New strategies for cyclization and bicyclization of oligonucleotides by click chemistry assisted by microwaves

Synthesis and structure-activity relationship studies of original cyclic diadenosine derivatives as nanomolar inhibitors of NAD kinase from pathogenic bacteria

Nicotinamide adenine dinucleotide kinases (NAD kinases) are essential and ubiquitous enzymes involved in the production of NADP(H) which is an essential cofactor in many metabolic pathways. Targeting NAD kinase (NADK), a rate limiting enzyme of NADP biosynthesis pathway, represents a new promising approach to treat bacterial infections. Previously, we have produced the first NADK inhibitor active against staphylococcal infection. From this linear di-adenosine derivative, namely NKI1, we designed macrocyclic analogues. Here, we describe the synthesis and evaluation of an original series of cyclic diadenosine derivatives as NADK inhibitors of two pathogenic bacteria, Listeria monocytogenes and Staphylococcus aureus. The nature and length of the link between the two adenosine units were examined leading to sub-micromolar inhibitors of NADK1 from L. monocytogenes, including its most potent in vitro inhibitor reported so far (with a 300-fold improvement compared to NKI1).

Small circular interfering RNAs (sciRNAs) as a potent therapeutic platform for gene-silencing

In order to achieve efficient therapeutic post-transcriptional gene-silencing mediated by the RNA interference (RNAi) pathway, small interfering RNAs (siRNAs) must be chemically modified. Several supra-RNA structures, with the potential to stabilize siRNAs metabolically have been evaluated for their ability to induce gene silencing, but all have limitations or have not been explored in therapeutically relevant contexts. Covalently closed circular RNA transcripts are prevalent in eukaryotes and have potential as biomarkers and disease targets, and circular RNA mimics are being explored for use as therapies. Here we report the synthesis and evaluation of small circular interfering RNAs (sciRNAs). To synthesize sciRNAs, a sense strand functionalized with the trivalent N-acetylgalactosamine (GalNAc) ligand and cyclized using ‘click’ chemistry was annealed to an antisense strand. This strategy was used for synthesis of small circles, but could also be used for synthesis of larger circular RNA mimics. We evaluated various sciRNA designs in vitro and in vivo. We observed improved metabolic stability of the sense strand upon circularization and off-target effects were eliminated. The 5′-(E)-vinylphosphonate modification of the antisense strand resulted in GalNAc-sciRNAs that are potent in vivo at therapeutically relevant doses. Physicochemical studies and NMR-based structural analysis, together with molecular modeling studies, shed light on the interactions of this novel class of siRNAs, which have a partial duplex character, with the RNAi machinery.

Triazole-Modified Nucleic Acids for the Application in Bioorganic and Medicinal Chemistry

This review covers studies which exploit triazole-modified nucleic acids in the range of chemistry and biology to medicine. The 1,2,3-triazole unit, which is obtained via click chemistry approach, shows valuable and unique properties. For example, it does not occur in nature, constitutes an additional pharmacophore with attractive properties being resistant to hydrolysis and other reactions at physiological pH, exhibits biological activity (i.e., antibacterial, antitumor, and antiviral), and can be considered as a rigid mimetic of amide linkage. Herein, it is presented a whole area of useful artificial compounds, from the clickable monomers and dimers to modified oligonucleotides, in the field of nucleic acids sciences. Such modifications of internucleotide linkages are designed to increase the hybridization binding affinity toward native DNA or RNA, to enhance resistance to nucleases, and to improve ability to penetrate cell membranes. The insertion of an artificial backbone is used for understanding effects of chemically modified oligonucleotides, and their potential usefulness in therapeutic applications. We describe the state-of-the-art knowledge on their implications for synthetic genes and other large modified DNA and RNA constructs including non-coding RNAs.

Covalent labeling of nucleic acids

Labeling of nucleic acids is required for many studies aiming to elucidate their functions and dynamics in vitro and in cells. Out of the numerous labeling concepts that have been devised, covalent labeling provides the most stable linkage, an unrivaled choice of small and highly fluorescent labels and - thanks to recent advances in click chemistry - an incredible versatility. Depending on the approach, site-, sequence- and cell-specificity can be achieved. DNA and RNA labeling are rapidly developing fields that bring together multiple areas of research: on the one hand, synthetic and biophysical chemists develop new fluorescent labels and isomorphic nucleobases as well as faster and more selective bioorthogonal reactions. On the other hand, the number of enzymes that can be harnessed for post-synthetic and site-specific labeling of nucleic acids has increased significantly. Together with protein engineering and genetic manipulation of cells, intracellular and cell-specific labeling has become possible. In this review, we provide a structured overview of covalent labeling approaches for nucleic acids and highlight notable developments, in particular recent examples. The majority of this review will focus on fluorescent labeling; however, the principles can often be readily applied to other labels. We will start with entirely chemical approaches, followed by chemo-enzymatic strategies and ribozymes, and finish with metabolic labeling of nucleic acids. Each section is subdivided into direct (or one-step) and two-step labeling approaches and will start with DNA before treating RNA.

Modified Galacto- or Fuco-Clusters Exploiting the Siderophore Pathway to Inhibit the LecA- or LecB-Associated Virulence of Pseudomonas aeruginosa

Galacto- and fuco-clusters conjugated with one to three catechol or hydroxamate motifs were synthesised to target LecA and LecB lectins of Pseudomonas aeruginosa (PA) localised in the outer membrane and inside the bacterium. The resulting glycocluster-pseudosiderophore conjugates were evaluated as Trojan horses to cross the outer membrane of PA by iron transport. The data suggest that glycoclusters with catechol moieties are able to hijack the iron transport, whereas those with hydroxamates showed strong nonspecific interactions. Mono- and tricatechol galactoclusters (G1C and G3C) were evaluated as inhibitors of infection by PA in comparison with the free galactocluster (G0). All of them exhibited an inhibitory effect between 46 to 75 % at 100 μM, with a higher potency than G0. This result shows that LecA localised in the outer membrane of PA is involved in the infection mechanism.

Fine-tuning the properties of the thrombin binding aptamer through cyclization: Effect of the 5'-3' connecting linker on the aptamer stability and anticoagulant activity

A small library of cyclic TBA analogues (named cycTBA I-IV), obtained by covalently connecting its 5'- and 3'-ends with flexible linkers, has been synthesized with the aim of improving its chemical and enzymatic stability, as well as its anticoagulant properties. Two chemical procedures have been exploited to achieve the desired cyclization, based on the oxime ligation method (providing cycTBA I and II) or on Cu(I)-assisted azide-alkyne cycloaddition (CuAAC) protocols (for cycTBA III and IV), leading to analogues containing circularizing linkers with different chemical nature and length, overall spanning from 22 to 48 atoms. The resulting cyclic TBAs have been characterized using a variety of biophysical methods (UV, CD, gel electrophoresis, SE-HPLC analyses) and then tested for their serum resistance and anticoagulant activity under in vitro experiments. A fine-tuning of the length and flexibility of the linker allowed identifying a cyclic analogue, cycTBA II, with improved anticoagulant activity, associated with a dramatically stabilized G-quadruplex structure (ΔT_m = +17 °C) and a 6.6-fold higher enzymatic resistance in serum compared to unmodified TBA.

Stability Is Not Everything: The Case of the Cyclisation of a Thrombin-Binding Aptamer

With the aim of developing a new approach to obtain improved aptamers, a cyclic thrombin-binding aptamer (TBA) analogue (cycTBA) has been prepared by exploiting a copper(I)-assisted azide-alkyne cycloaddition. The markedly increased serum resistance and exceptional thermal stability of the G-quadruplex versus TBA were associated with halved thrombin inhibition, which suggested that some flexibility in the TBA structure was necessary for protein recognition.

Phosphodiester photo-tethers for the (multi-)cyclic conformational caging of oligonucleotides

The ability to address the function of oligonucleotides with light is highly desirable since they are often used experimentally in the regulation of biological processes that need to be controlled in time, space and activation level. Here we present an extension of our initial approach of using photo-tethers that force single strands of nucleic acids into a circle, thus making them unable to form a duplex with a complementary DNA- or RNA-strand. Due to the persistence length a single strand can form a circle of, for example, 30 nucleotides, but a duplex cannot. We show that these new photo-tethers can also be easily installed on the phosphodiester backbone. This simplifies the approach considerably and leads to temporarily inhibited oligonucleotides that can only form a duplex after linearization by photoactivation.

Nucleoside macrocycles formed by intramolecular click reaction: efficient cyclization of pyrimidine nucleosides decorated with 5'-azido residues and 5-octadiynyl side chains

Copper(I)-promoted "click" cyclization in the presence of TBTA afforded nucleoside macrocycles in very high yields (≈70%) without using protecting groups. To this end, dU and dC derivatives functionalized at the 5-position of the nucleobase with octadiynyl side chains and with azido groups at the 5'-position of the sugar moieties were synthesized. The macrocycles display freely accessible Watson-Crick recognition sites. The conformation of the 16-membered macrocycle was deduced from X-ray analysis and ¹H,¹H-NMR coupling constants. The sugar conformation (N vs S) was different in solution as compared to the solid state.

Fluorescent Thrombin Binding Aptamer-Tagged Nanoparticles for an Efficient and Reversible Control of Thrombin Activity

Progress in understanding and treatment of thrombotic diseases requires new effective methods for the easy, rapid, and reversible control of coagulation processes. In this framework, the use of aptamers, and particularly of the thrombin binding aptamer (TBA), has aroused strong interest, due to its enormous therapeutic potential, associated with a large number of possible applications in biotechnological and bioanalytical fields. Here, we describe a new TBA analogue (named tris-mTBA), carrying three different pendant groups: a dansyl residue at the 3'- and a β-cyclodextrin moiety at the 5'-end-providing a host-guest system which exhibits a marked fluorescence enhancement upon TBA G-quadruplex folding-and a biotin tag, allowing the attachment of the aptamer onto biocompatible streptavidin-coated silica nanoparticles (NPs) of 50 nm hydrodynamic diameter (Sicastar). The use of nanoparticles for the in vivo delivery of TBA, expected to induce per se increased nuclease resistance and improved pharmacokinetic properties of this oligonucleotide, offers as an additional advantage the possibility to exploit multivalency effects, due to the presence of multiple copies of TBA on a single scaffold. In addition, the selected fluorescent system allows monitoring both the presence of TBA on the functionalized NPs and its correct folding upon immobilization, also conferring enhanced enzymatic resistance and bioactivity. The anticoagulant activity of the new tris-mTBA, free or conjugated to Sicastar NPs, was evaluated by dynamic light scattering experiments. Highly effective and reversible inhibition of thrombin activity toward fibrinogen was found for the free tris-mTBA and especially for the tris-mTBA-conjugated NPs, demonstrating great potential for the biomedical control of blood clotting.

Stabilization of RNA hairpins using non-nucleotide linkers and circularization

An RNA hairpin is an essential structural element of RNA. Hairpins play crucial roles in gene expression and intermolecular recognition but are also involved in the pathogenesis of some congenital diseases. Structural studies of the hairpin motifs are impeded by their thermodynamic instability, as they tend to unfold to form duplexes, especially at high concentrations required for crystallography or nuclear magnetic resonance spectroscopy. We have elaborated techniques to stabilize the RNA hairpins by linking the free ends of the RNA strand at the base of the hairpin stem. One method involves stilbene diether or hexaethylene glycol linkers and circularization by T4 RNA ligase. Another method uses click chemistry to stitch the RNA ends with a triazole linker. Both techniques are efficient and easy to perform. They should be useful in making stable, biologically relevant RNA constructs for structural studies.

"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.

The "Clickable" Method for Oligonucleotide Immobilization Onto Azide-Functionalized Microarrays

The DNA microarray technique was supposed to help identifying and analyzing the expression level of tens of thousands of genes in the whole genome. But there is a serious problem concerning fabrication of the microarrays by chemical synthesis, such as specific and efficient linking of probes to a solid support. Therefore, we reckon that applying "click" chemistry to covalently anchor oligonucleotides on chemically modified supports may help construct microarrays in applications such as gene identification. Silanization of the glass support with organofunctional silane makes it possible to link azide groups on glass surface and the nucleic acid probe that is equipped with a pentynyl group. This is followed by direct spotting of the nucleic acid on the azide-modified glass support in the presence of copper ions, and this is a frequently applied method of "click" chemistry.

Concise Synthesis of Triazole-Linked 5'-Peptide-Oligonucleotide Conjugates by Click Chemistry

A concise synthesis of oligonucleotide 5'-peptide-conjugates via copper(I)-catalyzed azide-alkyne 1,3-dipolar cycloaddition in aqueous solution is described. Synthesis of reagents was accomplished by on-column derivatization of corresponding peptides and oligonucleotides. This method is well suited for the preparation of peptide-oligonucleotide conjugates containing 1,2,3-triazole linkage between the 5'-position of an oligonucleotide and the N-terminus of a peptide.

Circular DNA by "Bis-Click" Ligation: Template-Independent Intramolecular Circularization of Oligonucleotides with Terminal Alkynyl Groups Utilizing Bifunctional Azides

A highly effective and convenient "bis-click" strategy was developed for the template-independent circularization of single-stranded oligonucleotides by employing copper(I)-assisted azide-alkyne cycloaddition. Terminal triple bonds were incorporated at both ends of linear oligonucleotides. Alkynylated 7-deaza-2'-deoxyadenosine and 2'-deoxyuridine residues with different side chains were used in solid-phase synthesis with phosphoramidite chemistry. The bis-click ligation of linear 9- to 36-mer oligonucleotides with 1,4-bis(azidomethyl)benzene afforded circular DNA in a simple and selective way; azido modification of the oligonucleotide was not necessary. Short ethynyl side chains were compatible with the circularization of longer oligonucleotides, whereas octadiynyl residues were used for short 9-mers. Compared with linear duplexes, circular bis-click constructs exhibit a significantly increased duplex stability over their linear counterparts. The intramolecular bis-click ligation protocol is not limited to DNA, but may also be suitable for the construction of other macrocycles, such as circular RNAs, peptides, or polysaccharides.

Importance of topology for glycocluster binding to Pseudomonas aeruginosa and Burkholderia ambifaria bacterial lectins

Pseudomonas aeruginosa (PA) and Burkholderia ambifaria (BA) are two opportunistic Gram negative bacteria and major infectious agents involved in lung infection of cystic fibrosis patients. Both bacteria can develop resistance to conventional antibiotherapies. An alternative strategy consists of targeting virulence factors in particular lectins with high affinity ligands such as multivalent glycoclusters. LecA (PA-IL) and LecB (PA-IIL) are two tetravalent lectins from PA that recognise galactose and fucose respectively. BambL lectin from BA is trimeric with 2 binding sites per monomer and is also specific for fucose. These three lectins are potential therapeutic targets in an anti-adhesive anti-bacterial approach. Herein, we report the synthesis of 18 oligonucleotide pentofuranose-centered or mannitol-centered glycoclusters leading to tri-, penta- or decavalent clusters with different topologies. The linker arm length between the core and the carbohydrate epitope was also varied leading to 9 galactoclusters targeting LecA and 9 fucoclusters targeting both LecB and BambL. Their dissociation constants (Kd) were determined using a DNA-based carbohydrate microarray technology. The trivalent xylo-centered galactocluster and the ribo-centered fucocluster exhibited the best affinity for LecA and LecB respectively while the mannitol-centered decafucocluster displayed the best affinity to BambL. These data demonstrated that the topology and nature of linkers were the predominant factors for achieving high affinity rather than valency.

Assessment of the Full Compatibility of Copper(I)-Catalyzed Alkyne-Azide Cycloaddition and Oxime Click Reactions for bis-Labelling of Oligonucleotides

The conjugation of oligonucleotides with reporters is of great interest for improving their intrinsic properties or endowing new ones. In this context, we report herein a new procedure for the bis-labelling of oligonucleotides through oxime ligation (Click-O) and copper(I)-catalyzed alkyne–azide cycloaddition (Click-H). 5′-Azido and 3′-aldehyde precursors were incorporated into oligonucleotides, and subsequent coupling reactions through Click-O and Click-H (or vice versa) were successfully achieved. In particular, we exhaustively investigated the full compatibility of each required step for both tethering strategies. The results demonstrate that click Huisgen and click oxime reactions are fully compatible. However, whilst both approaches can deliver the targeted doubly conjugated oligonucleotide, the route involving click oxime ligation prior to click Huisgen is significantly more successful. Thus the reactions investigated here can be considered to be key elements of the chemical toolbox for the synthesis of highly sophisticated bioconjugates.

Synthesis of a multibranched porphyrin-oligonucleotide scaffold for the construction of DNA-based nano-architectures

The interest in the functionalization of oligonucleotides with organic molecules has grown considerably over the last decade. In this work, we report on the synthesis and characterization of porphyrin-oligonucleotide hybrids containing one to four DNA strands (P1-P4). The hybrid P4, which inserts one porphyrin and four DNA fragments, was combined with gold nanoparticles and imaged by transmission electron microscopy.

Structure binding relationship of galactosylated Glycoclusters toward Pseudomonas aeruginosa lectin LecA using a DNA-based carbohydrate microarray

Pseudomonas aeruginosa (PA) is a major public health issue due to its impact on nosocomial infections as well as its impact on cystic fibrosis patient mortality. One of the main concerns is its ability to develop antibiotic resistance. Therefore, inhibition of PA virulence has been proposed as an alternative strategy to tackle PA based infections. LecA (or PA-IL), a galactose binding lectin from PA, is involved in its virulence. Herein, we aimed at designing high affinity synthetic ligands toward LecA for its inhibition and at understanding the key parameters governing the binding of multivalent galactosylated clusters. Twenty-five glycoclusters were synthesized and their bindings were studied on a carbohydrate microarray. Monosaccharide centered clusters and linear comb-like clusters were synthesized with different linkers separating the core and the galactosyl residues. Their length, flexibility, and aromaticity were varied. Our results showed that the binding profile of LecA to galactosylated clusters was dependent on both the core and the linker and also that the optimal linker was different for each core. Nevertheless, an aryl group in the linker structure drastically improved the binding to LecA. Our results also suggest that optimal distances are preferred between the core and the aromatic group and the core and the galactose.

Fluorescence enhancement upon G-quadruplex folding: synthesis, structure, and biophysical characterization of a dansyl/cyclodextrin-tagged thrombin binding aptamer

A novel fluorescent thrombin binding aptamer (TBA), conjugated with the environmentally sensitive dansyl probe at the 3'-end and a β-cyclodextrin residue at the 5'-end, has been efficiently synthesized exploiting Cu(I)-catalyzed azide-alkyne cycloaddition procedures. Its conformation and stability in solution have been studied by an integrated approach, combining in-depth NMR, CD, fluorescence, and DSC studies. ITC measurements have allowed us to analyze in detail its interaction with human thrombin. All the collected data show that this bis-conjugated aptamer fully retains its G-quadruplex formation ability and thrombin recognition properties, with the terminal appendages only marginally interfering with the conformational behavior of TBA. Folding of this modified aptamer into the chairlike, antiparallel G-quadruplex structure, promoted by K(+) and/or thrombin binding, typical of TBA, is associated with a net fluorescence enhancement, due to encapsulation of dansyl, attached at the 3'-end, into the apolar cavity of the β-cyclodextrin at the 5'-end. Overall, the structural characterization of this novel, bis-conjugated TBA fully demonstrates its potential as a diagnostic tool for thrombin recognition, also providing a useful basis for the design of suitable aptamer-based devices for theranostic applications, allowing simultaneously both detection and inhibition or modulation of the thrombin activity.

Solid-phase supports for oligonucleotide synthesis

This unit attempts to provide a reasonably complete inventory of over 280 solid supports available to oligonucleotide chemists for preparation of natural and 3'-modified oligonucleotides. Emphasis is placed on non-nucleosidic solid supports. The relationship between the structural features of linkers and their behavior in oligonucleotide synthesis and deprotection is discussed wherever the relevant observations are available.

Oligonucleotide cyclization: the thiol-maleimide reaction revisited

A novel method to synthesize cyclic oligonucleotides (5- to 26-mer) using the thiol-maleimide reaction is described. The target molecules were obtained after subsequent removal of thiol and maleimide protecting groups from 5'-maleimido-3'-thiol-derivatized linear precursors. Retro-Diels-Alder conditions deprotecting the maleimide simultaneously promoted cyclization cleanly and in high yield.

Synthesis of a library of fucosylated glycoclusters and determination of their binding toward Pseudomonas aeruginosa lectin B (PA-IIL) using a DNA-based carbohydrate microarray

Pseudomonas aeruginosa (PA) is a Gram negative opportunistic pathogen and is the major pathogen encounter in the cystic fibrosis (CF) lung airways. It often leads to chronic respiratory infection despite aggressive antibiotic therapy due to the emergence of resistant strains and to the formation of biofilm. The lectin PA-IIL (LecB) is a fucose-specific lectin from PA suspected to be involved in host recognition/adhesion and in biofilm formation. Thus, it can be foreseen as a potential therapeutic target. Herein, 16 fucosylated glycoclusters with antenna-like, linear, or crown-like spatial arrangements were synthesized using a combination of DNA solid-phase synthesis and alkyne azide 1,3-dipolar cycloaddition (CuAAC). Their binding properties toward PA-IIL were then evaluated based on DNA directed immobilization (DDI) carbohydrate microarray. Our results suggested that the antenna-like scaffold was preferred to linear or crown-like glycoclusters. Among the crown-like carbohydrate centered fucosylated glycoclusters, mannose-based core was better than glucose- and galactose-based ones. The influence of the linker arm was also evaluated, and long linkers between fucoses and the core led to a slight better binding than the short ones.

Alkynyl phosphonate DNA: a versatile "click"able backbone for DNA-based biological applications

Major hurdles associated with DNA-based biological applications include, among others, targeted cell delivery, undesirable nonspecific effects, toxicity associated with various analogues or the reagents used to deliver oligonucleotides to cells, and stability toward intracellular enzymes. Although a plethora of diverse analogues have been investigated, a versatile methodology that can systematically address these challenges has not been developed. In this contribution, we present a new, Clickable, and versatile chemistry that can be used to rapidly introduce diverse functionality for studying these various problems. As a demonstration of the approach, we synthesized the core analogue, which is useful for introducing additional functionality, the triazolylphosphonate, and present preliminary data on its biological properties. We have developed a new phosphoramidite synthon--the alkynyl phosphinoamidite, which is compatible with conventional solid-phase oligonucleotide synthesis. Postsynthesis, the alkynylphosphonate can be functionalized via "Click" chemistry to generate the 1,2,3-triazolyl or substituted 1,2,3-triazolyl phosphonate-2'-deoxyribonucleotide internucleotide linkage. This manuscript describes the automated, solid-phase synthesis of mixed backbone oligodeoxyribonucleotides (ODNs) having 1,2,3-triazolylphosphonate (TP) as well as phosphate or thiophosphate internucleotide linkages and also 2'-OMe ribonucleotides and locked nucleic acids (LNAs) at selected sites. Nuclease stability assays demonstrate that the TP linkage is highly resistant toward 5'- and 3'-exonucleases, whereas melting studies indicate a slight destabilization when a TP-modified ODN is hybridized to its complementary RNA. A fluorescently labeled 16-mer ODN modified with two TP linkages shows efficient cellular uptake during passive transfection. Of particular interest, the subcellular distribution of TP-modified ODNs is highly dependent on cell type; a significant nuclear uptake is observed in HeLa cells, whereas diffuse cytoplasmic fluorescence is found in the WM-239A cell line. Cytoplasmic distribution is also present in human neuroblastoma cells (SK-N-F1), but Jurkat cells show both diffuse and punctate cytoplasmic uptake. Our results demonstrate that triazolylphosphonate ODNs are versatile additions to the oligonucleotide chemist's toolbox relative to designing new biological research reagents.

Construction and assembly of branched Y-shaped DNA: "click" chemistry performed on dendronized 8-aza-7-deazaguanine oligonucleotides

Branched DNA was synthesized from tripropargylated oligonucleotides by the Huisgen-Meldal-Sharpless cycloaddition using "stepwise and double click" chemistry. Dendronized oligonucleotides decorated with 7-tripropargylamine side chains carrying two terminal triple bonds were further functionalized with bis-azides to give derivatives with two terminal azido groups. Then, the branched side chains with two azido groups or two triple bonds were combined with DNA-fragments providing the corresponding clickable function. Both concepts afforded branched (Y-shaped) three-armed DNA. Annealing of branched DNA with complementary oligonucleotides yielded supramolecular assemblies. The concept of "stepwise and double click" chemistry combined with selective hybridization represents a flexible tool to generate DNA nanostructures useful for various purposes in DNA diagnostics, delivery, and material science applications.

Application of click chemistry to the production of DNA microarrays

The copper-catalyzed alkyne-azide cycloaddition (CuAAC) reaction was applied as the novel method of DNA immobilization on a modified solid support. The CuAAC click reaction enables the covalent binding of DNA modified with pentynyl groups at its 5'-end to azide-loaded slides. Click microarrays were produced using this approach and successfully employed in biological/model experiments.

Versatile introduction of azido moiety into oligonucleotides through diazo transfer reaction

The use of a diazo transfer (DZT) reagent enables clean and efficient conversion of aminated oligodeoxyribonucleotides (ODNs) into their azido counterparts under mild conditions. ODNs bearing an amino tether at the 3', 5', or any internal position could be modified in this manner thus demonstrating the versatility of this reaction. Easy access to such azido-modified ODNs is of great interest for conjugation in particular through copper catalyzed 1,3-dipolar cycloaddition (CuAAC reaction).

RETRACTED: Unclicking the click: mechanically facilitated 1,3-dipolar cycloreversions

The specific targeting of covalent bonds in a local, anisotropic fashion using mechanical methods offers useful opportunities to direct chemical reactivity down otherwise prohibitive pathways. Here, we report that embedding the highly inert 1,2,3-triazole moiety (which is often prepared using the canonical "click" coupling of azides and alkynes) within a poly(methyl acrylate) chain renders it susceptible to ultrasound-induced cycloreversion, as confirmed by comprehensive spectroscopic and chemical analyses. Such reactivity offers the opportunity to develop triazoles as mechanically labile protecting groups or for use in readily accessible materials that respond to mechanical force.

Stepwise "click" chemistry for the template independent construction of a broad variety of cross-linked oligonucleotides: influence of linker length, position, and linking number on DNA duplex stability

Cross-linked DNA was constructed by a "stepwise click" reaction using a bis-azide. The reaction is performed in the absence of a template, and a monofunctionalized oligonucleotide bearing an azido-function is formed as intermediate. For this, an excess of the bis-azide has to be used compared to the alkynylated oligonucleotide. The cross-linking can be carried out with any alkynylated DNA having a terminal triple bond at any position of the oligonucleotide, independent of chain length or sequence with identical or nonidentical chains. Short and long linkers with terminal triple bonds were introduced in the 7-position of 8-aza-7-deaza-2'-deoxyguanosine (1 or 2), and the outcome of the "stepwise" click and the "bis-click" reaction was compared. The cross-linked DNAs form cross-linked duplexes when hybridized with single-stranded complementary oligonucleotides. The stability of these cross-linked duplexes is as high as respective individual duplexes when they were ligated at terminal positions with linkers of sufficient length. The stability decreases when the linkers are incorporated at central positions. The highest duplex stability was reached when two complementary cross-linked oligonucleotides were hybridized.

Strain-promoted "click" chemistry for terminal labeling of DNA

1,3-Dipolar [3 + 2] cycloaddition between azides and alkynes--an archetypal "click" chemistry--has been used increasingly for the functionalization of nucleic acids. Copper(I)-catalyzed 1,3-dipolar cycloaddition reactions between alkyne-tagged DNA molecules and azides work well, but they require optimization of multiple reagents, and Cu ions are known to mediate DNA cleavage. For many applications, it would be preferable to eliminate the Cu(I) catalyst from these reactions. Here, we describe the solid-phase synthesis and characterization of 5'-dibenzocyclooctyne (DIBO)-modified oligonucleotides, using a new DIBO phosphoramidite, which react with azides via copper-free, strain-promoted alkyne-azide cycloaddition (SPAAC). We found that the DIBO group not only survived the standard acidic and oxidative reactions of solid-phase oligonucleotide synthesis (SPOS), but that it also survived the thermal cycling and standard conditions of the polymerase chain reaction (PCR). As a result, PCR with DIBO-modified primers yielded "clickable" amplicons that could be tagged with azide-modified fluorophores or immobilized on azide-modified surfaces. Given its simplicity, SPAAC on DNA could streamline the bioconjugate chemistry of nucleic acids in a number of modern biotechnologies.

Microwave promoted simple, efficient and regioselective synthesis of trisubstituted imidazo[1,2-a]benzimidazoles on soluble support

An efficient microwave-assisted and soluble polymer-supported synthesis of medicinally important imidazole-fused benzimidazoles has been developed. The protocol involves the rapid condensation of polymer-bound amino benzimidazoles with various α-bromoketones and subsequent in situ intramolecular cyclization under microwave irradiation resulting in a one pot synthesis of imidazole interlacing benzimidazole polymer conjugates. The condensed product was obtained with excellent regioselectivity. The biologically interesting imidazo[1,2-a]benzimidazoles was released from polymer support at ambient temperature. Diversity in the triheterocyclic nucleus was achieved by the different substitutions at its 2, 3, and 9 positions. The new protocol has the advantages of short reaction time, easy workup process, excellent yields, reduced environmental impact, wide substrate scope and convenient procedure.

Versatile site-specific conjugation of small molecules to siRNA using click chemistry

We have previously demonstrated that conjugation of small molecule ligands to small interfering RNAs (siRNAs) and anti-microRNAs results in functional siRNAs and antagomirs in vivo. Here we report on the development of an efficient chemical strategy to make oligoribonucleotide-ligand conjugates using the copper-catalyzed azide-alkyne cycloaddition (CuAAC) or click reaction. Three click reaction approaches were evaluated for their feasibility and suitability for high-throughput synthesis: the CuAAC reaction at the monomer level prior to oligonucleotide synthesis, the solution-phase postsynthetic "click conjugation", and the "click conjugation" on an immobilized and completely protected alkyne-oligonucleotide scaffold. Nucleosides bearing 5'-alkyne moieties were used for conjugation to the 5'-end of the oligonucleotide. Previously described 2'- and 3'-O-propargylated nucleosides were prepared to introduce the alkyne moiety to the 3' and 5' termini and to the internal positions of the scaffold. Azido-functionalized ligands bearing lipophilic long chain alkyls, cholesterol, oligoamine, and carbohydrate were utilized to study the effect of physicochemical characteristics of the incoming azide on click conjugation to the alkyne-oligonucleotide scaffold in solution and on immobilized solid support. We found that microwave-assisted click conjugation of azido-functionalized ligands to a fully protected solid-support bound alkyne-oligonucleotide prior to deprotection was the most efficient "click conjugation" strategy for site-specific, high-throughput oligonucleotide conjugate synthesis tested. The siRNA conjugates synthesized using this approach effectively silenced expression of a luciferase gene in a stably transformed HeLa cell line.

Oligonucleotide carbohydrate-centered galactosyl cluster conjugates synthesized by click and phosphoramidite chemistries

Oligonucleotide glycoconjugates with a mannose or galactose core bearing four galactose residues introduced by phosphoramidite chemistry and copper catalyzed azide alkyne 1,3-dipolar cycloaddition (click chemistry) have been synthesized. A first click reaction allowed the introduction on a solid support of a mannose core on which four pentynyl linkers were introduced using a phosphoramidite derivative. After the elongation of the oligonucleotide, a second click reaction performed either on solid support or in solution allowed the introduction of four galactose azide derivatives. Repeating the phosphoramidite and click chemistries afforded an oligonucleotide glycoconjugate dendrimer bearing 16 galactoses on its periphery.

5'-Bis-conjugation of oligonucleotides by amidative oxidation and click chemistry

A pent-4-ynyl tert-butyl N,N-diisopropyl phosphoramidite was coupled at the 5'-end of oligonucleotides to give a phosphite triester linkage, which forms an H-phosphonate diester linkage during treatment with dichloroacetic acid. Then an amidative oxidation with CCl(4) in the presence of an amine and a 1,3-dipolar cycloaddition with an azide under copper(I) catalysis afforded the bis-conjugated oligonucleotides with high efficiency. The introduction of a bromoalkyl group as a precursor of azidoalkyl by amidative oxidation allowed the performance of two selective 1,3-dipolar cycloadditions.

Cross-linked DNA generated by "bis-click" reactions with bis-functional azides: site independent ligation of oligonucleotides via nucleobase alkynyl chains

Template-free cross-linking of single-stranded DNA bearing octadiynyl side chains at the 7-position of 8-aza-7-deazapurine moieties with bisfunctional azides is reported employing a Cu(I)-catalyzed azide-alkyne "bis-click" reaction. Bis-adducts were formed when the bis-azide:oligonucleotide ratio was 1:1; monofunctionalization occurred when the ratio was 15:1. Four-stranded DNA consisting of two cross-linked duplexes was obtained after hydridization. Cross-linked duplexes are as stable as individual duplexes when ligation was introduced at terminal positions; ligation at a central position led to a slight duplex destabilization.