Multilabeled pyrene-functionalized 2'-amino-LNA probes for nucleic acid detection in homogeneous fluorescence assays

Homogeneous fluorescence assays for detection of nucleic acids are widely used in biological sciences. Typically, probes such as molecular beacons that rely on distance-dependent fluorescence quenching are used for such assays. Less attention has been devoted to tethering a single kind of fluorophores to oligonucleotides and exploiting hybridization-induced modulation of fluorescence intensity for nucleic acid detection. Herein, thermal denaturation experiments and fluorescence properties of oligodeoxyribonucleotides containing one or more 2'-N-(pyren-1-yl)carbonyl-2'-amino-LNA monomer(s) X are described. These pyrene-functionalized 2'-amino-LNAs display large increases in thermal stability against DNA/RNA complements with excellent Watson-Crick mismatch discrimination. Upon duplex formation of appropriately designed 2'-N-(pyren-1-yl)carbonyl-2'-amino-LNA probes and complementary DNA/RNA, intensive fluorescence emission with quantum yields between 0.28 and 0.99 are observed. Quantum yields of such magnitudes are unprecedented among pyrene-labeled oligonucleotides. Molecular modeling studies suggest that the dioxabicyclo[2.2.1]heptane skeleton and amide linkage of monomer X fix the orientation of the pyrene moiety in the minor groove of a nucleic acid duplex. Interactions between pyrene and nucleobases, which typically lead to quenching of fluorescence, are thereby reduced. Duplexes between multiple modified probes and DNA/RNA complements exhibit additive increases in fluorescence intensity, while the fluorescence of single stranded probes becomes increasingly quenched. Up to 69-fold increase in fluorescence intensity (measured at lambda(em) = 383 nm) is observed upon hybridization to DNA/RNA. The emission from duplexes of multiple modified probes and DNA/RNA at concentrations down to less than 500 nM can easily be seen by the naked eye using standard illumination intensities.     

Peptide nucleic acid stabilized perovskite nanoparticles for nucleic acid sensing

Nanostructural hybrid organic-inorganic metal halide perovskites offer a wide range of potential applications including photovoltaics, solar cells, and light emitting diodes. Up to now the surface stabilizing ligands were used solely to obtain the optimal properties of nanoparticles in terms of dimensionality and stability, however their possible additional functionality was rarely considered. In the present work, hybrid lead bromide perovskite nanoparticles (PNP) were prepared using a unique approach where a peptide nucleic acid is used as a surface ligand. Methylammonium lead bromide perovskite colloidal nanoparticles stabilized by thymine-based peptide nucleic acid monomer (PNA-M) and relevant trimer (PNA-T) were prepared exhibiting the size below 10 nm. Perovskite structure and crystallinity were verified by X-ray powder diffraction spectroscopy and high resolution transmission electron microscopy. PNP-PNA-M and PNP-PNA-T colloidal dispersions in chloroform and toluene possessed green-blue fluorescence, while Fourier-transform infrared spectroscopy (FT-IR) and quantum chemical calculations showed that the PNA coordinates to the PNP surface through the primary amine group. Additionally, the sensing ability of the PNA ligand for adenine nucleic acid was demonstrated by photoluminescence quenching via charge transfer. Furthermore, PNP thin films were effectively produced by the centrifugal casting. We envision that combining the unique, tailored structure of peptide nucleic acids and the prospective optical features of lead halide perovskite nanoparticles could expand the field of applications of such hybrids exploiting analogous ligand chemistry.     

Fluorescent hybridization probes for nucleic acid detection

Due to their high sensitivity and selectivity, minimum interference with living biological systems, and ease of design and synthesis, fluorescent hybridization probes have been widely used to detect nucleic acids both in vivo and in vitro. Molecular beacons (MBs) and binary probes (BPs) are two very important hybridization probes that are designed based on well-established photophysical principles. These probes have shown particular applicability in a variety of studies, such as mRNA tracking, single nucleotide polymorphism (SNP) detection, polymerase chain reaction (PCR) monitoring, and microorganism identification. Molecular beacons are hairpin oligonucleotide probes that present distinctive fluorescent signatures in the presence and absence of their target. Binary probes consist of two fluorescently labeled oligonucleotide strands that can hybridize to adjacent regions of their target and generate distinctive fluorescence signals. These probes have been extensively studied and modified for different applications by modulating their structures or using various combinations of fluorophores, excimer-forming molecules, and metal complexes. This review describes the applicability and advantages of various hybridization probes that utilize novel and creative design to enhance their target detection sensitivity and specificity.     

Sandwich probes: two simultaneous reactions for templated nucleic acid detection

Fluorescence-quenched nucleic acid probes with reactive moieties at both the 5' and 3' ends are synthesized and tested for reaction with two adjacent nucleophile-containing DNAs. These probes improve signal to background over singly reactive probes and can discriminate single nucleotide polymorphisms in the target DNA or RNA.     

Advances in nucleic acid architectures for electrochemical sensing

Nucleic acid–based electrochemical sensors are ideally suited to the detection of molecular targets for which enzymatic detection or direct electrochemical oxidation – reduction reactions are not possible. Moreover, the versatility of nucleic acids in their ability to bind a great variety of target types, from small molecules to single-entity mesoscopic targets, makes them attractive receptors for the development of electrochemical biosensors. In this brief opinion piece, we discuss field advances from the past two years. We hope the works highlighted here will inspire the community to pursue creative designs enabling the detection of larger and more complex targets with a specific focus on analytical validation and translation into preclinical or clinical applications.     

Hybridization and reaction-based fluorogenic nucleic acid probes

We developed fluorogenic probes, which are photoactivated in the presence of specific nucleic acid templates with the release of fluorescent dyes. This templated reaction can be used to target specific nucleic acids in complex mixtures. We further demonstrate that this reaction can be monitored by single-molecule fluorescence imaging.     

Sensitive determination of nucleic acids using organic nanoparticle fluorescence probes

This paper describes the preparation of organic nanoparticles by reprecipitation method under sonication and vigorous stirring. Transmission electron microscopy (TEM) was used to characterize the size and size distribution of the luminescent nanoparticles. Their average diameter was about 25 nm with a size variation of +/-18%. The fluorescence decay lifetime of the nanoparticles also was determined on a self-equipped fluorospectrometer with laser light source. The lifetime (approximately 0.09 micros) of nanoparticles is about three times long as that of the monomer. The nanoparticles were in abundant of hydrophilic groups, which increased their miscibility in aqueous solution. These organic nanoparticles have high photochemical stability, excellent resistance to chemical degradation and photodegradation, and a good fluorescence quantum yield (25%). The fluorescence can be efficiently quenched by nucleic acids. Based on the fluorescence quenching of nanoparticles, a fluorescence quenching method was developed for determination of microamounts of nucleic acids by using the nanoparticles as a new fluorescent probe. Under optimal conditions, maximum fluorescence quenching is produced, with maximum excitation and emission wavelengths of 345 and 402 nm, respectively. Under optimal conditions, the calibration graphs are linear over the range 0.4-19.0 microgml(-1) for calf thymus DNA (ct-DNA) and 0.3-19.0 microgml(-1) for fish sperm DNA (fs-DNA). The corresponding detection limits are 0.25 microgml(-1) for ct-DNA and 0.17 microgml(-1) for fs-DNA. The relative standard deviation of six replicate measurements is 1.3-2.1%. The method is simple, rapid and sensitive with wide linear range. The recovery and relative standard deviation are very satisfactory.     

DNA nanostructure-based nucleic acid probes: construction and biological applications

In recent years, DNA has been widely noted as a kind of material that can be used to construct building blocks for biosensing, in vivo imaging, drug development, and disease therapy because of its advantages of good biocompatibility and programmable properties. However, traditional DNA-based sensing processes are mostly achieved by random diffusion of free DNA probes, which were restricted by limited dynamics and relatively low efficiency. Moreover, in the application of biosystems, single-stranded DNA probes face challenges such as being difficult to internalize into cells and being easily decomposed in the cellular microenvironment. To overcome the above limitations, DNA nanostructure-based probes have attracted intense attention. This kind of probe showed a series of advantages compared to the conventional ones, including increased biostability, enhanced cell internalization efficiency, accelerated reaction rate, and amplified signal output, and thus improved in vitro and in vivo applications. Therefore, reviewing and summarizing the important roles of DNA nanostructures in improving biosensor design is very necessary for the development of DNA nanotechnology and its applications in biology and pharmacology. In this perspective, DNA nanostructure-based probes are reviewed and summarized from several aspects: probe classification according to the dimensions of DNA nanostructures (one, two, and three-dimensional nanostructures), the common connection modes between nucleic acid probes and DNA nanostructures, and the most important advantages of DNA self-assembled nanostructures in the applications of biosensing, imaging analysis, cell assembly, cell capture, and theranostics. Finally, the challenges and prospects for the future development of DNA nanostructure-based nucleic acid probes are also discussed.

In recent years, DNA has been widely noted as a kind of material that can be used to construct building blocks for biosensing, in vivo imaging, drug development, and disease therapy because of its advantages of good biocompatibility and programmable properties.     

CdS quantum dots as a fluorescent sensing platform for nucleic acid detection

We demonstrate that CdS quantum dots (QDs) can be applied to fluorescence-enhanced detection of nucleic acids in a two-step protocol. In step one, a fluorescently labeled single-stranded DNA probe is adsorbed on the QDs to quench its luminescence. In step two, the hybridization of the probe with its target ssDNA produces a double-stranded DNA which detaches from the QD. This, in turn, leads to the recovery of the fluorescence of the label. The lower detection limit of the assay is as low as 1?nM. The scheme (that was applied to detect a target DNA related to the HIV) is simple and can differentiate between perfectly complementary targets and mismatches.

The alpha-helical peptide nucleic acid concept: merger of peptide secondary structure and codified nucleic acid recognition

A novel platform for nucleic acid recognition that integrates the alpha-helix secondary structure of peptides with the codified base-pairing capability of nucleic acids is reported. The resulting alpha-helical peptide nucleic acids (alpha PNAs) are composed of a repeating tetrapeptidyl unit, aa(1)-aa(2)-aa(3)-Ser(B), where aa(1) through aa(3) represent generic ancillary amino acids and B = nucleobases linked to Ser via a methylene bridge. Effective syntheses of constituent Fmoc-protected nucleoamino acids (Fmoc-Ser(B)-OH, where B = thymine, cytosine, and uracil) are described along with a protocol for the solid-phase synthesis of 21mer alpha PNAs containing five such nucleobases. By varying the ancillary amino acids, two distinct classes of alpha PNAs were constructed, having a net charge of -1 or +6, respectively, at physiological pH. The modular nature of the alpha PNA platform was illustrated by the synthesis of symmetrical disulfide-bridged alpha PNA dimers containing 10 nucleobases. Hybridization of these alpha PNAs with ssDNA has been examined by thermal denaturation, gel electrophoresis, and circular dichroism (CD) and the data indicated that alpha PNA binds to ssDNA in a cooperative manner with high affinity and sequence specificity. In general, b2 alpha PNAs bind faster and more strongly with ssDNA than do the corresponding b1 alpha PNAs. Parallel alpha PNA-DNA complexes are more stable than their antiparallel counterparts. CD studies also revealed that the hybridization event involves the folding of both species into their helical conformations. Finally, NMR experiments provided conclusive evidence of Watson-Crick base pairing in alpha PNA-ssDNA hybrids.     

Fluorescence-enhanced nucleic acid detection: using coordination polymer colloids as a sensing platform

Coordination polymer colloids have been used as an effective fluorescent sensing platform for multiplexing nucleic acid detection capable of distinguishing complementary and mismatched target sequences for the first time.     

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.     

Quencher-free molecular beacons: a new strategy in fluorescence based nucleic acid analysis

Molecular beacons (MBs) have been used as viable fluorescent probes in nucleic acid analysis. Many researchers around the world continue to modify the MBs to suit their needs. As a result, a number of nucleic acid probing systems with close resemblance to the MBs are being reported from time to time. Quencher-free molecular beacons (QF-MBs) are a significant modification of the conventional MB; in QF-MBs the quencher part has been eliminated. Despite the absence of the quencher, the QF-MBs can identify specific target DNA. They can also be used in SNP typing and in real-time PCR analysis for quantification of DNAs. The design, factors behind functioning and applications of different types of QF-MBs and closely related quencher-free nucleic acid probing systems (QF-NAPs) have been described in this tutorial review.     

Amplified nucleic acid sensing using programmed self-cleaving DNAzyme

A newly designed target-assisted self-cleavage (TASC) probe composed of a target-binding site and a DNAzyme domain undergoes TASC when activated via hybridization with a target DNA/RNA. This self-splicing or self-dissociation reaction occurs in a catalytic manner with the probe as a substrate and the target as a catalyst, since the fragmented products are automatically released from the target, thus amplifying the sequence information of the latter under non-PCR, i.e., isothermal and enzyme/reagent-free, conditions. A fluorescence-reporting TASC probe having a fluorescein/dabsyl FRET pair across the cleavage site allows a mix-and-read discrimination of single-nucleotide differences in the target.     

Chlamydomonas reinhardtii genetic variants as probes for fluorescence sensing system in detection of pollutants

The unicellular green alga Chlamydomonas reinhardtii is employed here for the setup of a biosensor demonstrator based on multibiomediators for the detection of herbicides. The detection is based on the activity of photosystem II, the multienzymatic chlorophyll–protein complex located in the thylakoid membrane that catalyzes the light-dependent photosynthetic primary charge separation and the electron transfer chain in cyanobacteria, algae, and higher plants. Several C. reinhardtii mutants modified on the D1 photosystem II protein are generated by site-directed mutagenesis and experimentally tested for the development of a biosensor revealing the modification of the fluorescence parameter (1 − V _J) in the presence of herbicides. The A250R, A250L, A251C, and I163N mutants are highly sensitive to the urea and triazine herbicide classes; the newly generated F255N mutant is shown to be especially resistant to the class of urea. It follows that the response of the multibiomediators is associated to a particular herbicide subclass and can be useful to monitor several species of pollutants.     

Simultaneous quantification of multiple nucleic acid targets using chemiluminescent probes

A novel method is described for simultaneous detection and quantification of attomoles or a few femtomoles of two (or potentially more) nucleic acid targets, without need for amplification. The technique depends on spectral-temporal resolution of chemiluminescence emitted from independent hybridization-induced chemiluminescent signal probes. The probes are internally quenched except in the presence of their specific targets, thereby allowing detection limits up to 10,000 times lower than with fluorescent probes. This is sufficient to obviate the need for amplification in many cases. The utility of the technique has been demonstrated by use of resolvable N-linked acridinium and 2,7-dimethoxyacridinium ester labeled probes in a homogeneous assay for sensitive and simultaneous independent quantification of pan-bacterial and pan-fungal target sequences in seawater.     

基于小分子的核酸结构探针最新研究进展

本文主要综述了基于小分子的核酸结构探针的最新研究进展.所探索的核酸结构主要有四链核酸（包括G-四链体以及i-motif）结构、三链核酸结构、左手螺旋DNA结构以及不规则核酸结构（包括突起结构以及环状结构）等;所探索的小分子包括过渡金属配合物、大环共轭化合物、环肽以及寡糖抗生素等.     

Pyrenyl-appended imidazolium receptor for selective fluorescence sensing of oxalic acid

A new fluorescent imidazolium-based cholestane receptor 4 bearing a pyrene moiety was synthesized. The binding ability of 4 toward various dicarboxylic acids was examined by UV-vis and fluorescence spectroscopy. Receptor 4 showed the highest binding constant for oxalic acid among all the tested dicarboxylic acids (K_a = 5.06 × 10⁴ M⁻¹). Oxalic acid formed a complex with 4 with a 1:2 ratio in ethanol.