Single-Molecule Detection of a Fluorescent Nucleobase Analogue via Multiphoton Excitation

Isomorphic Fluorescent Nucleosides

In 1960, Weber prophesied that "There are many ways in which the properties of the excited state can be utilized to study points of ignorance of the structure and function of proteins". This has been realized, illustrating that an intrinsic and highly responsive fluorophore such as tryptophan can alter the course of an entire scientific discipline. But what about RNA and DNA? Adapting Weber's protein photophysics prophecy to nucleic acids requires the development of intrinsically emissive nucleoside surrogates as, unlike Trp, the canonical nucleobases display unusually low emission quantum yields, which render nucleosides, nucleotides, and oligonucleotides practically dark for most fluorescence-based applications.Over the past decades, we have developed emissive nucleoside surrogates that facilitate the monitoring of nucleoside-, nucleotide-, and nucleic acid-based transformations at a nucleobase resolution in real time. The premise underlying our approach is the identification of minimal atomic/structural perturbations that endow the synthetic analogs with favorable photophysical features while maintaining native conformations and pairing. As illuminating probes, the photophysical parameters of such isomorphic nucleosides display sensitivity to microenvironmental factors. Responsive isomorphic analogs that function similarly to their native counterparts in biochemical contexts are defined as isofunctional.Early analogs included pyrimidines substituted with five-membered aromatic heterocycles at their 5 position and have been used to assess the polarity of the major groove in duplexes. Polarized quinazolines have proven useful in assembling FRET pairs with established fluorophores and have been used to study RNA-protein and RNA-small-molecule binding. Completing a fluorescent ribonucleoside alphabet, composed of visibly emissive purine (thA, thG) and pyrimidine (thU, thC) analogs, all derived from thieno[3,4-d]pyrimidine as the heterocyclic nucleus, was a major breakthrough. To further augment functionality, a second-generation emissive RNA alphabet based on an isothiazolo[4,3-d]pyrimidine core (thA, tzG, tzU, and tzC) was fabricated. This single-atom "mutagenesis" restored the basic/coordinating nitrogen corresponding to N7 in the purine skeleton and elevated biological recognition.The isomorphic emissive nucleosides and nucleotides, particularly the purine analogs, serve as substrates for diverse enzymes. Beyond polymerases, we have challenged the emissive analogs with metabolic and catabolic enzymes, opening optical windows into the biochemistry of nucleosides and nucleotides as metabolites as well as coenzymes and second messengers. Real-time fluorescence-based assays for adenosine deaminase, guanine deaminase, and cytidine deaminase have been fabricated and used for inhibitor discovery. Emissive cofactors (e.g., SthAM), coenzymes (e.g., NtzAD+), and second messengers (e.g., c-di-tzGMP) have been enzymatically synthesized, using xyNTPs and native enzymes. Both their biosynthesis and their transformations can be fluorescently monitored in real time.Highly isomorphic and isofunctional emissive surrogates can therefore be fabricated and judiciously implemented. Beyond their utility, side-by-side comparison to established analogs, particularly to 2-aminopurine, the workhorse of nucleic acid biophysics over 5 decades, has proven prudent as they refined the scope and limitations of both the new analogs and their predecessors. Challenges, however, remain. Associated with such small heterocycles are relatively short emission wavelengths and limited brightness. Recent advances in multiphoton spectroscopy and further structural modifications have shown promise for overcoming such barriers.

Synthesis of Thiazoloindole α-Amino Acids: Chromophores Amenable to One- and Two-Photon Induced Fluorescence

Thiazoloindole α-amino acids have been synthesized in four steps from tryptophan using a dual-catalytic thiolation reaction and a copper-mediated intramolecular N-arylation process. Late-stage diversification of the thiazoloindole core with electron-deficient aryl substituents produced chromophores that on one-photon excitation displayed blue-green emission, mega-Stokes shifts, and high quantum yields. The thiazoloindole amino acids could also be excited via two-photon absorption in the near-infrared, demonstrating their potential for biomedical imaging applications.

Screening for Novel Fluorescent Nucleobase Analogues Using Computational and Experimental Methods: 2-Amino-6-chloro-8-vinylpurine (2A6Cl8VP) as a Case Study

Novel fluorescent nucleic acid base analogues (FBAs) with improved optical properties are needed in a variety of biological applications. 2-Amino-6-chloro-8-vinylpurine (2A6Cl8VP) is structural analogue of two existing highly fluorescent FBAs, 2-aminopurine (2AP) and 8-vinyladenine (8VA), and can therefore be expected to have similar base pairing as well as better optical properties compared to its counterparts. In order to determine the absorption and fluorescence properties of 2A6Cl8VP, as a first step, we used TD-DFT calculations and the polarizable continuum model for simulating the solvents and computationally predicted absorption and fluorescence maxima. To test the computational predictions, we also synthesized 2A6Cl8VP and measured its UV/vis absorbance, fluorescence emission, and fluorescence lifetime. The computationally predicted absorbance and fluorescence maxima of 2A6Cl8VP are in reasonable agreement to the experimental values and are significantly redshifted compared to 2AP and 8VA, allowing for its specific excitation. The fluorescence quantum yield of 2A6Cl8VP, however, is significantly lower than those of 2AP and 8VA. Overall, 2A6Cl8VP is a novel fluorescent nucleobase analogue, which can be useful in studying structural, biophysical, and biochemical applications.

A computational two-photon fluorescence approach for revealing label-free the 3D image of viruses and bacteria

Current solutions for bacteria and viruses identification are based on time-consuming technics with complex preparation procedures. In the present work, we revealed label-free the presence of free viral particles and bacteria with a computational two-photon fluorescence (C-TPF) strategy. Six bacteria were tested: Escherichia coli, Staphylococcus epidermidis, Proteus vulgaris, Pseudomonas fluorescens, Bacillus subtilis, and Clostridium perfringens. The two families of viral particles were the herpes virus with the cytomegalovirus (CMV, 300 nm of diameter) and the coronavirus with the SARS-CoV-2 (100 nm of diameter). The instrumental and computational pipeline FAMOUS optimized the produced 3D images. The origin of the fluorescence emission was discussed for bacteria regarding to their two-photon excitation spectra and attributed to the metabolic indicators (FAD and NADH). The optical and computational strategy constitute a new approach for imaging label-free viral particles and bacteria and paves the way to a new understanding of viral or bacterial ways of infection.

Pulse-shaped broadband multiphoton excitation for single-molecule fluorescence detection in the far field

Multiphoton excitation of fluorescence has many potential advantages over resonant (one-photon) excitation, but the method has not found widespread use for ultrasensitive applications. We recently described an approach to the multiphoton excitation of single molecules that uses a pulse shaper to compress and tailor pulses from an ultrafast broadband laser in order to optimise the brightness and signal-to-background ratio following non-linear excitation. Here we provide a detailed description of the setup and illustrate its use and potential by optimising two-photon fluorescence of a common fluorophore, rhodamine 110, at the single-molecule level. We also show that a DNA oligonucleotide labelled with a fluorescent nucleobase analogue, tC, can be detected using two-photon FCS, whereas one-photon excitation causes rapid photobleaching. The ability to improve the signal-to-background ratio and to reduce the incident power required to attain a given brightness can be applied to the multiphoton excitation of any fluorescent species, from small molecules with low multiphoton cross sections to the brightest nanoparticles.

Enhancement of optical properties of new purine nucleobases containing electron-donating and -withdrawing peripheral groups

Nowadays, a great deal of attention has been focused on synthesizing highly fluorescent unnatural base analogs. This has been motivated by the need to overcome the lack of fluorescence of nucleic acids' natural bases. Fluorescent unnatural base analogs, such as purines, may be used in several applications, such as DNA or RNA optical spectroscopy studies. Moreover, for purines base analogs, the optical properties, for example, emission, can be tunable through molecular engineering, improving their applications as fluorescent probes. Looking in this direction, the synthesis and optical spectroscopic studies of a new set of purines base analogs are of foremost relevance. Here, an increase in the fluorescence quantum yield was observed in molecules with NH-π-CN arrangement. The two-photon absorption (2PA) cross-sections also increased for the lower energy 2PA state. The enhancement of both properties results in a two-photon brightness of 5 and 10 times higher than in compounds lacking the NH-π-CN arrangement. For the higher energy 2PA state, an excited state absorption contribution to the 2PA cross-section values was observed, that was verified through ultrafast transient absorption measurements. The higher 2PA brightness makes the purines base analogs promising candidates as fluorescent probes in RNA and DNA spectroscopic studies.

Single molecule detection; from microscopy to sensors

Single molecule detection is necessary to find out physical, chemical properties and their mechanism involved in the normal functioning of body cells. In this way, they can provide a new direction to the healthcare system. Various techniques have been developed and employed for their successful detection. Herein, we have emphasized various traditional methods as well as biosensing technology which offer single molecule sensitivity. The various methods including plasmonic resonance, nanopores, whispering gallery mode, Simoa assay and recognition tunneling are discussed in the initial part which has been followed by a discussion about biosensor-based detection. Plasmonic, SERS, CRISPR/Cas, and other types of biosensors are focused in this review and found to be highly sensitive for single molecule detection. This review provides an overview of progression in different techniques employed for single molecule detection.

Mechanical characterization of base analogue modified nucleic acids by force spectroscopy

We use mechanical unfolding of single DNA hairpins with modified bases to accurately assess intra- and intermolecular forces in nucleic acids. As expected, the modification stabilizes the hybridized hairpin, but we also observe intriguing stacking interactions in the unfolded hairpin. Our study highlights the benefit of using base-modified nucleic acids in force-spectroscopy.

The thermochemical, structural, and spectroscopic analyses of the tautomers of sulfur and selenium modified emissive nucleobases

In this study, density functional theory (DFT) and time dependent density functional theory (TD-DFT) are used to investigate the stabilities and spectral properties [IR, UV–vis, and two-photon absorption (2PA)] of two sets of modified RNA nucleobase tautomers. The modifications introduce either a sulfur or selenium atom to form an isothiazolo[4,3-d]pyrimidine or isoselenazolo[4,3-d]pyrimidine heterocyclic core, respectively. The relative stabilities of both sets of modified tautomers determined with B3LYP/6-31++G(d,p) reveal that in water (with a polarizable continuum model), the 6-keto-2-amino tautomer of guanine and the rare 4-imino-2-keto tautomer of cytosine may be present at significant populations, whereas the 6-enol-2-amino tautomer of guanine is more common in the gas phase. The identification of these modified tautomers is possible due to the natural differences in their vibrational modes and hence IR spectra. Furthermore, the photophysical properties of both these sets of modified tautomers indicate that excitation and emission energies are shifted relative to their more abundant form in both one photon absorption and emission and 2PA spectra, as determined at the B3LYP/6-31++G(d,p) and CAM-B3LYP/aug-cc-pVDZ levels of theory, respectively. Even though the 2PA cross sections in water for all of the species are small (0.3–2.3 GM), the modified cytosine tautomer shows promise, as its cross section is larger than the more dominant form. The spectral separation between the dominant form and the tautomers of isoselenazole and isothiazole modified cytosine and guanine are relatively similar, suggesting both modifications could be useful in elucidating the tautomers from their more abundant counterparts.

Structure and luminescence of DNA-templated silver clusters

DNA serves as a versatile template for few-atom silver clusters and their organized self-assembly. These clusters possess unique structural and photophysical properties that are programmed into the DNA template sequence, resulting in a rich palette of fluorophores which hold promise as chemical and biomolecular sensors, biolabels, and nanophotonic elements. Here, we review recent advances in the fundamental understanding of DNA-templated silver clusters (Ag N -DNAs), including the role played by the silver-mediated DNA complexes which are synthetic precursors to Ag N -DNAs, structure-property relations of Ag N -DNAs, and the excited state dynamics leading to fluorescence in these clusters. We also summarize the current understanding of how DNA sequence selects the properties of Ag N -DNAs and how sequence can be harnessed for informed design and for ordered multi-cluster assembly. To catalyze future research, we end with a discussion of several opportunities and challenges, both fundamental and applied, for the Ag N -DNA research community. A comprehensive fundamental understanding of this class of metal cluster fluorophores can provide the basis for rational design and for advancement of their applications in fluorescence-based sensing, biosciences, nanophotonics, and catalysis.

Single-molecule fluorescence detection of a tricyclic nucleoside analogue

Fluorescent nucleobase surrogates capable of Watson–Crick hydrogen bonding are essential probes of nucleic acid structure and dynamics, but their limited brightness and short absorption and emission wavelengths have rendered them unsuitable for single-molecule detection. Aiming to improve on these properties, we designed a new tricyclic pyrimidine nucleoside analogue with a push–pull conjugated system and synthesized it in seven sequential steps. The resulting C-linked 8-(diethylamino)benzo[b][1,8]naphthyridin-2(1H)-one nucleoside, which we name ABN, exhibits ε₄₄₂ = 20 000 M⁻¹ cm⁻¹ and Φ_em,540 = 0.39 in water, increasing to Φ_em = 0.50–0.53 when base paired with adenine in duplex DNA oligonucleotides. Single-molecule fluorescence measurements of ABN using both one-photon and two-photon excitation demonstrate its excellent photostability and indicate that the nucleoside is present to > 95% in a bright state with count rates of at least 15 kHz per molecule. This new fluorescent nucleobase analogue, which, in duplex DNA, is the brightest and most red-shifted known, is the first to offer robust and accessible single-molecule fluorescence detection capabilities.

Fluorescent nucleoside analogue ABN is readily detected at the single-molecule level and retains a quantum yield >50% in duplex DNA oligonucleotides.

Getting DNA and RNA out of the dark with 2CNqA: a bright adenine analogue and interbase FRET donor

With the central role of nucleic acids there is a need for development of fluorophores that facilitate the visualization of processes involving nucleic acids without perturbing their natural properties and behaviour. Here, we incorporate a new analogue of adenine, 2CNqA, into both DNA and RNA, and evaluate its nucleobase-mimicking and internal fluorophore capacities. We find that 2CNqA displays excellent photophysical properties in both nucleic acids, is highly specific for thymine/uracil, and maintains and slightly stabilises the canonical conformations of DNA and RNA duplexes. Moreover, the 2CNqA fluorophore has a quantum yield in single-stranded and duplex DNA ranging from 10% to 44% and 22% to 32%, respectively, and a slightly lower one (average 12%) inside duplex RNA. In combination with a comparatively strong molar absorptivity for this class of compounds, the resulting brightness of 2CNqA inside double-stranded DNA is the highest reported for a fluorescent base analogue. The high, relatively sequence-independent quantum yield in duplexes makes 2CNqA promising as a nucleic acid label and as an interbase Förster resonance energy transfer (FRET) donor. Finally, we report its excellent spectral overlap with the interbase FRET acceptors qAnitro and tCnitro, and demonstrate that these FRET pairs enable conformation studies of DNA and RNA.

A Highly Fluorescent Nucleobase Molecular Rotor

Fluorescent base analogs (FBAs) are powerful probes of nucleic acids' structures and dynamics. However, previously reported FBAs exhibit relatively low brightness and therefore limited sensitivity of detection. Here we report the hitherto brightest FBA that has ideal molecular rotor properties for detecting local dynamic motions associated with base pair mismatches. The new trans-stilbene annulated uracil derivative "^tsT" exhibits bright fluorescence emissions in various solvents (ε × Φ = 3400-29 700 cm^-1 M^-1) and is highly sensitive to mechanical motions in duplex DNA (ε × Φ = 150-4250 cm^-1 M^-1). ^tsT is thereby a "smart" thymidine analog, exhibiting a 28-fold brighter fluorescence intensity when base paired with A as compared to T or C. Time-correlated single photon counting revealed that the fluorescence lifetime of ^tsT (τ = 4-11 ns) was shorter than its anisotropy decay in well-matched duplex DNA (θ = 20 ns), yet longer than the dynamic motions of base pair mismatches (0.1-10 ns). These properties enable unprecedented sensitivity in detecting local dynamics of nucleic acids.

Conformationally rigid pyrazoloquinazoline α-amino acids: one- and two-photon induced fluorescence

Conformationally rigid unnatural α-amino acids bearing a pyrazoloquinazoline ring system that are amenable to both one- and two-photon excitation have been developed as new fluorescent probes.