A Highly Sensitive, Excimer-Controlled Molecular Beacon

Stabilization of DNA Duplexes and Hairpins by Charge-Transfer Interactions Using DAN:NDI Pairs

Electron-rich 1,5-dialkoxynaphthalene (DAN) and electron-deficient 1,8,4,5-naphthalenetetracarboxylic diimide (NDI) are known to interact through the formation of charge-transfer complexes. The introduction of DAN and NDI into various DNA duplexes and hairpins was investigated by ultraviolet (UV) melting curve analysis. The positioning of the DAN:NDI pair was found to strongly influence the stability of DNA duplex and hairpins. In particular, while the introduction of one DAN/NDI pair in front of each other in the center of a DNA duplex led to a decrease of the thermal stability (ΔTm - 6 °C), the addition of a second pair restored or even increased the stability. In contrast, the introduction of DAN:NDI pairs at the end of a duplex always induced a strong stabilization (ΔTm up to +20 °C). Finally, a DAN:NDI pair positioned in the loop of a hairpin induced a stronger stabilization than a T4 loop (ΔTm + 10 °C). Based on charge-transfer interactions, the strong stabilizations observed allow the preparation of highly stabilized DNA nanostructures opening the way to numerous applications in nanotechnology.

Comparative chloroplast genome analyses of Paraboea (Gesneriaceae): Insights into adaptive evolution and phylogenetic analysis

Paraboea (Gesneriaceae) distributed in the karst areas of South and Southwest China and Southeast Asia, is an ideal genus to study the phylogeny and adaptive evolution of karst plants. In this study, the complete chloroplast genomes of twelve Paraboea species were sequenced and analyzed. Twelve chloroplast genomes ranged in size from 153166 to 154245 bp. Each chloroplast genome had a typical quartile structure, and relatively conserved type and number of gene components, including 131 genes which are composed of 87 protein coding genes, 36 transfer RNAs and 8 ribosomal RNAs. A total of 600 simple sequence repeats and 389 non-overlapped sequence repeats were obtained from the twelve Paraboea chloroplast genomes. We found ten divergent regions (trnH-GUG-psbA, trnM-CAU, trnC-GCA, atpF-atpH, ycf1, trnK-UUU-rps16, rps15, petL, trnS-GCU-trnR-UCU and psaJ-rpl33) among the 12 Paraboea species to be potential molecular markers. In the phylogenetic tree of 31 Gesneriaceae plants including twelve Paraboea species, all Paraboea species clustered in a clade and confirmed the monophyly of Paraboea. Nine genes with positive selection sites were detected, most of which were related to photosynthesis and protein synthesis, and might played crucial roles in the adaptability of Paraboea to diverse karst environments. These findings are valuable for further study of the phylogeny and karst adaptability of Gesneriaceae plants.

Stacked Thiazole Orange Dyes in DNA Capable of Switching Emissive Behavior in Response to Structural Transitions

Functional nucleic acids with the capability of generating fluorescence in response to hybridization events, microenvironment or structural changes are valuable as structural probes and chemical sensors. We now demonstrate the enzyme-assisted preparation of nucleic acids possessing multiple thiazole orange (TO) dyes and their fluorescent behavior, that show a spectral change from the typical monomer emission to the excimer-type red-shifted emission. We found that the fluorescent response and emission wavelength of the TO dyes were dependent on both the state of the DNA structure (single- or double-stranded DNA) and the arrangement of the TO dyes. We showed that the fluorescent behavior of the TO dyes can be applied for the detection of RNA molecules, suggesting that our approach for preparing the fluorescent nucleic acids functionalized with multiple TO dyes could be useful to design a fluorescence bioimaging and detection technique of biomolecules.

Recent Advances in Nucleic Acid Targeting Probes and Supramolecular Constructs Based on Pyrene-Modified Oligonucleotides

In this review, we summarize the recent advances in the use of pyrene-modified oligonucleotides as a platform for functional nucleic acid-based constructs. Pyrene is of special interest for the development of nucleic acid-based tools due to its unique fluorescent properties (sensitivity of fluorescence to the microenvironment, ability to form excimers and exciplexes, long fluorescence lifetime, high quantum yield), ability to intercalate into the nucleic acid duplex, to act as a π-π-stacking (including anchoring) moiety, and others. These properties of pyrene have been used to construct novel sensitive fluorescent probes for the sequence-specific detection of nucleic acids and the discrimination of single nucleotide polymorphisms (SNPs), aptamer-based biosensors, agents for binding of double-stranded DNAs, and building blocks for supramolecular complexes. Special attention is paid to the influence of the design of pyrene-modified oligonucleotides on their properties, i.e., the structure-function relationships. The perspectives for the applications of pyrene-modified oligonucleotides in biomolecular studies, diagnostics, and nanotechnology are discussed.

Peptide-Based Probes with an Artificial Anion-Binding Motif for Direct Fluorescence "Switch-On" Detection of Nucleic Acid in Cells

This work reports two new peptide-based fluorescence probes (1 and 2) for the detection of ds-DNA at physiological pH. Probes 1 and 2 contain a fluorophore, either amino-naphthalimide or diethyl-aminocoumarin, respectively, and two identical peptide arms each equipped with a guanidiniocarbonylpyrrole (GCP) anion-binding motif. These probes show "switch-on" fluorescence response upon binding to ds-DNA, whereby they can differentiate between various types of polynucleotides. For instance, they exhibit more pronounced fluorescence response for AT-rich polynucleotides than GC-rich polynucleotides, and both give only negligible response to ds-RNA. The fluorimetric response of 1 is proportional to the AT-basepair content in DNA, whereas the fluorescence of 2 is sensitive to the secondary structure of the polynucleotide. Fluorescence experiments, thermal melting experiments and circular dichroism studies suggest that 1 interacts with ds-DNA in a combined intercalation and minor groove binding, whereas 2 interacts mainly with the outer surface of DNA/RNA. As 1 and 2 have a very low cytotoxicity, 1 can be applied for the imaging of nuclear DNA in cells.

Long-stem shaped multifunctional molecular beacon for highly sensitive nucleic acids determination via intramolecular and intermolecular interactions based strand displacement amplification

LS-MMB based intra-SDA and inter-SDA for amplified gene signaling.

Revealing Nucleic Acid Mutations Using Förster Resonance Energy Transfer-Based Probes

Nucleic acid mutations are of tremendous importance in modern clinical work, biotechnology and in fundamental studies of nucleic acids. Therefore, rapid, cost-effective and reliable detection of mutations is an object of extensive research. Today, Förster resonance energy transfer (FRET) probes are among the most often used tools for the detection of nucleic acids and in particular, for the detection of mutations. However, multiple parameters must be taken into account in order to create efficient FRET probes that are sensitive to nucleic acid mutations. In this review; we focus on the design principles for such probes and available computational methods that allow for their rational design. Applications of advanced, rationally designed FRET probes range from new insights into cellular heterogeneity to gaining new knowledge of nucleic acid structures directly in living cells.

A single thiazole orange molecule forms an exciplex in a DNA i-motif

A fluorescent exciplex of thiazole orange (TO) is formed in a single-dye conjugated DNA i-motif. The exciplex fluorescence exhibits a large Stokes shift, high quantum yield, robust response to pH oscillation and little structural disturbance to the DNA quadruplex, which can be used to monitor the folding of high-order DNA structures.

A FRET-enabled molecular peptide beacon with a significant red shift for the ratiometric detection of nucleic acids

A cationic molecular peptide beacon NAP1 functionalized with a fluorescence resonance energy transfer-pair at its ends allows the ratiometric detection of ds-DNA with a preference for AT rich sequences. NAP1 most likely binds in a folded form into the minor groove of ds-DNA, which results in a remarkable change in its fluorescence properties. As NAP1 exhibits quite low cytotoxicity, it can also be used for imaging of nuclear DNA in cells.

Double-stem Hairpin Probe and Ultrasensitive Colorimetric Detection of Cancer-related Nucleic Acids

The development of a versatile biosensing platform to screen specific DNA sequences is still an essential issue of molecular biology research and clinic diagnosis of genetic disease. In this work, we for the first time reported a double-stem hairpin probe (DHP) that was simultaneously engineered to incorporate a DNAzyme, DNAzyme's complementary fragment and nicking enzyme recognition site. The important aspect of this hairpin probe is that, although it is designed to have a long ds DNA fragment, no intermolecular interaction occurs, circumventing the sticky-end pairing-determined disadvantages encountered by classic molecular beacon. For the DHP-based colorimetric sensing system, as a model analyte, cancer-related DNA sequence can trigger a cascade polymerization/nicking cycle on only one oligonucleotide probe. This led to the dramatic accumulation of G-quadruplexes directly responsible for colorimetric signal conversion without any loss. As a result, the target DNA is capable of being detected to 1 fM (six to eight orders of magnitude lower than that of catalytic molecular beacons) and point mutations are distinguished by the naked eye. The described DHP as a-proof-of-concept would not only promote the design of colorimetric biosensors but also open a good way to promote the diagnosis and treatment of genetic diseases.

LNA-enhanced DNA FIT-probes for multicolour RNA imaging

The simultaneous imaging of different RNA molecules in homogeneous solution is a challenge and requires optimisation to enable unambiguous staining of intracellular RNA targets. Our approach relies on single dye forced intercalation (FIT) probes, in which a visco-sensitive reporter of the thiazole orange (TO) family serves as a surrogate nucleobase and provides enhancements of fluorescence upon hybridisation. Previous FIT probes spanned the cyan and green emission range. Herein, we report for the first time chromophores for FIT probes that emit in the red range (above 600 nm). Such probes are valuable to overcome cellular auto-fluorescent background and enable multiplexed detection. In order to find suitable chromophores, we developed a submonomer approach that facilitated the rapid analysis of different TO family dyes in varied sequence positions. A carboxymethylated 4,4'-methine linked cyanine, which we named quinoline blue (QB), provided exceptional response characteristics at the 605 nm emission maximum. Exceeding previously reported base surrogates, the emission of the QB nucleotide intensified by up to 195-fold upon binding of complementary RNA. Owing to large extinction coefficients and quantum yields (up to ε = 129.000 L mol-1 cm-1 and Φ = 0.47, respectively) QB-FIT probes enable imaging of intracellular mRNA. A mixture of BO-, TO- and QB-containing FIT probes allowed the simultaneous detection of three different RNA targets in homogenous solution. TO- and QB-FIT probes were used to localize oskar mRNA and other polyadenylated mRNA molecules in developing oocytes from Drosphila melanogaster by means of wash-free fluorescent in situ hybridisation and super resolution microscopy (STED).

Hetero-Selective DNA-Like Duplex Stabilized by Donor-Acceptor Interactions

We report on the characterization of a novel hetero-selective DNA-like duplex of pyrene and anthraquinone pseudo base pairs. The pyrene/anthraquinone pairs showed excellent selectivity in hetero-recognition and even trimers were found to form a hetero-duplex. Pyrene and anthraquinone moieties were tethered on acyclic D-threoninol linkers and linked to adjacent residues by using standard phosphoramidite chemistry. When pyrene and anthraquinone were incorporated at pairing positions in complementary strands of natural DNA oligonucleotides, the duplex was stabilized significantly. Moreover, a pyrene hexamer and an anthraquinone hexamer formed a stable artificial hetero-duplex without the assistance of natural base pairs. The pyrene/anthraquinone pair was so stable that even trimers formed a hetero-duplex under conditions in which natural DNA strands of three residues do not.

Cytosine-bulge-dependent fluorescence quenching for the real-time hairpin primer PCR

The progress of a polymerase chain reaction (PCR) was sensitively monitored based on the increase in fluorescence of N,N'-bis(3-aminopropyl)-2,7-diamino-1,8-naphthyridine, which was covalently anchored on the cytosine bulge directly neighbouring the 5'-T_G-3'/5'-CCA-3' sequence in the hairpin tag at the 5' end of the PCR primer.

Rationally designed molecular beacons for bioanalytical and biomedical applications

Nucleic acids hold promise as biomolecules for future applications in biomedicine and biotechnology. Their well-defined structures and compositions afford unique chemical properties and biological functions. Moreover, the specificity of hydrogen-bonded Watson-Crick interactions allows the construction of nucleic acid sequences with multiple functions. In particular, the development of nucleic acid probes as essential molecular engineering tools will make a significant contribution to advancements in biosensing, bioimaging and therapy. The molecular beacon (MB), first conceptualized by Tyagi and Kramer in 1996, is an excellent example of a double-stranded nucleic acid (dsDNA) probe. Although inactive in the absence of a target, dsDNA probes can report the presence of a specific target through hybridization or a specific recognition-triggered change in conformation. MB probes are typically fluorescently labeled oligonucleotides that range from 25 to 35 nucleotides (nt) in length, and their structure can be divided into three components: stem, loop and reporter. The intrinsic merit of MBs depends on predictable design, reproducibility of synthesis, simplicity of modification, and built-in signal transduction. Using resonance energy transfer (RET) for signal transduction, MBs are further endowed with increased sensitivity, rapid response and universality, making them ideal for chemical sensing, environmental monitoring and biological imaging, in contrast to other nucleic acid probes. Furthermore, integrating MBs with targeting ligands or molecular drugs can substantially support their in vivo applications in theranositics. In this review, we survey advances in bioanalytical and biomedical applications of rationally designed MBs, as they have evolved through the collaborative efforts of many researchers. We first discuss improvements to the three components of MBs: stem, loop and reporter. The current applications of MBs in biosensing, bioimaging and therapy will then be described. In particular, we emphasize recent progress in constructing MB-based biosensors in homogeneous solution or on solid surfaces. We expect that such rationally designed and functionalized MBs will open up new and exciting avenues for biological and medical research and applications.

Molecular size, shape, and electric charges: essential for perylene bisimide-based DNA intercalator to localize in cell nuclei and inhibit cancer cell growth

The molecular properties concerning size, shape, and electric charges of the planar aromatic DNA intercalators are still poorly understood. Herein, a series of water-soluble perylene bisimide (PBI) derivatives containing a rigid and planar aromatic nanoscaffold with different size, shape, and electric charges were synthesized. Using histochemistry and cell viability assays on animal tissues and cancer cells, we revealed the molecular properties required for successful DNA intercalators to localize in cell nuclei and inhibit cancer cells. Small molecular size and the strong polarity of hydrophilic substituents are prerequisites for PBI-based DNA intercalators. A large number of charges facilitate the nucleic accumulation of these DNA intercalators, while fewer charges and planar aromatic nanoscaffold more efficiently inhibit cancer cell growth.

Photophysical and structural investigation of a PyA-modified adenine cluster: its potential use for fluorescent DNA probes exhibiting distinct emission color changes

A ^PyA-modified adenine cluster, exhibiting a large Stokes shift based on interstrand stacking interactions of adenines, was investigated and exploited as signaling parts of fluorescent DNA probes.

Fluorescence detection of DNA hybridization based on the aggregation-induced emission of a perylene-functionalized polymer

A perylene-functionalized polycation was synthesized by quaternization of poly(4-vinylpyridine) with bromomethyl-perylene and methyl iodide, which exhibited a unique aggregation-induced emission (AIE) effect. The synthesized polycation and polyanion ssDNA could form a complex probe to detect DNA hybridization. Upon adding noncomplementary ssDNA, the fluorescence of the complex probe increased due to the AIE effect; upon adding complementary ssDNA, the fluorescence intensity changed little due to the combined effects of AIE and duplex-quenching resulting from the intercalation of perylene into the duplex.

Pyrene-modified PNAs: Stacking interactions and selective excimer emission in PNA2DNA triplexes

Pyrene derivatives can be incorporated into nucleic acid analogs in order to obtain switchable probes or supramolecular architectures. In this paper, peptide nucleic acids (PNAs) containing 1 to 3 1-pyreneacetic acid units (PNA1–6) with a sequence with prevalence of pyrimidine bases, complementary to cystic fibrosis W1282X point mutation were synthesized. These compounds showed sequence-selective switch-on of pyrene excimer emission in the presence of target DNA, due to PNA₂DNA triplex formation, with stability depending on the number and positioning of the pyrene units along the chain. An increase in triplex stability and a very high mismatch-selectivity, derived from combined stacking and base-pairing interactions, were found for PNA2, bearing two distant pyrene units.

Photonic crystal enhancement of a homogeneous fluorescent assay using submicron fluid channels fabricated by E-jet patterning

We demonstrate the enhancement of a liquid-based homogenous fluorescence assay using the resonant electric fields from a photonic crystal (PC) surface. Because evanescent fields are confined to the liquid volume nearest to the photonic crystal, we developed a simple approach for integrating a PC fabricated on a silicon substrate within a fluid channel with submicron height, using electrohydrodynamic jet (e-jet) printing of a light-curable epoxy adhesive to define the fluid channel pattern. The PC is excited by a custom-designed compact instrument that illuminates the PC with collimated light that precisely matches the resonant coupling condition when the PC is covered with aqueous media. Using a molecular beacon nucleic acid fluorescence resonant energy transfer (FRET) probe for a specific miRNA sequence, we demonstrate an 8× enhancement of the fluorescence emission signal, compared to performing the same assay without exciting resonance in the PC detecting a miRNA sequence at a concentration of 62 nM from a liquid volume of only ∼20 nL. The approach may be utilized for any liquid-based fluorescence assay for applications in point-of-care diagnostics, environmental monitoring, or pathogen detection.

A supramolecular nanobiological hybrid as a PET sensor for bacterial DNA isolated from Streptomyces sanglieri

A ‘turn on–off–on’ sensor for highly sensitive detection of ds DNA with an excellent ‘limit of detection’ is reported.

Fluorescent aptasensor based on aggregation-induced emission probe and graphene oxide

Recently, a great variety of aggregation-induced emission (AIE)-active molecules has been utilized to design bioprobes for label-free fluorescent turn-on aptasensing with high sensitivity. However, due to nonspecific binding interaction between aptamer and AIE probe, these AIE-based aptasensors have nearly no selectivity, thereby significantly limiting the development. In this work, a 9,10-distyrylanthracene with two ammonium groups (DSAI) is synthesized as a novel AIE probe, and the fluorescent aptasensor based on DSAI and graphene oxide (GO) is developed for selective and sensitive sensing of targeted DNA and thrombin protein. Given its AIE property and high selectivity and sensitivity, this aptasensor can be also exploited to detect other targets.

Brightness enhanced DNA FIT-probes for wash-free RNA imaging in tissue

Fluorogenic oligonucleotides enable RNA imaging in cells and tissues. A high responsiveness of fluorescence is required when unbound probes cannot be washed away. Furthermore, emission should be bright in order to enable detection against autofluorescent background. The development of fluorescence-quenched hybridization probes has led to remarkable improvement of fluorescence responsiveness. Yet, comparably little attention has been paid to the brightness of smart probes. We describe hybridization probes that combine responsiveness with a high brightness of the measured signal. The method relies upon quencher-free DNA forced intercalation (FIT)-probes, in which two (or more) intercalator dyes of the thiazole orange (TO) family serve as nucleobase surrogates. Initial experiments on multi-TO-labeled probes led to improvements of responsiveness, but self-quenching limited their brightness. To enhance both brightness and responsiveness the highly responsive TO nucleoside was combined with the highly emissive oxazolopyridine analogue JO. Single-stranded TO/JO FIT-probes are dark. In the probe-target duplex, quenching caused by torsional twisting and dye-dye contact is prevented. The TO nucleoside appears to serve as a light collector that increases the extinction coefficient and transfers excitation energy to the JO emitter. This leads to very bright JO emission upon hybridization (F/F0 = 23, brightness = 43 mL mol(-1) cm(-1) at λex = 516 nm). TO/JO FIT-probes allowed the direct fluorescence microscopic imaging of oskar mRNA within a complex tissue. Of note, RNA imaging was feasible under wide-field excitation conditions. The described protocol enables rapid RNA imaging in tissue without the need for cutting-edge equipment, time-consuming washing, or signal amplification.

Dual fluorophore PNA FIT-probes--extremely responsive and bright hybridization probes for the sensitive detection of DNA and RNA

Fluorescently labeled oligonucleotides are commonly employed as probes to detect specific DNA or RNA sequences in homogeneous solution. Useful probes should experience strong increases in fluorescent emission upon hybridization with the target. We developed dual labeled peptide nucleic acid probes, which signal the presence of complementary DNA or RNA by up to 450-fold enhancements of fluorescence intensity. This enabled the very sensitive detection of a DNA target (40 pM LOD), which was detectable at less than 0.1% of the beacon concentration. In contrast to existing DNA-based molecular beacons, this PNA-based method does not require a stem sequence to enforce dye-dye communication. Rather, the method relies on the energy transfer between a "smart" thiazole orange (TO) nucleotide, which requires formation of the probe-target complex in order to become fluorescent, and terminally appended acceptor dyes. To improve upon fluorescence responsiveness the energy pathways were dissected. Hydrophobic, spectrally mismatched dye combinations allowed significant (99.97%) decreases of background emission in the absence of a target. By contrast, spectral overlap between TO donor emission and acceptor excitation enabled extremely bright FRET signals. This and the large apparent Stokes shift (82 nm) suggests potential applications in the detection of specific RNA targets in biogenic matrices without the need of sample pre-processing prior to detection.

An elegant biosensor molecular beacon probe: challenges and recent solutions

Molecular beacon (MB) probes are fluorophore- and quencher-labeled short synthetic DNAs folded in a stem-loop shape. Since the first report by Tyagi and Kramer, it has become a widely accepted tool for nucleic acid analysis and triggered a cascade of related developments in the field of molecular sensing. The unprecedented success of MB probes stems from their ability to detect specific DNA or RNA sequences immediately after hybridization with no need to wash out the unbound probe (instantaneous format). Importantly, the hairpin structure of the probe is responsible for both the low fluorescent background and improved selectivity. Furthermore, the signal is generated in a reversible manner; thus, if the analyte is removed, the signal is reduced to the background. This paper highlights the advantages of MB probes and discusses the approaches that address the challenges in MB probe design. Variations of MB-based assays tackle the problem of stem invasion, improve SNP genotyping and signal-to-noise ratio, as well as address the challenges of detecting folded RNA and DNA.

Oligonucleotide Conjugates for Detection of Specific Nucleic Acid Sequences

In this chapter, we summarise the designs of fluorophore-modified nucleic acids used as probes for the detection of target DNA/RNA. Recently, there has been an increasing demand for the sequence-specific detection of DNA and RNA in biology and biotechnology. Fluorescent probes based on nucleic acids are useful because of their simplicity and ease of handling. Here, we described three types of fluorescent probe: 1) linear probes, 2) binary probes, and 3) molecular beacons. Each can have one or more fluorophores. Mechanisms for the fluorescence responses of these probes are also discussed in detail. These fluorescent probes have been used in real-time polymerase chain reaction (PCR), genetic analyses, and messenger RNA (mRNA) imaging in living cells. Improvements in sensitivity, selectivity, and nuclease resistance of these probes will lead to more widespread applications in chemical biology, biotechnology, and medicine.

Molecular assemblies of perylene bisimide dyes in water

Perylene bisimides are among the most valuable functional dyes and have numerous potential applications. As a result of their chemical robustness, photostability, and outstanding optical and electronic properties, these dyes have been applied as pigments, fluorescence sensors, and n-semiconductors in organic electronics and photovoltaics. Moreover, the extended quadrupolar π system of this class of dyes has facilitated the construction of numerous supramolecular architectures with fascinating photophysical properties. However, the supramolecular approach to the formation of perylene bisimide aggregates has been restricted mostly to organic media. Pleasingly, considerable progress has been made in the last few years in developing water-soluble perylene bisimides and their application in aqueous media. This Review provides an up-to-date overview on the self-assembly of perylene bisimides through π-π interactions in aqueous media. Synthetic strategies for the preparation of water-soluble perylene bisimides and the influence of water on the π-π stacking of perylene bisimides as well as the resulting applications are discussed.