Pyrenemethyl ara-uridine-2'-carbamate: a strong interstrand excimer in the major groove of a DNA duplex

Factors Impacting Invader-Mediated Recognition of Double-Stranded DNA

The development of chemically modified oligonucleotides enabling robust, sequence-unrestricted recognition of complementary chromosomal DNA regions has been an aspirational goal for scientists for many decades. While several groove-binding or strand-invading probes have been developed towards this end, most enable recognition of DNA only under limited conditions (e.g., homopurine or short mixed-sequence targets, low ionic strength, fully modified probe strands). Invader probes, i.e., DNA duplexes modified with +1 interstrand zippers of intercalator-functionalized nucleotides, are predisposed to recognize DNA targets due to their labile nature and high affinity towards complementary DNA. Here, we set out to gain further insight into the design parameters that impact the thermal denaturation properties and binding affinities of Invader probes. Towards this end, ten Invader probes were designed, and their biophysical properties and binding to model DNA hairpins and chromosomal DNA targets were studied. A Spearman's rank-order correlation analysis of various parameters was then performed. Densely modified Invader probes were found to result in efficient recognition of chromosomal DNA targets with excellent binding specificity in the context of denaturing or non-denaturing fluorescence in situ hybridization (FISH) experiments. The insight gained from the initial phase of this study informed subsequent probe optimization, which yielded constructs displaying improved recognition of chromosomal DNA targets. The findings from this study will facilitate the design of efficient Invader probes for applications in the life sciences.

Serine-γPNA, Invader probes, and chimeras thereof: three probe chemistries that enable sequence-unrestricted recognition of double-stranded DNA

Three probe chemistries are evaluated with respect to thermal denaturation temperatures, UV-Vis and fluorescence characteristics, recognition of complementary and mismatched DNA hairpin targets, and recognition of chromosomal DNA targets in the context of non-denaturing fluorescence in situ hybridization (nd-FISH) experiments: (i) serine-γPNAs (SγPNAs), i.e., single-stranded peptide nucleic acid (PNA) probes that are modified at the γ-position with (R)-hydroxymethyl moieties, (ii) Invader probes, i.e., DNA duplexes modified with +1 interstrand zippers of 2'-O-(pyren-1-yl)methyl-RNA monomers, a molecular arrangement that results in a violation of the neighbor exclusion principle, and (iii) double-stranded chimeric SγPNAs:Invader probes, i.e., duplexes between complementary SγPNA and Invader strands, which are destabilized due to the poor compatibility between intercalators and PNA:DNA duplexes. Invader probes resulted in efficient, highly specific, albeit comparatively slow recognition of the model DNA hairpin targets. Recognition was equally efficient and faster with the single-stranded SγPNA probes but far less specific, whilst the double-stranded chimeric SγPNAs:Invader probes displayed recognition characteristics that were intermediate of the parent probes. All three probe chemistries demonstrated the capacity to target chromosomal DNA in nd-FISH experiments, with Invader probes resulting in the most favorable and consistent characteristics (signals in >90% of interphase nuclei against a low background and no signal in negative control experiments). These probe chemistries constitute valuable additions to the molecular toolbox needed for DNA-targeting applications.

Nicked Invader probes: multistranded and sequence-unrestricted recognition of double-stranded DNA

Major efforts have been devoted to the development of constructs that enable sequence-specific recognition of double-stranded (ds) DNA, fueled by the promise for enabling tools for applications in molecular biology, diagnostics, and medicine. Towards this end, we have previously introduced Invader probes, i.e., short DNA duplexes with +1 interstrand zipper arrangements of intercalator-functionalized nucleotides. The individual strands of these labile probes display high affinity towards complementary DNA (cDNA), which drives sequence-unrestricted dsDNA-recognition. However, recognition of long targets is challenging due to the high stability of the corresponding probes. To address this, we recently introduced toehold Invader probes, i.e., Invader probes with 5'-single-stranded overhangs. The toehold architecture allows for shorter double-stranded segments to be used, which facilitates probe dissociation and dsDNA-recognition. As an extension thereof, we here report the biophysical and dsDNA-targeting properties of nicked Invader probes. In this probe architecture, the single-stranded overhangs of toehold Invader probes are hybridized to short intercalator-modified auxiliary strands, leading to formation of additional labile segments. The extra binding potential from the auxiliary strands imparts nicked Invader probes with greater dsDNA-affinity than the corresponding toehold or blunt-ended probes. Recognition of chromosomal DNA targets, refractory to recognition by conventional Invader probes, is demonstrated for nicked Invader probes in the context of non-denaturing FISH experiments, which highlights their utility as dsDNA-targeting tools.

Recognition of double-stranded DNA using LNA-modified toehold Invader probes

Development of molecules capable of binding to specific sequences of double-stranded (ds) DNA continues to attract considerable interest, as this may yield useful tools for applications in life science, biotechnology, and medicine. We have previously demonstrated sequence-unrestricted of dsDNA using Invader probes, i.e., DNA duplexes that are energetically activated through incorporation of +1 interstrand zipper arrangements of O2'-intercalator-functionalized RNA monomers. Nonetheless, recognition of extended dsDNA target regions remains challenging due to the high stability of the corresponding probes. To address this, we introduce toehold Invader probes, i.e., Invader probes with 5'-single-stranded overhangs. This design provides access to probes with shortened double-stranded segments, which facilitates probe denaturation. The single-stranded overhangs can, furthermore, be modified with affinity-enhancing modifications like LNA (locked nucleic acid) monomers to additionally increase target affinity. Herein, we report the biophysical and dsDNA-targeting properties of different toehold Invader designs and compare them to conventional Invader probes. LNA-modified toehold Invader probes display promising recognition characteristics, including greatly improved affinity to dsDNA, excellent binding specificity, and fast recognition kinetics, which enabled recognition of chromosomal DNA targets that have proven refractory to recognition by conventional Invader probes. Thus, toehold Invader probes represent another step toward a robust, oligonucleotide-based approach for sequence-unrestricted dsDNA-recognition.

Fluorescence Lifetime Imaging Microscopy (FLIM) of Intracellular Transport by Means of Doubly Labelled siRNA Architectures

For monitoring the intracellular pathway of small interfering RNA (siRNA), both strands were labelled at internal positions by two ATTO dyes as an interstrand Förster resonance energy transfer pair. siRNA double strands show red emission and a short donor lifetime as readout, whereas siRNA antisense single strands show green emission and a long donor lifetime. This readout signals if GFP silencing can be expected (green) or not (red). We attached both dyes to three structurally different alkyne anchors by postsynthetic modifications. There is only a slight preference for the ribofuranoside anchors with the dyes at their 2'-positions. For the first time, the delivery and fate of siRNA in live HeLa cells was tracked by fluorescence lifetime imaging microscopy (FLIM), which revealed a clear relationship between intracellular transport using different transfection methods and knockdown of GFP expression, which demonstrates the potential of our siRNA architectures as a tool for future development of effective siRNA.

Impact of non-nucleotidic bulges on recognition of mixed-sequence dsDNA by pyrene-functionalized Invader probes

Invader probes, i.e., DNA duplexes modified with +1 interstrand zippers of intercalator-functionalized nucleotides like 2'-O-(pyren-1-yl)methyl-RNA monomers, are energetically activated for sequence-unrestricted recognition of double-stranded DNA (dsDNA) as they are engineered to violate the neighbor exclusion principle, while displaying high affinity towards complementary DNA sequences. The impact on Invader-mediated dsDNA-recognition upon additional modification with different non-nucleotidic bulges is studied herein, based on the hypothesis that bulge-containing Invader probes will display additionally disrupted base-stacking, more extensive denaturation, and improved dsDNA-recognition efficiency. Indeed, Invader probes featuring a single central large bulge - e.g., a nonyl (C9) monomer - display improved recognition of model DNA hairpin targets vis-à-vis conventional Invader probes (C50 values ∼1.5 μM vs. ∼3.9 μM). In contrast, probes with two opposing central bulges display less favorable binding characteristics. Remarkably, C9-modified Invader probes display perfect discrimination between fully complementary dsDNA and dsDNA differing in only one of eighteen base-pairs, underscoring the high binding specificity of double-stranded probes. Cy3-labeled bulge-containing Invader probes are demonstrated to signal the presence of gender-specific DNA sequences in fluorescent in situ hybridization assays (FISH) performed under non-denaturing conditions, highlighting one potential application of dsDNA-targeting Invader probes.

Head-to-head comparison of LNA, MPγPNA, INA and Invader probes targeting mixed-sequence double-stranded DNA

Four probe chemistries are characterized and compared with respect to thermal denaturation temperatures (Tms), thermodynamic parameters associated with duplex formation, and recognition of mixed-sequence double-stranded (ds) DNA targets: (i) oligodeoxyribonucleotides (ONs) modified with Locked Nucleic Acid (LNA) monomers, (ii) MPγPNAs, i.e., single-stranded peptide nucleic acid (PNA) probes that are functionalized at the γ-position with (R)-diethylene glycol (mini-PEG, MP) moieties, (iii) Invader probes, i.e., DNA duplexes modified with +1 interstrand zipper arrangements of 2'-O-(pyren-1-yl)methyl-RNA monomers, and (iv) intercalating nucleic acids (INAs), i.e., DNA duplexes with opposing insertions of 1-O-(1-pyrenylmethyl)glycerol bulges. Invader and INA probes, which are designed to violate the nearest-neighbor exclusion principle, denature readily, whereas the individual probe strands display exceptionally high affinity towards complementary DNA (cDNA) as indicated by increases in Tms of up to 8 °C per modification. Optimized Invader and INA probes enable efficient and highly specific recognition of mixed-sequence dsDNA targets with self-complementary regions (C50 = 30-50 nM), whereas recognition is less efficient with LNA-modified ONs and fully modified MPγPNAs due to lower cDNA affinity (LNA) and a proclivity for dimerization (LNA and MPγPNA). A Cy3-labeled Invader probe is shown to stain telomeric DNA of individual chromosomes in metaphasic spreads under non-denaturing conditions with excellent specificity.

Chimeric γPNA-Invader probes: using intercalator-functionalized oligonucleotides to enhance the DNA-targeting properties of γPNA

Gamma peptide nucleic acids (γPNAs), i.e., single-stranded PNA strands that are modified at the γ-position with (R)-diethylene glycol, and Invader probes, i.e., DNA duplexes with +1 interstrand zipper arrangements of 2'-O-(pyren-1-yl)methyl-RNA monomers, are two types of nucleic acid mimics that are showing promise for sequence-unrestricted recognition of double-stranded (ds) DNA targets. We recently demonstrated that recognition of dsDNA targets with self-complementary regions is challenging for single-stranded high-affinity probes like γPNAs due to their proclivity for secondary structure formation, but not so for Invader probes, which are engineered to form readily denaturing duplexes irrespective of the target sequence context. In the present study, we describe an approach that mitigates these limitations and improves the dsDNA-recognition properties of γPNAs in partially self-complementary target contexts. Chimeric probes between γPNAs and individual Invader strands are shown to form metastable duplexes that (i) are energetically activated for recognition of complementary mixed-sequence dsDNA target regions, (ii) reduce γPNA dimerization, and (iii) substantially improve the fidelity of the dsDNA-recognition process. Chimeric γPNA-Invader probes are characterized with respect to thermal denaturation properties, thermodynamic parameters associated with duplex formation, UV-Vis and fluorescence trends to establish pyrene binding modes, and dsDNA-recognition properties using DNA hairpin model targets.

Synthesis and biophysical characterization of oligonucleotides modified with O2'-alkylated RNA monomers featuring substituted pyrene moieties

Over the past three decades, a wide range of pyrene-functionalized oligonucleotides have been developed and explored for potential applications in material science and nucleic acid diagnostics. Our efforts have focused on their possible use as components of Invader probes, i.e., DNA duplexes with +1 interstrand zipper arrangements of intercalator-functionalized nucleotides. We have previously demonstrated that Invader probes based on 2'-O-(pyren-1-yl)methyl-RNA monomers are energetically activated for sequence-unrestricted recognition of chromosomal DNA targets under non-denaturing conditions. As part of ongoing efforts towards delineating structure-property relationships and optimizing Invader probes, we report the synthesis and biophysical characterization of oligodeoxyribonucleotides (ONs) modified with 2'-O-(7-neo-pentylpyren-1-yl)methyl-uridine monomer V and 2'-O-(7-tert-butyl-1-methoxypyren-5-yl)methyl-uridine monomer Y. ONs modified with monomer V display increased DNA affinity (ΔTm up to +10.5 °C), while Y-modified ONs display lower DNA affinity and up to 22-fold increases in fluorescence emission upon RNA binding. Although these monomers display limited potential as building blocks for Invader probes, their photophysical properties render them of interest for diagnostic RNA-targeting applications.

Conformational studies of 10-23 DNAzyme in solution through pyrenyl-labeled 2'-deoxyadenosine derivatives

10-23 DNAzyme is a small catalytic DNA molecule. Studies on its conformation in solution are critical for understanding its catalytic mechanism and functional optimization. Based on our previous research, two fluorescent nucleoside analogues 1 and 2 were designed for the introduction of a pyrenyl group at one of the five dA residues in the catalytic core and the unpaired adenosine residue in its full-DNA substrate, respectively. Ten pyrenyl-pyrenyl pairs are formed in the DNAzyme-substrate complexes in solution for sensing the spacial positions of the five dA residues relative to the cleavage site using fluorescence spectra. The position-dependent quenching effect of pyrene emission fluorescence by nucleobases, especially the pyrenyl-pyrenyl interaction, was observed for some positions. The adenine residues in the 3'-part of the catalytic loop seem to be closer to the cleavage site than the adenine residues in the 5'-part, which is consistent with the molecular dynamics simulation result. The catalytic activities and T_m changes also confirmed the effect of the pyrenyl-nucleobase and pyrenyl-pyrenyl pair interactions. Together with functional group mutations, catalytically relevant nucleobases will be identified for understanding the catalytic mechanism of 10-23 DNAzyme.

A postsynthetically 2'-"clickable" uridine with arabino configuration and its application for fluorescent labeling and imaging of DNA

The arabino-configured analog of uridine with a propargyl group at the 2’-position was synthesized and incorporated into DNA by solid-phase chemistry. The fluorescence quantum yields of DNA strands that were postsynthetically modified by blue and green emitting cyanine-styryl dyes were improved due to the arabino-configured anchor. These oligonucleotides were used as energy transfer donors in hybrids with oligonucleotides modified with acceptor dyes that emit in the yellow-red range. These combinations give energy transfer pairs with blue–yellow, blue–red and green–red emission color changes. All combinations of arabino- and ribo-configured donor strands with arabino- and ribo-configured acceptor strands were evaluated. This array of doubly modified hybrids was screened by their emission color contrast and fluorescence quantum yield. Especially mixed combinations, that means donor dyes with arabino-configured anchor with acceptor dyes with ribo-configured anchor, and vice versa, showed significantly improved fluorescence properties. Those were successfully applied for fluorescent imaging of DNA after transport into living cells.

Microenvironmental Effect of 2'-O-(1-Pyrenylmethyl)uridine Modified Fluorescent Oligonucleotide Probes on Sensitive and Selective Detection of Target RNA

2'-O-(1-Pyrenylmethyl)uridine modified oligoribonucleotides provide highly sensitive pyrene fluorescent probes for detecting specific nucleotide mutation of RNA targets. To develop more stable and cost-effective oligonucleotide probes, we investigated the local microenvironmental effects of nearby nucleobases on pyrene fluorescence in duplexes of RNAs and 2'-O-(1-pyrenylmethyl)uridine modified oligonucleotides. By incorporation of deoxyribonucleotides, ribonucleotides, 2'-MeO-nucleotides and 2'-F-nucleotides at both sides of 2'-O-(1-pyrenylmethyl)uridine (U(p)) in oligodeoxynucleotide probes, we synthesized a series of pyrene modified oligonucleotide probes. Their pyrene fluorescence emission spectra indicated that only two proximal nucleotides have a substantial effect on the pyrene fluorescence properties of these oligonucleotide probes hybridized with target RNA with an order of fluorescence sensitivity of 2'-F-nucleotides > 2'-MeO-nucleotides > ribonucleotides ≫ deoxyribonucleotides. While based on circular dichroism spectra, overall helix conformations (either A- or B-form) of the duplexes have marginal effects on the sensitivity of the probes. Instead, the local substitution reflected the propensity of the nucleotide sugar ring to adopt North type conformation and, accordingly, shifted their helix geometry toward a more A-type like conformation in local microenvironments. Thus, higher enhancement of pyrene fluorescence emission favored local A-type helix structures and more polar and hydrophobic environments (F > MeO > OH at 2' substitution) of duplex minor grooves of probes with the target RNA. Further dynamic simulation revealed that local microenvironmental effect of 2'-F-nucleotides or ribonucleotides was enough for pyrene moiety to move out of nucleobases to the minor groove of duplexes; in addition, 2'-F-nucleotide had less effect on π-stack of pyrene-modified uridine with upstream and downstream nucleobases. The present oligonucleotide probes successfully distinguished target RNA from single-mutated RNA analyte during an in vitro assay of RNA synthesis.

Merging Two Strategies for Mixed-Sequence Recognition of Double-Stranded DNA: Pseudocomplementary Invader Probes

The development of molecular strategies that enable recognition of specific double-stranded DNA (dsDNA) regions has been a longstanding goal as evidenced by the emergence of triplex-forming oligonucleotides, peptide nucleic acids (PNAs), minor groove binding polyamides, and—more recently—engineered proteins such as CRISPR/Cas9. Despite this progress, an unmet need remains for simple hybridization-based probes that recognize specific mixed-sequence dsDNA regions under physiological conditions. Herein, we introduce pseudocomplementary Invader probes as a step in this direction. These double-stranded probes are chimeras between pseudocomplementary DNA (pcDNA) and Invader probes, which are activated for mixed-sequence dsDNA-recognition through the introduction of pseudocomplementary base pairs comprised of 2-thiothymine and 2,6-diaminopurine, and +1 interstrand zipper arrangements of intercalator-functionalized nucleotides, respectively. We demonstrate that certain pseudocomplementary Invader probe designs result in very efficient and specific recognition of model dsDNA targets in buffers of high ionic strength. These chimeric probes, therefore, present themselves as a promising strategy for mixed-sequence recognition of dsDNA targets for applications in molecular biology and nucleic acid diagnostics.

Sensitive Detection of Single-Nucleotide Mutation in the BRAF Mutation Site (V600E) of Human Melanoma Using Phosphate-Pyrene-Labeled DNA Probes

A series of novel nucleotide phosphoramidites were rationally designed and synthesized and were then site-specifically incorporated in DNA oligonucleotide probes with pyrene-modified phosphate. These oligodeoxynucleotide (ODN) probes almost have no inherent fluorescence emission with pyrene modification at 3' phosphate of corresponding nucleotides as a result of the photoinduced electron-transfer quenching effect of nucleobases (thymidine ∼ cytidine > guanosine ≫ adenosine). However, strong fluorescence emission was observed only with the perfectly matched duplex for the probes with pyrene modified at 3' phosphate of thymidine and cytidine. These rationally designed ODN probes successfully worked as "turn on" fluorescence oligonucleotide sensors for single-nucleotide polymorphism (SNP) and were used for detecting a single BRAF mutation site (V600E) of human melanoma.

Synthesis and characterization of oligodeoxyribonucleotides modified with 2'-thio-2'-deoxy-2'-S-(pyren-1-yl)methyluridine

Pyrene-functionalized oligonucleotides are intensively explored for applications in materials science and diagnostics. Here, we describe a short synthetic route to 2'-S-(pyren-1-yl)methyl-2'-thiouridine monomer S, its incorporation into oligodeoxyribonucleotides (ONs), and biophysical characterization thereof. Pseudorotational analysis reveals that the furanose ring of this monomer has a slight preference for South-type conformations. ONs modified with monomer S display high cDNA affinity but decreased binding specificity. Hybridization is associated with bathochromic shifts of pyrene absorption bands and quenching of pyrene fluorescence consistent with an intercalative binding mode of the pyrene moiety. Monomer S was also evaluated as a building block for mixed-sequence recognition of double-stranded DNA via the Invader strategy. However, probes with +1 interstrand arrangements of monomer S were found to be less efficient than Invader probes based on 2'-O-(pyren-1-yl)methyluridine or 2'-N-(pyren-1-yl)methyl-2'-N-methyl-2'-aminouridine.

Mixed-Sequence Recognition of Double-Stranded DNA Using Enzymatically Stable Phosphorothioate Invader Probes

Development of probes that allow for sequence-unrestricted recognition of double-stranded DNA (dsDNA) continues to attract much attention due to the prospect for molecular tools that enable detection, regulation, and manipulation of genes. We have recently introduced so-called Invader probes as alternatives to more established approaches such as triplex-forming oligonucleotides, peptide nucleic acids and polyamides. These short DNA duplexes are activated for dsDNA recognition by installment of +1 interstrand zippers of intercalator-functionalized nucleotides such as 2'-N-(pyren-1-yl)methyl-2'-N-methyl-2'-aminouridine and 2'-O-(pyren-1-yl)methyluridine, which results in violation of the nearest neighbor exclusion principle and duplex destabilization. The individual probes strands have high affinity toward complementary DNA strands, which generates the driving force for recognition of mixed-sequence dsDNA regions. In the present article, we characterize Invader probes that are based on phosphorothioate backbones (PS-DNA Invaders). The change from the regular phosphodiester backbone furnishes Invader probes that are much more stable to nucleolytic degradation, while displaying acceptable dsDNA-recognition efficiency. PS-DNA Invader probes therefore present themselves as interesting probes for dsDNA-targeting applications in cellular environments and living organisms.

Recognition of Double-Stranded DNA Using Energetically Activated Duplexes Modified with N2'-Pyrene-, Perylene-, or Coronene-Functionalized 2'-N-Methyl-2'-amino-DNA Monomers

Invader probes have been proposed as alternatives to polyamides, triplex-forming oligonucleotides, and peptide nucleic acids for recognition of chromosomal DNA targets. These double-stranded probes are activated for DNA recognition by +1 interstrand zippers of pyrene-functionalized nucleotides. This particular motif forces the intercalating pyrene moieties into the same region, resulting in perturbation and destabilization of the probe duplex. In contrast, the two probe strands display very high affinity toward complementary DNA. The energy difference between the probe duplexes and recognition complexes provides the driving force for DNA recognition. In the present study, we explore the properties of Invader probes based on larger intercalators, i.e., perylene and coronene, expecting that the larger π-surface area will result in additional destabilization of the probe duplex and further stabilization of probe-target duplexes, in effect increasing the thermodynamic driving force for DNA recognition. Toward this end, we developed protocols for 2'-N-methyl-2'-amino-2'-deoxyuridine phosphoramidites that are functionalized at the N2'-position with pyrene, perylene, or coronene moieties and incorporated these monomers into oligodeoxyribonucleotides (ONs). The resulting ONs and Invader probes are characterized by thermal denaturation experiments, analysis of thermodynamic parameters, absorption and fluorescence spectroscopy, and DNA recognition experiments. Invader probes based on large intercalators efficiently recognize model targets.

Recognition of double-stranded DNA using energetically activated duplexes with interstrand zippers of 1-, 2- or 4-pyrenyl-functionalized O2'-alkylated RNA monomers

Despite advances with triplex-forming oligonucleotides, peptide nucleic acids, polyamides and--more recently--engineered proteins, there remains an urgent need for synthetic ligands that enable specific recognition of double-stranded (ds) DNA to accelerate studies aiming at detecting, regulating and modifying genes. Invaders, i.e., energetically activated DNA duplexes with interstrand zipper arrangements of intercalator-functionalized nucleotides, are emerging as an attractive approach toward this goal. Here, we characterize and compare Invaders based on 1-, 2- and 4-pyrenyl-functionalized O2'-alkylated uridine monomers X-Z by means of thermal denaturation experiments, optical spectroscopy, force-field simulations and recognition experiments using DNA hairpins as model targets. We demonstrate that Invaders with +1 interstrand zippers of X or Y monomers efficiently recognize mixed-sequence DNA hairpins with single nucleotide fidelity. Intercalator-mediated unwinding and activation of the double-stranded probe, coupled with extraordinary stabilization of probe-target duplexes (ΔT(m)/modification up to +14.0 °C), provides the driving force for dsDNA recognition. In contrast, Z-modified Invaders show much lower dsDNA recognition efficiency. Thus, even very conservative changes in the chemical makeup of the intercalator-functionalized nucleotides used to activate Invader duplexes, affects dsDNA-recognition efficiency of the probes, which highlights the importance of systematic structure-property studies. The insight from this study will guide future design of Invaders for applications in molecular biology and nucleic acid diagnostics.

Scaffolding along nucleic acid duplexes using 2'-amino-locked nucleic acids

CONSPECTUS: Incorporation of chemically modified nucleotide scaffolds into nucleic acids to form assemblies rich in function is an innovative area with great promise for nanotechnology and biomedical and material science applications. The intrinsic biorecognition potential of nucleic acids combined with advanced properties of the locked nucleic acids (LNAs) provide opportunities to develop new nanomaterials and devices like sensors, aptamers, and machines. In this Account, we describe recent research on preparation and investigation of the properties of LNA/DNA hybrids containing functionalized 2'-amino-LNA nucleotides. By application of different chemical reactions, modification of 2'-amino-LNA scaffolds can be efficiently performed in high yields and with various tags, postsynthetically or during the automated oligonucleotide synthesis. The choice of a synthetic method for scaffolding along 2'-amino-LNA mainly depends on the chemical nature of the modification, its price, its availability, and applications of the product. One of the most useful applications of the product LNA/DNA scaffolds containing 2'-amino-LNA is to detect complementary DNA and RNA targets. Examples of these applications include sensing of clinically important single-nucleotide polymorphisms (SNPs) and imaging of nucleic acids in vitro, in cell culture, and in vivo. According to our studies, 2'-amino-LNA scaffolds are efficient within diagnostic probes for DNA and RNA targets and as therapeutics, whereas both 2'-amino- and isomeric 2'-α-l-amino-LNA scaffolds have promising properties for stabilization and detection of DNA nanostructures. Attachment of fluorescent groups to the 2'-amino group results in very high fluorescent quantum yields of the duplexes and remarkable sensitivity of the fluorescence signal to target binding. Notably, fluorescent LNA/DNA probes bind nucleic acid targets with advantages of high affinity and specificity. Thus, molecular motion of nanodevices and programmable self-assembly of chemically modified LNA/DNA nanomaterials can be followed by bright fluorescence signaling from the functionalized LNA units. Another appealing aspect of the amino-LNA scaffolds is specific targeting of nucleic acids and proteins for therapeutic applications. 2'-Amino-LNA/DNA conjugates containing peptide and polyaromatic hydrocarbon (PAH) groups are promising in this context as well as for advanced imaging and diagnostic purposes in vivo. For imaging applications, photostability of fluorescence dyes is of crucial importance. Chemically stable and photostable fluorescent PAH molecules attached to 2'-amino functionality of the 2'-amino-LNA are potent for in vitro and in vivo imaging of DNA and RNA targets. We believe that rational evolution of the biopolymers of Nature may solve the major challenges of the future material science and biomedicine. However, this requires strong scientific progress and efficient interdisciplinary research. Examples of this Account demonstrate that among other synthetic biopolymers, synthetic nucleic acids containing functionalized 2'-amino-LNA scaffolds offer great opportunities for material science, diagnostics, and medicine of the future.

C5-alkynyl-functionalized α-L-LNA: synthesis, thermal denaturation experiments and enzymatic stability

Major efforts are currently being devoted to improving the binding affinity, target specificity, and enzymatic stability of oligonucleotides used for nucleic acid targeting applications in molecular biology, biotechnology, and medicinal chemistry. One of the most popular strategies toward this end has been to introduce additional modifications to the sugar ring of affinity-inducing conformationally restricted nucleotide building blocks such as locked nucleic acid (LNA). In the preceding article in this issue, we introduced a different strategy toward this end, i.e., C5-functionalization of LNA uridines. In the present article, we extend this strategy to α-L-LNA: i.e., one of the most interesting diastereomers of LNA. α-L-LNA uridine monomers that are conjugated to small C5-alkynyl substituents induce significant improvements in target affinity, binding specificity, and enzymatic stability relative to conventional α-L-LNA. The results from the back-to-back articles therefore suggest that C5-functionalization of pyrimidines is a general and synthetically straightforward approach to modulate biophysical properties of oligonucleotides modified with LNA or other conformationally restricted monomers.

Recognition of mixed-sequence DNA duplexes: design guidelines for invaders based on 2'-O-(pyren-1-yl)methyl-RNA monomers

The development of agents that recognize mixed-sequence double-stranded DNA (dsDNA) is desirable because of their potential as tools for detection, regulation, and modification of genes. Despite progress with triplex-forming oligonucleotides, peptide nucleic acids, polyamides, and other approaches, recognition of mixed-sequence dsDNA targets remains challenging. Our laboratory studies Invaders as an alternative approach toward this end. These double-stranded oligonucleotide probes are activated for recognition of mixed-sequence dsDNA through modification with +1 interstrand zippers of intercalator-functionalized nucleotides such as 2'-O-(pyren-1-yl)methyl-RNA monomers and have recently been shown to recognize linear dsDNA, DNA hairpins, and chromosomal DNA. In the present work, we systematically studied the influence that the nucleobase moieties of the 2'-O-(pyren-1-yl)methyl-RNA monomers have on the recognition efficiency of Invader duplexes. Results from thermal denaturation, binding energy, and recognition experiments using Invader duplexes with different +1 interstrand zippers of the four canonical 2'-O-(pyren-1-yl)methyl-RNA A/C/G/U monomers show that incorporation of these motifs is a general strategy for activation of probes for recognition of dsDNA. Probe duplexes with interstrand zippers comprising C and/or U monomers result in the most efficient recognition of dsDNA. The insight gained from this study will drive the design of efficient Invaders for applications in molecular biology, nucleic acid diagnostics, and biotechnology.

Identification and characterization of second-generation invader locked nucleic acids (LNAs) for mixed-sequence recognition of double-stranded DNA

The development of synthetic agents that recognize double-stranded DNA (dsDNA) is a long-standing goal that is inspired by the promise for tools that detect, regulate, and modify genes. Progress has been made with triplex-forming oligonucleotides, peptide nucleic acids, and polyamides, but substantial efforts are currently devoted to the development of alternative strategies that overcome the limitations observed with the classic approaches. In 2005, we introduced Invader locked nucleic acids (LNAs), i.e., double-stranded probes that are activated for mixed-sequence recognition of dsDNA through modification with "+1 interstrand zippers" of 2'-N-(pyren-1-yl)methyl-2'-amino-α-l-LNA monomers. Despite promising preliminary results, progress has been slow because of the synthetic complexity of the building blocks. Here we describe a study that led to the identification of two simpler classes of Invader monomers. We compare the thermal denaturation characteristics of double-stranded probes featuring different interstrand zippers of pyrene-functionalized monomers based on 2'-amino-α-l-LNA, 2'-N-methyl-2'-amino-DNA, and RNA scaffolds. Insights from fluorescence spectroscopy, molecular modeling, and NMR spectroscopy are used to elucidate the structural factors that govern probe activation. We demonstrate that probes with +1 zippers of 2'-O-(pyren-1-yl)methyl-RNA or 2'-N-methyl-2'-N-(pyren-1-yl)methyl-2'-amino-DNA monomers recognize DNA hairpins with similar efficiency as original Invader LNAs. Access to synthetically simple monomers will accelerate the use of Invader-mediated dsDNA recognition for applications in molecular biology and nucleic acid diagnostics.

Progress and prospects of pyrrole-imidazole polyamide-fluorophore conjugates as sequence-selective DNA probes

Recently, the versatility of N-methylpyrrole (Py)-N-methylimidazole (Im) polyamide conjugates, which have been developed from the DNA-binding antibiotics distamycin A and netropsin, has been shown. These synthetic small molecules can permeate cells to bind with duplex DNA in a sequence-specific manner, and hence can influence gene expression in vivo. Accordingly, several reports demonstrating the sequence specificity and biological activity of Py-Im polyamides have accumulated. However, the benefits of Py-Im polyamides, in particular those conjugated with fluorophores, has been overlooked. Moreover, clear directions for the employment of these attractive artificial small molecules have not yet been shown. Here, we present a detailed overview of the current and prospective applications of Py-Im polyamide-fluorophore conjugates, including sequence-specific recognition with fluorescence emission properties, and their potential roles in biological imaging.

Pyrene-functionalized oligonucleotides and locked nucleic acids (LNAs): tools for fundamental research, diagnostics, and nanotechnology

Pyrene-functionalized oligonucleotides (PFOs) are increasingly explored as tools in fundamental research, diagnostics and nanotechnology. Their popularity is linked to the ability of pyrenes to function as polarity-sensitive and quenchable fluorophores, excimer-generating units, aromatic stacking moieties and nucleic acid duplex intercalators. These characteristics have enabled development of PFOs for detection of complementary DNA/RNA targets, discrimination of single nucleotide polymorphisms (SNPs), and generation of π-arrays on nucleic acid scaffolds. This critical review will highlight the physical properties and applications of PFOs that are likely to provide high degree of positional control of the chromophore in nucleic acid complexes. Particular emphasis will be placed on pyrene-functionalized Locked Nucleic Acids (LNAs) since these materials display interesting properties such as fluorescence quantum yields approaching unity and recognition of mixed-sequence double stranded DNA (144 references).

LNA for optimization of fluorescent oligonucleotide probes: improved spectral properties and target binding

Mixmer LNA/DNA fluorescent probes containing the 1-(phenylethynyl)pyrene fluorophore attached to 2'-arabino-uridine were synthesized and studied. The conjugates displayed significantly higher hybridization affinity to target DNA, increased fluorescence quantum yields of single-stranded oligonucleotides and their duplexes, and improved ability to form an interstrand excimer compared to analogous non-LNA probes.

Invader LNA: efficient targeting of short double stranded DNA

Despite progress with triplex-forming oligonucleotides or helix-invading peptide nucleic acids (PNAs), there remains a need for probes facilitating sequence-unrestricted targeting of double stranded DNA (dsDNA) at physiologically relevant conditions. Invader LNA probes, i.e., DNA duplexes with "+1 interstrand zipper arrangements" of intercalator-functionalized 2'-amino-alpha-l-LNA monomers, are demonstrated herein to recognize short mixed sequence dsDNA targets. This approach, like pseudo-complementary PNA (pcPNA), relies on relative differences in stability between probe duplexes and the corresponding probe:target duplexes for generation of a favourable thermodynamic gradient. Unlike pcPNA, Invader LNA probes take advantage of the "nearest neighbour exclusion principle", i.e., intercalating units of Invader LNA monomers are poorly accommodated in probe duplexes but extraordinarily well tolerated in probe-target duplexes (DeltaT(m)/modification up to +11.5 degrees C). Recognition of isosequential dsDNA-targets occurs: a) at experimental temperatures much lower than the thermal denaturation temperatures (T(m)'s) of Invader LNAs or dsDNA-targets, b) at a wide range of ionic strengths, and c) with good mismatch discrimination. Recognition of dsDNA is monitored in real-time using inherent pyrene-pyrene excimer signals of Invader LNA probes, which provides insights into reaction kinetics and enables rational design of probes. These properties render Invader LNAs as promising probes for biomedical applications entailing sequence-unrestricted recognition of dsDNA.

Synthesis and biophysical studies of coronene functionalized 2'-amino-LNA: a novel class of fluorescent nucleic acids

Incorporation of 2'-N-(coronen-1-yl)methyl-2'-amino-LNA monomer X or 2'-N-4-(coronen-1-yl)-4-oxobutanoyl-2'-amino-LNA monomer Y into short DNA strands induces high binding affinity toward DNA or RNA and a marked red-shift in steady-state fluorescence emission upon hybridization to cDNA or RNA.

Nucleic acid-guided assembly of aromatic chromophores

The rational formation of aromatic chromophore arrays is an intriguing challenge since ordered collectives of chromophores possess properties that are largely different from those of the individual molecules. Therefore, nucleic acids are increasingly used as scaffolds for the construction of multi-chromophore arrays. This tutorial review provides an introduction to the field of nucleic acid-guided chromophore assemblies for non-specialists and a reference point for those familiar with the area by highlighting the recent developments and describing some of the spectroscopic methods used for the study of oligonucleotide-chromophore conjugates.

[Oligo(2'-O-methylribonucleotides) and their derivatives: III. 5'-mono- and 5'-bispyrenyl derivatives of oligo(2'-O-methylribonucleotides) and their 3'-modified analogues: synthesis and properties]

5'-Pyrenylmethylphosphoramidite and 5'-bispyrenylmethylphosphordiamidite derivatives of oligo(2'-O-methylribonucleotides) and their analogues with thymidine attached at their 3'-termini by a 3'-3'-phosphodiester internucleotide bond (inverted thymidine) were synthesized. The effect of the pyrene residue(s) on the thermal stability of duplexes of the modified oligonucleotides with RNA and DNA was studied. A possibility of detection of hybridization of 5'-mono- and 5'-bispyrenyl derivatives with RNA and DNA targets in solution was demonstrated according to the changes in fluorescence. 5'-Pyrenylmethylphosphoramidite derivatives of oligo(2'-O-methylribonucleotides) and their inverted analogues were shown to serve as sensitive probes for the detection of oligonucleotide substitutions in RNA and DNA by the method of thermal denaturation of the formed duplexes detected according to changes in their fluorescence.

Functionalized 2'-amino-alpha-L-LNA: directed positioning of intercalators for DNA targeting

Chemically modified oligonucleotides are increasingly applied in nucleic acid based therapeutics and diagnostics. LNA (locked nucleic acid) and its diastereomer alpha-L-LNA are two promising examples thereof that exhibit increased thermal and enzymatic stability. Herein, the synthesis, biophysical characterization, and molecular modeling of N2'-functionalized 2'-amino-alpha-L-LNA is described. Chemoselective N2'-functionalization of protected amino alcohol 1 followed by phosphitylation afforded a structurally varied set of target phosphoramidites, which were incorporated into oligodeoxyribonucleotides. Incorporation of pyrene-functionalized building blocks such as 2'-N-(pyren-1-yl)carbonyl-2'-amino-alpha-L-LNA (monomer X) led to extraordinary increases in thermal affinity of up to +19.5 degrees C per modification against DNA targets in particular. In contrast, incorporation of building blocks with small nonaromatic N2'-functionalities such as 2'-N-acetyl-2'-amino-alpha-L-LNA (monomer V) had detrimental effects on thermal affinity toward DNA/RNA complements with decreases of as much as -16.5 degrees C per modification. Extensive thermal DNA selectivity, favorable entropic contributions upon duplex formation, hybridization-induced bathochromic shifts of pyrene absorption maxima and increases in circular dichroism signal intensity, and molecular modeling studies suggest that pyrene-functionalized 2'-amino-alpha-L-LNA monomers W-Y having short linkers between the bicyclic skeleton and the pyrene moiety allow high-affinity hybridization with DNA complements and precise positioning of intercalators in nucleic acid duplexes. This rigorous positional control has been utilized for the development of probes for emerging therapeutic and diagnostic applications focusing on DNA targeting.

Highly fluorescent conjugated pyrenes in nucleic acid probes: (phenylethynyl)pyrenecarbonyl-functionalized locked nucleic acids

In recent years, fluorescently labeled oligonucleotides have become a widely used tool in diagnostics, DNA sequencing, and nanotechnology. The recently developed (phenylethynyl)pyrenes are attractive dyes for nucleic acid labeling, with the advantages of long-wave emission relative to the parent pyrene, high fluorescence quantum yields, and the ability to form excimers. Herein, the synthesis of six (phenylethynyl)pyrene-functionalized locked nucleic acid (LNA) monomers M(1)-M(6) and their incorporation into DNA oligomers is described. Multilabeled duplexes display higher thermal stabilities than singly modified analogues. An increase in the number of phenylethynyl substituents attached to the pyrene results in decreased binding affinity towards complementary DNA and RNA and remarkable bathochromic shifts of absorption/emission maxima relative to the parent pyrene fluorochrome. This bathochromic shift leads to the bright fluorescence colors of the probes, which differ drastically from the blue emission of unsubstituted pyrene. The formation of intra- and interstrand excimers was observed for duplexes that have monomers M(1)-M(6) in both complementary strands and in numerous single-stranded probes. If more phenylethynyl groups are inserted, the detected excimer signals become more intense. In addition, (phenylethynyl)pyrenecarbonyl-LNA monomers M(4), M(5), and M(6) proved highly useful for the detection of single mismatches in DNA/RNA targets.

Novel strategies for the site-specific covalent labelling of nucleic acids

To broaden the scope of applications in DNA nano- and biotechnology, material science, diagnostics and molecular recognition the functionalization of DNA is of utmost importance. In the last decade many new methods have been developed to achieve this goal. Apart from the direct chemical synthesis of modified DNA by automated phosphoramidite chemistry incorporation of labelled triphosphates and the post-synthetic labelling approach evolved as valuable methods. New bioorthogonal reactions as Diels-Alder, click and Staudinger ligations pushed forward the post-synthetic approach as new insights into DNA polymerase substrate specificity allowed generation and amplification of labelled DNA strands. These novel developments are summarized herein.

Perylene attached to 2'-amino-LNA: synthesis, incorporation into oligonucleotides, and remarkable fluorescence properties in vitro and in cell culture

During recent years, fluorescently labeled oligonucleotides have been extensively investigated within diagnostic approaches. Among a large variety of available fluorochromes, the polyaromatic hydrocarbon perylene is an object of increasing interest due to its high fluorescence quantum yield, long-wave emission compared to widely used pyrene, and photostability. These properties make perylene an attractive label for fluorescence-based detection in vitro and in vivo. Herein, the synthesis of 2'- N-(perylen-3-yl)carbonyl-2'-amino-LNA monomer X and its incorporation into oligonucleotides is described. Modification X induces high thermal stability of DNA:DNA and DNA:RNA duplexes, high Watson-Crick mismatch selectivity, red-shifted fluorescence emission compared to pyrene, and high fluorescence quantum yields. The thermal denaturation temperatures of duplexes involving two modified strands are remarkably higher than those for double-stranded DNAs containing modification X in only one strand, suggesting interstrand communication between perylene moieties in the studied 'zipper' motifs. Fluorescence of single-stranded oligonucleotides having three monomers X is quenched compared to modified monomer (quantum yields Phi F = 0.03-0.04 and 0.67, respectively). However, hybridization to DNA/RNA complements leads to Phi F increase of up to 0.20-0.25. We explain it by orientation of the fluorochrome attached to the 2'-position of 2'-amino-LNA in the minor groove of the nucleic acid duplexes, thus protecting perylene fluorescence from quenching with nucleobases or from the environment. At the same time, the presence of a single mismatch in DNA or RNA targets results in up to 8-fold decreased fluorescence intensity of the duplex. Thus, distortion of the duplex geometry caused by even one mismatched nucleotide induces remarkable quenching of fluorescence. Additionally, a perylene-LNA probe is successfully applied for detection of mRNA in vivo providing excitation wavelength, which completely eliminates cell autofluorescence.

Spectroscopic properties of pyrene-containing DNA mimics

DNA mimics containing non-nucleosidic pyrene building blocks are described. The modified oligomers form stable hybrids, although a slight reduction in hybrid stability is observed in comparison to the unmodified DNA duplex. The nature of the interaction between the pyrene residues in single and double stranded oligomers is analyzed spectroscopically. Intra- and interstrand stacking interactions of pyrenes are monitored by UV-absorbance as well as fluorescence spectroscopy. Excimer formation is observed in both single and double strands. In general, intrastrand excimers show fluorescence emission at shorter wavelengths (approx. 5-10 nm) than excimers formed by interstrand interactions. The existence of two different forms of excimers (intra- vs. interstrand) is also revealed in temperature dependent UV-absorbance spectra.

1-Phenylethynylpyrene (1-PEPy) as Refined Excimer Forming Alternative to Pyrene: Case of DNA Major Groove Excimer

1-Phenylethynylpyrene fluorochrome was studied as meta- and para-derivatives of arabino-uridine-2'-carbamates in ss and dsDNA. 1-PEPy showed red-shifted emission and increased fluorescence quantum yield compared to pyrene. Although 1-PEPy has very short excited lifetime (<2.5 ns), it is able to form inter- and intrastrand excimers on DNA, probably resulting from spatial preorganization of two dye molecules.

Detection of CAG repeat DNA sequences by pyrene-functionalized pyrrole-imidazole polyamides

Five N-methylpyrrole-N-methylimidazole (Py-Im) polyamides possessing a fluorescent pyrene were synthesized by Fmoc solid-phase synthesis using Py/Im monomers and pyrenylbutyl-pyrrole monomer compound 9. The steady state fluorescence of conjugates 1-5 was examined in the presence and absence of (CAG)(12)-containing oligodeoxynucleotides (ODNs) 1 and 2. Of the conjugates, conjugate 1 showed no background emission around 470 nm in the absence of ODNs, and a clear increase of emission at 475 nm was observed upon addition of ODNs 1 and 2. The emission of conjugate 1 at 475 nm increased linearly with the concentration of ODN and the number of CAG repeats. The results indicate that conjugate 1 efficiently forms a pyrene excimer upon binding in the minor groove of DNA.

Ultrafast energy transfer and structural dynamics in DNA

Ultrafast structural dynamics concomitant to excitation energy transfer in DNA has been studied using a pair of pyrene-labeled DNA bases. The temporal evolution of the femtosecond pump-probe spectra reveals the existence of two electronic coupling pathways, through-base stack and through-space, which lead to excitation energy transfer and excimer formation even when the labeled DNA bases are separated by one AT base pair. The electronic coupling which mediates through-base stack energy transfer is so strong that a new absorption band arises in the excited-state absorption spectrum within 300 fs. From the analysis of time-dependent spectral shifts due to through-space excimer formation, the local structural dynamics and flexibility of DNA are characterized on the picosecond and nanosecond time scale.

Selectivity in DNA interstrand-stacking

The synthesis and spectroscopic investigation of DNA hybrids containing non-nucleosidic pyrene and phenanthrene building blocks is reported. It was found that interstrand-stacking of the polyaromatic compounds in the DNA duplex takes place with a high degree of selectivity.

Targeting of mixed sequence double-stranded DNA using pyrene-functionalized 2′-amino-α-l-LNA

Incorporation of a single pyrene-functionalized 2'-amino-alpha-l-LNA monomer X into short DNA strands induces extraordinarily high binding affinity towards complementary DNA (up to 16 degrees C increase per modification), whereas labile duplexes, suitable as probes for targeting of double stranded DNA, are formed upon positioning of two monomers X in an interstrand +1 zipper motif.

DNA mismatch detection using a pyrene–excimer-forming probe

A pyrene-excimer-forming probe allowed the easy and sensitive detection of a single base mismatch in target DNA. This was due to the faster strand exchange rate compared to a fully-matched target.