H-indazole derivatives as promising antileishmanial candidates: synthesis, biological activity, and molecular modelling studies

An efficient pathway was disclosed for the synthesis of 3-chloro-6-nitro-1H-indazole derivatives by 1,3-dipolar cycloaddition on dipolarophile compounds 2 and 3. Faced the problem of separation of two regioisomers, a click chemistry method has allowed us to obtain regioisomers of triazole-1,4 with good yields from 82 to 90% were employed. Also, the antileishmanial biological potency of the compounds was achieved using an MTT assay that reported compound 13 as a promising growth inhibitor of Leishmania major. Molecular docking demonstrated highly stable binding with the Leishmania trypanothione reductase enzyme and produced a network of hydrophobic and hydrophilic interactions. Molecular dynamics simulations were performed for TryR-13 complex to understand its structural and intermolecular affinity stability in a biological environment. The studied complex remained in good equilibrium with a structure deviation of ∼1-3 Å. MM/GBSA binding free energies illustrated the high stability of TryR-13 complex. The studied compounds are promising leads for structural optimisation to enhance the antileishmanial activity.

Novel Double-Modified Colchicine Derivatives Bearing 1,2,3-Triazole: Design, Synthesis, and Biological Activity Evaluation

A series of 1,4-disubstituted 1,2,3-triazoles having 10-demethoxy-10-N-methylaminocolchicine core were designed and synthesized via the Cu(I)-catalyzed "click" reaction and screened for their in vitro cytotoxicity against four cancer cell lines (A549, MCF-7, LoVo, LoVo/DX) and one noncancerous cell line (BALB/3T3). Indexes of resistance (RI) and selectivity (SI) were also determined to assess the potential of the analogues to break drug resistance of the LoVo/DX cells and to verify their selectivity toward killing cancer cells over normal cells. The compounds with an ester or amide moiety in the fourth position of 1,2,3-triazole of 10-N-methylaminocolchicine turned out to have the greatest therapeutic potential (low IC₅₀ values and favorable SI values), much better than that of unmodified colchicine or doxorubicin and cisplatin. Thus, they make a valuable clue for the further search for a drug having a colchicine scaffold.

Synthesis and Biological Evaluation of 1,2,3-Triazole Tethered Thymol-1,3,4-Oxadiazole Derivatives as Anticancer and Antimicrobial Agents

A library of 1,2,3-triazole-incorporated thymol-1,3,4-oxadiazole derivatives (6–18) hasbeen synthesized and tested for anticancer and antimicrobial activities. Compounds 7, 8, 9, 10, and 11 exhibited significant antiproliferative activity. Among these active derivatives, compound 2-(4-((5-((2-isopropyl-5-methylphenoxy)methyl)-1,3,4-oxadiazol-2-ylthio)methyl)-1H-1,2,3-triazol-1-yl)phenol (9) was the best compound against all three tested cell lines, MCF-7 (IC₅₀ 1.1 μM), HCT-116 (IC₅₀ 2.6 μM), and HepG2 (IC₅₀ 1.4 μM). Compound 9 was found to be better than the standard drugs, doxorubicin and 5-fluorouracil. These compounds showed anticancer activity through thymidylate synthase inhibition as they displayed significant TS inhibitory activity with IC₅₀ in the range 1.95–4.24 μM, whereas the standard drug, Pemetrexed, showed IC₅₀ 7.26 μM. The antimicrobial results showed that some of the compounds (6, 7, 9, 16, and 17) exhibited good inhibition on Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus). The molecular docking and simulation studies supported the anticancer and antimicrobial data. It can be concluded that the synthesized 1,2,3-triazole tethered thymol-1,3,4-oxadiazole conjugates have both antiproliferative and antimicrobial potential.

CDATA[Recent Advances in the Development of 1,2,3-Triazole-containing Derivatives as Potential Antifungal Agents and Inhibitors of Lanoster ol 14α-Demethylase

1,2,3-Triazole, a five-membered heterocyclic nucleus, is widely recognized as a key chromophore of great value in medicinal chemistry for delivering compounds possessing innumerable biological activities, including antimicrobial, antitubercular, antidiabetic, antiviral, antitumor, antioxidants, and anti-inflammatory activities. Mainly, in the past years, diverse conjugates carrying this biologically valuable core have been reported due to their attractive fungicidal potential and potent effects on various infective targets. Hence, hybridization of 1,2,3-triazole with other antimicrobial pharmacophores appears to be a judicious strategy to develop new effective anti-fungal candidates to combat the emergence of drug-sensitive and drug-resistant infectious diseases. Thus, the current review highlights the recent advances of this promising category of 1,2,3-triazole-containing hybrids incorporating diverse varieties of bioactive heterocycles such as conozole, coumarin, imidazole, benzimidazole, pyrazole, indole, oxindole, chromene, pyrane, quinazoline, chalcone, isoflavone, carbohydrates, and amides. It underlies their inhibition behavior against a wide array of infectious fungal species during 2015-2020.

Cu(I)-Catalyzed Click Chemistry in Glycoscience and Their Diverse Applications

Copper(I)-catalyzed 1,3-dipolar cycloaddition between organic azides and terminal alkynes, commonly known as CuAAC or click chemistry, has been identified as one of the most successful, versatile, reliable, and modular strategies for the rapid and regioselective construction of 1,4-disubstituted 1,2,3-triazoles as diversely functionalized molecules. Carbohydrates, an integral part of living cells, have several fascinating features, including their structural diversity, biocompatibility, bioavailability, hydrophilicity, and superior ADME properties with minimal toxicity, which support increased demand to explore them as versatile scaffolds for easy access to diverse glycohybrids and well-defined glycoconjugates for complete chemical, biochemical, and pharmacological investigations. This review highlights the successful development of CuAAC or click chemistry in emerging areas of glycoscience, including the synthesis of triazole appended carbohydrate-containing molecular architectures (mainly glycohybrids, glycoconjugates, glycopolymers, glycopeptides, glycoproteins, glycolipids, glycoclusters, and glycodendrimers through regioselective triazole forming modular and bio-orthogonal coupling protocols). It discusses the widespread applications of these glycoproducts as enzyme inhibitors in drug discovery and development, sensing, gelation, chelation, glycosylation, and catalysis. This review also covers the impact of click chemistry and provides future perspectives on its role in various emerging disciplines of science and technology.

Design and synthesis, biological evaluation of bis-(1,2,3- and 1,2,4)-triazole derivatives as potential antimicrobial and antifungal agents

A new series of bis-1,2,3- and 1,2,4-triazoles (10a-m) were designed and efficiently synthesized using methyl salicylate as potential antimicrobial agents. All compounds were characterized by their proton & ¹³C NMR, IR, Mass spectral data, and elemental analysis. The final compounds 10a-m were in vitro screened for antimicrobial and antifungal activity against gram negative Pseudomonas aeruginosa, Escherichia coli, gram positive Bacillus subtilis, Staphylococcus aureus strains and Aspergillus niger & Saccharomyces cerevisiae. Majority of the synthesized compounds exhibited potent antimicrobial activity (MIC 3.9 µg/mL) and promising antifungal activity with the zone of inhibition (ZOI) 1.5-8.2 mm. Compounds like 10d and 10f exhibited best antimicrobial activity against S. aureus. The molecular docking analysis revealed that all the synthesized derivatives shown better binding affinities. Among all, compound 10f exhibited best scores. Hence, there was an assumption that introduction of para-chloro and bromo-phenyl aromatic groups on triazole moiety could result excellent antimicrobial activity. This substantial growth inhibitory activity of bis-1,2,3- and 1,2,4-triazole derivatives suggested these compounds could assist a new way in the development of lead molecules against microbial infection and antimicrobial resistance investigations.

1,2,3-Triazoles as Biomimetics in Peptide Science

Natural peptides are an important class of chemical mediators, essential for most vital processes. What limits the potential of the use of peptides as drugs is their low bioavailability and enzymatic degradation in vivo. To overcome this limitation, the development of new molecules mimicking peptides is of great importance for the development of new biologically active molecules. Therefore, replacing the amide bond in a peptide with a heterocyclic bioisostere, such as the 1,2,3-triazole ring, can be considered an effective solution for the synthesis of biologically relevant peptidomimetics. These 1,2,3-triazoles may have an interesting biological activity, because they behave as rigid link units, which can mimic the electronic properties of amide bonds and show bioisosteric effects. Additionally, triazole can be used as a linker moiety to link peptides to other functional groups.

Synthesis, Anticancer Evaluation, Computer-Aided Docking Studies, and ADMET Prediction of 1,2,3-Triazolyl-Pyridine Hybrids as Human Aurora B Kinase Inhibitors

A novel series of 1,2,3-triazolyl-pyridine hybrids were prepared through the reaction of the triazole derivative (1) with the appropriate aldehyde (2a-g) and malononitrile or ethyl cyanoacetate in the presence of ammonium acetate in refluxed acetic acid. The chemical composition of the products was established on the basis of spectral and elemental analyses. Aurora B kinase is a protein with diverse biological actions in controlling tumorigenesis by inhibiting apoptosis and promoting proliferation and metastasis, making it an emerging target for diseases such as hepatocellular carcinoma (HCC). Alteration in the target protein expression causes unequal distribution of genetic information, causing HCC. The new compounds were tested for their antihepatic cancer activity, and some of them had strong efficacy against human hepatoblastoma (HepG2) cell lines.

Fluorescent-Labeled Octasilsesquioxane Nanohybrids as Potential Materials for Latent Fingerprinting Detection

The recent demand for fluorescent-labeled materials (FLMs) in forensic security concepts such as latent fingerprints (LFs) that encode information for anti-counterfeiting and encryption of confidential data makes necessary the development of building new and innovative materials. Here, novel FLMs based on polyhedral oligomeric silsesquioxanes (POSS) functionalized with fluorophores via "click" reactions have been successfully synthesized and fully characterized. A comprehensive study of their photophysical properties has displayed large Stokes's shift together with good photostability in all cases, fulfilling the fundamental requisites for any legible LF detection on various surfaces. The excellent performance of the hetero-bifunctional FLM in the visualization of LF is emphasized by their legibility, selectivity, sensitivity and temporal photostability. In this study, development mechanisms have been proposed and the overall concept constitute a novel approach for vis-à-vis forensic investigations to trace an individual's identity.

1,4-Disubstituted 1,2,3-Triazoles as Amide Bond Surrogates for the Stabilisation of Linear Peptides with Biological Activity

Peptides represent an important class of biologically active molecules with high potential for the development of diagnostic and therapeutic agents due to their structural diversity, favourable pharmacokinetic properties, and synthetic availability. However, the widespread use of peptides and conjugates thereof in clinical applications can be hampered by their low stability in vivo due to rapid degradation by endogenous proteases. A promising approach to circumvent this potential limitation includes the substitution of metabolically labile amide bonds in the peptide backbone by stable isosteric amide bond mimetics. In this review, we focus on the incorporation of 1,4-disubstituted 1,2,3-triazoles as amide bond surrogates in linear peptides with the aim to increase their stability without impacting their biological function(s). We highlight the properties of this heterocycle as a trans-amide bond surrogate and summarise approaches for the synthesis of triazole-containing peptidomimetics via the Cu(I)-catalysed azide-alkyne cycloaddition (CuAAC). The impacts of the incorporation of triazoles in the backbone of diverse peptides on their biological properties such as, e.g., blood serum stability and affinity as well as selectivity towards their respective molecular target(s) are discussed.

Single Peptide Backbone Surrogate Mutations to Regulate Angiotensin GPCR Subtype Selectivity

Mutating the side-chains of amino acids in a peptide ligand, with unnatural amino acids, aiming to mitigate its short half-life is an established approach. However, it is hypothesized that mutating specific backbone peptide bonds with bioisosters can be exploited not only to enhance the proteolytic stability of parent peptides, but also to tune its receptor subtype selectivity. Towards this end, four [Y]⁶ -Angiotensin II analogues are synthesized where amide bonds have been replaced by 1,4-disubstituted 1,2,3-triazole isosteres in four different backbone locations. All the analogues possessed enhanced stability in human plasma in comparison with the parent peptide, whereas only two of them achieved enhanced AT₂ R/AT₁ R subtype selectivity. This diversification has been studied through 2D NMR spectroscopy and unveiled a putative more structured microenvironment for the two selective ligands accompanied with increased number of NOE cross-peaks. The most potent analogue, compound 2, has been explored regarding its neurotrophic potential and resulted in an enhanced neurite growth with respect to the established agent C21.

β-Turn mimetic synthetic peptides as amyloid-β aggregation inhibitors

Aggregation of amyloid peptides results in severe neurodegenerative diseases. While the fibril structures of Aβ₄₀ and Aβ₄₂ have been described recently, resolution of the aggregation pathway and evaluation of potent inhibitors still remains elusive, in particular in view of the hairpin-region of Aβ₄₀. We here report the preparation of beta-turn mimetic conjugates containing synthetic turn mimetic structures in the turn region of Aβ₄₀ and Aβ_16-35, replacing 2 amino acids in the turn-region G25 - K28. The structure of the turn mimic induces both, acceleration of fibrillation and the complete inhibition of fibrillation, confirming the importance of the turn region on the aggregation. Replacing position G25-S26 provided the best inhibition effect for both beta-turn mimetics, the bicyclic BTD 1 and the aromatic TAA 2, while positions N27-K28 and V24-G25 showed only weaker or no inhibitory effects. When comparing different turn mimetics at the same position (G25-S26), conjugate 1a bearing the BTD turn showed the best inhibition of Aβ₄₀ aggregation, while 5-amino-valeric acid 4a showed the weakest effect. Thus there is a pronounced impact on fibrillation with the chemical nature of the embedded beta-turn-mimic: the conformationally constrained turns 1 and 2 lead to a significantly reduced fibrillation, even inhibiting fibrillation of native Aβ₄₀ when added in amounts down to 1/10, whereas the more flexible beta-turn-mimics 4-amino-benzoic acid 3a and 5-amino-valeric acid 4a lead to enhanced fibrillation. Toxicity-testing of the most successful conjugate showed only minor toxicity in cell-viability assays using the N2a cell line. Structural downsizing lead to the short fragment BTD/peptide Aβ_16-35 as inhibitor of the aggregation of Aβ₄₀, opening large potential for further small peptide based inhibitors.

Cu(II)-Hydrazide Coordination Compound Supported on Silica Gel as an Efficient and Recyclable Heterogeneous Catalyst for Green Click Synthesis of β-Hydroxy-1,2,3-triazoles in Water

A hydrazone ligand, (E)-6-(2-((2-hydroxynaphthalen-1-yl)methylene)hydrazinyl)nicotinohydrazide (H₂L), was synthesized and characterized by spectroscopic methods. The reaction of H₂L with CuCl₂·2H₂O in methanol gave Cu(II) coordination compound, [Cu(HL')(Cl)]·CH₃OH (1), which was characterized by elemental analysis and spectroscopic methods (Fourier transform infrared (FT-IR) and UV-vis). The structure of 1 was also determined by single-crystal X-ray analysis. Structural studies confirmed the formation of esteric group during the synthesis of 1. Compound 1 was immobilized on 3-aminopropyltriethoxysilane (APTS)-functionalized silica gel through the amidification reaction and the obtained heterogeneous coordination compound was utilized as a catalyst for the three-component azide-epoxide-alkyne cycloaddition reaction in water as a green solvent. The structural properties of the heterogeneous catalyst were characterized by a combination of FT-IR, UV-vis, thermogravimetric analysis (TGA), scanning electron microscopy (SEM) and energy-dispersive spectrometry (EDS) analyses. The effect of the amount of catalyst and temperature on the cycloaddition reaction was studied, and the obtained 1,2,3-triazoles were characterized by spectroscopic studies and single-crystal X-ray analysis. The catalytic investigations revealed that this catalytic system has high activity in the synthesis of β-hydroxy-1,2,3-triazoles. It was also found that the aromatic and aliphatic substituents on the alkyne and epoxide together with the reaction temperature have considerable effects on the activity and regioselectivity of this catalytic system.

Azo-triazolide bis-cyclometalated Ir(iii) complexes via cyclization of 3-cyanodiarylformazanate ligands

In this work we describe the synthesis of sterically encumbered 1,5-diaryl-3-cyanoformazanate bis-cyclometalated iridium(iii) complexes, two of which undergo redox-neutral cyclization during the reaction to produce carbon-bound 2-aryl-4-arylazo-2H-1,2,3-triazolide ligands. This transformation offers a method for accessing 2-aryl-4-arylazo-2H-1,2,3-triazolide ligands, a heretofore unreported class of chelating ligands. One formazanate complex and both triazolide complexes are structurally characterized by single-crystal X-ray diffraction, with infrared spectroscopy being the primary bulk technique to distinguish the formazanate and triazolide structures. All complexes are further characterized by UV-Vis absorption spectroscopy and cyclic voltammetry, with the triazolide compounds having similar frontier orbital energies to the formazanate complexes but much less visible absorption.

Design and synthesis of 4,5-diphenyl-imidazol-1,2,3-triazole hybrids as new anti-diabetic agents: in vitro α-glucosidase inhibition, kinetic and docking studies

Fourteen novel 4,5-diphenyl-imidazol-1,2,3-triazole hybrids 8a-n were synthesized with good yields by performing click reaction between the 4,5-diphenyl-2-(prop-2-yn-1-ylthio)-1H-imidazole and various benzyl azides. The synthesized compounds 8a-n were evaluated against yeast α-glucosidase, and all these compounds exhibited excellent inhibitory activity (IC₅₀ values in the range of 85.6 ± 0.4-231.4 ± 1.0 μM), even much more potent than standard drug acarbose (IC₅₀ = 750.0 μM). Among them, 4,5-diphenyl-imidazol-1,2,3-triazoles possessing 2-chloro and 2-bromo-benzyl moieties (compounds 8g and 8i) demonstrated the most potent inhibitory activities toward α-glucosidase. The kinetic study of the compound 8g revealed that this compound inhibited α-glucosidase in a competitive mode. Furthermore, docking calculations of these compounds were performed to predict the interaction mode of the synthesized compounds in the active site of α-glucosidase. A novel series of 4,5-diphenyl-imidazol-1,2,3-triazole hybrids 8a-n was synthesized with good yields by performing click reaction between the 4,5-diphenyl-2-(prop-2-yn-1-ylthio)-1Himidazole and various benzyl azides. The synthesized compounds 8a-n were evaluated against yeast α-glucosidase and all these compounds exhibited excellent inhibitory activity (IC50 values in the range of 85.6 ± 0.4-231.4 ± 1.0 μM), even much more potent than standard drug acarbose (IC50 = 750.0 μM).

Copper-Catalyzed Decarboxylative Cycloaddition of Propiolic Acids, Azides, and Arylboronic Acids: Construction of Fully Substituted 1,2,3-Triazoles

A copper-catalyzed decarboxylative cycloaddition of propiolic acids, azides, and arylboronic acids is described. The present reaction provides an efficient and convenient method for the synthesis of various fully substituted 1,2,3-triazoles from readily available starting materials. A possible mechanism is proposed.

CuAAC-ensembled 1,2,3-triazole-linked isosteres as pharmacophores in drug discovery: review

The review lays emphasis on the significance of 1,2,3-triazoles synthesized via CuAAC reaction having potential to act as anti-microbial, anti-cancer, anti-viral, anti-inflammatory, anti-tuberculosis, anti-diabetic, and anti-Alzheimer drugs. The importance of click chemistry is due to its 'quicker' methodology that has the capability to create complex and efficient drugs with high yield and purity from simple and cheap starting materials. The activity of different triazolyl compounds was compiled considering MIC, IC50, and EC50 values against different species of microbes. In addition to this, the anti-oxidant property of triazolyl compounds have also been reviewed and discussed.

Pharmacological and Cellular Significance of Triazole-Surrogated Compounds

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Heterocyclic compounds have been at the hierarchy position in academia, and industrial arena, particularly the compounds containing triazole-core are found to be potent with a broad range of biological activities. The resistance of triazole ring towards chemical (acid and base) hydrolysis, oxidative and reductive reaction conditions, metabolic degradation and its higher aromatic stabilization energy makes it a better heterocyclic core as therapeutic agents. These triazole-linked compounds are used for clinical purposes for antifungal, anti-mycobacterium, anticancer, anti-migraine and antidepressant drugs. Triazole scaffolds are also found to act as a spacer for the sake of covalent attachment of the high molecular weight bio-macromolecules with an experimental building blocks to explore structure-function relationships. Herein, several methods and strategies for the synthesis of compounds with 1,2,3-triazole moiety exploring Hüisgen, Meldal and Sharpless 1,3-dipolar cycloaddition reaction between azide and alkyne derivatives have been deliberated for a series of representative compounds. Moreover, this review article highlights in-depth applications of the [3+2]-cycloaddition reaction for the advances of triazole-containing antibacterial as well as metabolic labelling agents for the in vitro and in vivo studies on cellular level.

Conformational and functional characterization of artificially conjugated non-canonical ubiquitin dimers

Ubiquitylation is an eminent posttranslational modification referring to the covalent attachment of single ubiquitin molecules or polyubiquitin chains to a target protein dictating the fate of such labeled polypeptide chains. Here, we have biochemically produced artificially Lys11-, and Lys27-, and Lys63-linked ubiquitin dimers based on click-chemistry generating milligram quantities in high purity. We show that the artificial linkage used for the conjugation of two ubiquitin moieties represents a fully reliable surrogate of the natural isopeptide bond by acquiring highly resolved nuclear magnetic resonance (NMR) spectroscopic data including ligand binding studies. Extensive coarse grained and atomistic molecular dynamics (MD) simulations allow to extract structures representing the ensemble of domain-domain conformations used to verify the experimental data. Advantageously, this methodology does not require individual isotopic labeling of both ubiquitin moieties as NMR data have been acquired on the isotopically labeled proximal moiety and complementary MD simulations have been used to fully interpret the experimental data in terms of domain-domain conformation. This combined approach intertwining NMR spectroscopy with MD simulations makes it possible to describe the conformational space non-canonically Lys11-, and Lys27-linked ubiquitin dimers occupy in a solution averaged ensemble by taking atomically resolved information representing all residues in ubiquitin dimers into account.

Extensive Structure-Activity Relationship Study of Albicidin's C-Terminal Dipeptidic p-Aminobenzoic Acid Moiety

Albicidin is a recently described natural product that strongly inhibits bacterial DNA gyrase. The pronounced activity, particularly against Gram-negative bacteria, turns it into a promising lead structure for an antibacterial drug. Hence, structure-activity relationship studies are key for the in-depth understanding of structural features/moieties affecting gyrase inhibition, antibacterial activity and overcoming resistance. The 27 newly synthesized albicidins give profound insights into possibilities for variations of the C-terminus. Furthermore, in the present study, a novel derivative has been identified as overcoming resistance posed by the Klebsiella-protease AlbD. Structural modifications include, for example, azahistidine replacing the previous instable cyanoalanine as the central amino acid, as well as a triazole amide bond isostere between building blocks D and E.

Expanding the limits of amide-triazole isosteric substitution in bisamide-based physical gels

Gelation of organic solvents using N,N'-((1S,2S)-cyclohexane-1,2-diyl)didodecanamide (C12-Cyc) is driven by its self-assembly via antiparallel hydrogen bonds and van der Waals intermolecular interactions. In this work we carried out a dual isosteric substitution of the two amide groups with 1,2,3-triazole rings affording the corresponding isosteric gelator (click-C12-Cyc). A detailed comparative study in terms of the gelation ability and gel properties demonstrated that the 1,2,3-triazoles can take over all of the functions derived from the amide groups offering a versatile strategy for tuning the properties of the corresponding gels. This is not an obvious outcome because the directional amide groups in C12-Cyc constitute the source of the hydrogen bonds to build the 3D self-assembled network. Furthermore, theoretical calculations revealed that click-C12-Cyc can adopt a wide variety of interacting patterns, whose relative stability depends on the polarity of the environment, this is in good agreement with the experimental data obtained regarding its gelation ability. Other important features of click-C12-Cyc for potential practical applications are its non-cytotoxic character and its phase-selective gelation of water-oil mixtures.

1,4-Disubstituted 1H-1,2,3-Triazole Containing Peptidotriazolamers: A New Class of Peptidomimetics With Interesting Foldamer Properties

Peptidotriazolamers are hybrid foldamers with features of peptides and triazolamers, containing alternation of amide bonds and 1,4-disubstituted 1H-1,2,3-triazoles with conservation of the amino acid side chains. We report on the synthesis of a new class of peptidomimetics, containing 1,4-disubstituted 1H-1,2,3-triazoles in alternation with amide bonds and the elucidation of their conformational properties in solution. Based on enantiomerically pure propargylamines bearing the stereogenic center in the propargylic position and α-azido esters, building blocks were obtained by copper-catalyzed azide-alkyne cycloaddition. With these building blocks the peptidotriazolamers were readily available by solution phase synthesis. A panel of homo- and heterochiral tetramers, hexamers, and heptamers was synthesized and the heptamer Boc-Ala-Val-Ψ[4Tz]Phe-LeuΨ[4Tz]Phe-LeuΨ[4Tz]Val-OAll as well as an heterochiral and a Gly-containing equivalent were structurally characterized by NMR-based molecular dynamics simulations using a specifically tailored force field to determine their conformational and solvation properties. All three variants adopt a compact folded conformation in DMSO as well as in water. In addition to the heptamers we predicted the conformational behavior of similar longer oligomers i.e., Boc-Ala-(AlaΨ[4Tz]Ala)₆-OAll as well as Boc-Ala-(d-AlaΨ[4Tz]Ala)₆-OAll and Boc-Ala-(GlyΨ[4Tz]Ala)₆-OAll. Our calculations predict a clear secondary structure of the first two molecules in DMSO that collapses in water due to the hydrophobic character of the side chains. The homochiral compound folds into a regular helical structure and the heterochiral one shows a twisted “S”-shape, while the Gly variant exhibits no clear secondary structure.

The application of click chemistry for targeting quadruplex nucleic acids

The Cu(i)-catalyzed azide and alkyne 1,3-dipolar cycloaddition (CuAAC), commonly known as the "click reaction", has emerged as a powerful and versatile synthetic tool that finds a broad spectrum of applications in chemistry, biology and materials science. The efficiency, selectivity and versatility of the CuAAC reactions have enabled the preparation of vast arrays of triazole compounds with biological and pharmaceutical applications. In this feature article, we outline the applications and future prospects of click chemistry in the synthesis and development of small molecules that target G-quadruplex nucleic acids and show promising biological activities. Furthermore, this article highlights the template-assisted in situ click chemistry for developing G-quadruplex specific ligands and the use of click chemistry for enhancing drug specificity as well as designing imaging and sensor systems to elucidate the biological functions of G-quadruplex nucleic acids in live cells.

Synthesis and antibacterial activity of new 1,2,3-triazolylmethyl-2H-1,4-benzothiazin-3(4H)-one derivatives

BACKGROUND

A novel series of 1,2,3-triazole derivatives containing 1,4-benzothiazin-3-one ring (7a-9a, 7b-9b), (10a-12a, 10b-12b) and (13-15) were synthesized by 1,3-dipolar cycloaddition reactions of azides α-D-galactopyranoside azide F, 2,3,4,6-tetra-O-acetyl-(D)-glucopyranosyl azide G and methyl-N-benzoyl-α-azidoglycinate H with compounds 4-6.

FINDINGS

Initially, the reactions were conducted under thermal conditions in ethanol. The reaction leads, each time, to the formation of two regioisomers: (Schemes 2, 3) with yields of 17 to 21% for 1,5-disubstituted 1,2,3-triazole-regioisomers (7b-12b) and yields ranging from 61 to 65% for the 1,4-disubstituted regioisomers (7a-12a). In order to report an unequivocal synthesis of the 1,4-regioisomers and confirm the structures of the two regioisomers obtained in thermal conditions (Huisgen reactions), the method click chemistry (Copper-Catalyzed Azide-Alkyne Cycloaddition) has been used.

CONCLUSIONS

The newly synthesized compounds using cycloaddition reactions were evaluated in vitro for their antibacterial activities against some Gram positive and Gram negative microbial strains. Among the compounds tested, the compound 8a showed excellent antibacterial activities against PA ATCC and Acin ESBL (MIC = 31.2 μg/ml).