Unexpected Cyclization Product Discovery from the Photoinduced Bioconjugation Chemistry between Tetrazole and Amine

Bioconjugation chemistry has emerged as a powerful tool for the modification of diverse biomolecules under mild conditions. Tetrazole, initially proposed as a bioorthogonal photoclick handle for 1,3-dipolar cyclization with alkenes, was later demonstrated to possess broader photoreactivity with carboxylic acids, serving as a versatile bioconjugation and photoaffinity labeling probe. In this study, we unexpectedly discovered and validated the photoreactivity between tetrazole and primary amine to afford a new 1,2,4-triazole cyclization product. Given the significance of functionalized N-heterocycles in medicinal chemistry, we successfully harnessed the serendipitously discovered reaction to synthesize both pharmacologically relevant DNA-encoded chemical libraries (DELs) and small molecule compounds bearing 1,2,4-triazole scaffolds. Furthermore, the mild reaction conditions and stable 1,2,4-triazole linkage found broad application in photoinduced bioconjugation scenarios, spanning from intramolecular peptide macrocyclization and templated DNA reaction cross-linking to intermolecular photoaffinity labeling of proteins. Triazole cross-linking products on lysine side chains were identified in tetrazole-labeled proteins, refining the comprehensive understanding of the photo-cross-linking profiles of tetrazole-based probes. Altogether, this tetrazole-amine bioconjugation expands the current bioconjugation toolbox and creates new possibilities at the interface of medicinal chemistry and chemical biology.

Harnessing PUF-Based Reporters for Noninvasive Imaging of the MicroRNA Dynamics in Differentiation

Precise characterization of miRNA expression patterns is critical to exploit the complexity of miRNA regulation in biology. Herein, we developed a Pumilio/FBF (PUF) protein-based engineering luciferase reporter system, PUF/miR, to quantitatively and non-invasively sense miRNA activity in living cells and animal models. We verified the feasibility of this reporter by monitoring the expression of several types of miRNAs (miRNA-9, 124a, 1, and 133a) in neural and muscle differentiated cells as well as subcutaneous or tibial anterior muscles in mice. The quantitative RT-PCR also validated the reliability and quantitative consistency of bioluminescence imaging in detecting miRNA expression. We further effectively employed this reporter system to visualize the expression of miRNA-1 and miRNA-133a in mouse models of skeletal muscle injury. As a non-invasive and convenient innovative approach, our results have realized the positive bioluminescence imaging of endogenous miRNAs in vitro and in vivo using the PUF/miR system. We believe that this approach would provide a potential means for noninvasive monitoring of disease-related miRNAs and could facilitate a deeper understanding of miRNA biology.

A tetrazole-ene photoactivatable fluorophore with improved brightness and stability in protic solution

The pyrazoline fluorophore, generated by photoinduced tetrazole-ene cycloaddition, shows faint fluorescence in protic solvents. To suppress this fluorescence-quenching, we rationally designed a series of substituted diaryl tetrazoles at the N-side phenyl ring to produce a tetrazole-ene based photoactivatable fluorophore. Spectroscopic and cellular imaging studies demonstrated that the version of the fluorophore with a bis(trifluoromethyl)benzene substituent exhibited significantly enhanced brightness and photostability.

Photoactivatable Fluorogenic Azide-Alkyne Click Reaction: A Dual-Activation Fluorescent Probe

Aryl azide and diaryl tetrazole are both photoactive molecules, which can form nitrene and nitrile imine intermediates respectively by photolysis. Depending on the new finding that the azide can suppress the photolysis of tetrazole in the azide-tetrazole conjugated system, we developed aryl azide-tetrazole probes for the photoactivatable fluorogenic azide alkyne click (PFAAC) reaction, in which the aryl azide-tetrazole probes were not phoroactivatable fluorogenic itself, but the triazole products after click reaction were prefluorophore that can be activated by light. Therefore, in PFAAC chemistry, the fluorescent probes can be activated by two orthogonal events: azide-alkyne click reaction and light, which leads to spatiotemporal resolution and high signal-to-noise ratio. This PFAAC process was proved in vitro by copper catalyzed or strain-promoted azide-alkyne reactions and in live cells by spatiotemporally controlled organelle imaging. By incorporation a linker to the azide-tetrazole conjugate, this PFAAC chemistry could covalently label extra probes to the biomolecules and spatiotemporally detecting this process by photoinduced fluorescence.

Lights on 2,5-diaryl tetrazoles: applications and limits of a versatile photoclick reaction

Recently, photoclick chemistry emerged as a powerful tool employed in several research fields, from medicinal chemistry and biology to material sciences. The growing interest in this type of chemical process is justified by the possibility to produce complex molecular systems using mild reaction conditions. However, the elevated spatio-temporal control offered by photoclick chemistry is highly intriguing, as it expands the range of applications. In this context, the light-triggered reaction of 2,5-diaryl tetrazoles with dipolarophiles emerged for its interesting features: excellent stability of the substrates, fast reaction kinetic, and the formation of a highly fluorescent product, fundamental for sensing applications. In the last years, 2,5-diaryl tetrazoles have been extensively employed, especially for bioorthogonal ligations, to label biomolecules and nucleic acids. In this review, we summarized recent applications of this interesting photoclick reaction, with a particular focus on biological fields. Moreover, we described the main limits that affect this system and current strategies proposed to overcome these issues. The general discussion here presented could prompt further optimization of the process and pave the way for the development of new original structures and innovative applications.

Graphical abstract

Molecular platforms based on biocompatible photoreactions for photomodulation of biological targets

Photoirradiation provides a convenient and biocompatible approach for spatiotemporal modulation of biological systems with photoresponsive components. The construction of molecular platforms with a photoresponse to be integrated into biomolecules for photomodulation has been of great research interest in optochemical biology. In this review, we summarize typical molecular platforms that are integratable with biomolecules for photomodulation purposes. We categorize these molecular platforms according to their excitation light source, namely ultraviolet (UV), visible (Vis) or near-infrared (NIR) light. The protype chemistry of these molecular platforms is introduced along with an overview of their most recent applications for spatiotemporal regulation of biomolecular function in living cells or mice models. Challenges and the outlook are also presented. We hope this review paper will contribute to further progress in the development of molecular platforms and their biomedical use.

Near-infrared light controlled fluorogenic labeling of glycoengineered sialic acids in vivo with upconverting photoclick nanoprobe

Sialic acid serves as an important determinant for profiling cell activities in diverse biological and pathological processes. The precise control of sialic acid labeling to visualize its biological pathways under endogenous conditions is significant but still challenging due to the lack of reliable methods. Herein, we developed an effective strategy to spatiotemporally label thesialic acids with a near-infrared (NIR) light activated upconverting nanoprobe (Tz-UCNP). With this photoclickable nanoprobe and a stable N-alkene-d-mannosamine (Ac4ManNIPFA), metabolically synthesized alkene sialic acids on the cell surface were labeled and imaged in real time through fluorogenic cycloaddition. More importantly, we achieved spatially selective visualization of sialic acids in specific tumor tissues of the mice under NIR light activation in a spatially controlled manner. This in situ controllable labeling strategy thus enables the metabolic labeling of specific sialic acids in complex biological systems.

Photo-controllable bioorthogonal chemistry for spatiotemporal control of bio-targets in living systems

The establishment of bioorthogonal chemistry is one of the most significant advances in chemical biology using exogenous chemistry to perturb and study biological processes. Photo-modulation of biological systems has realized temporal and spatial control on biomacromolecules in living systems. The combination of photo-modulation and bioorthogonal chemistry is therefore emerging as a new direction to develop new chemical biological tools with spatiotemporal resolution. This minireview will focus on recent development of bioorthogonal chemistry subject to spatiotemporal control through photo-irradiation. Different strategies to realize photo-control on bioorthogonal bond-forming reactions and biological applications of photo-controllable bioorthogonal reactions will be summarized to give a perspective on how the innovations on photo-chemistry can contribute to the development of optochemical biology. Future trends to develop more optochemical tools based on novel photochemistry will also be discussed to envision the development of chemistry-oriented optochemical biology.

A photoactivatable microRNA probe for identification of microRNA targets and light-controlled suppression of microRNA target expression

Here, we report a novel dual-functional microRNA (miRNA) probe, PA-miRNA, for miRNA target identification and light control of miRNA target expression. PA-miRNA is a miRNA mimic with a 3'-biotin tag linked via a photo-cleavable linker. Using PA-miR-34a, intracellular targets of miR-34a in HeLa cells were isolated and confirmed. Moreover, PA-miR-34a upon transfection into HeLa cells was inactive until light irradiation to break the photo-cleavable linker to release functional miR-34a. We demonstrated that miR-34a target expression as well as miR-34a-promoted cell apoptosis were regulated by PA-miR-34a in a photo-controllable manner.

A Dual-Signal Twinkling Probe for Fluorescence-SERS Dual Spectrum Imaging and Detection of miRNA in Single Living Cell via Absolute Value Coupling of Reciprocal Signals

Imaging and detecting microRNAs (miRNAs) is of central importance in tumor cell analysis. It stays challenging to establish simple, accurate, and sensitive analytical assays for imaging and detection of miRNA in a single living cell, because of intracellular complex environment and miRNA sequence similarity. Herein, we designed a dual-signal twinkling probe (DSTP) with triplex-stem structure which employed a fluorescence-SERS signal reciprocal switch. The spatiotemporal dynamics of the miRNA molecular and intracellular uptake of the probe are monitored by fluorescence-SERS signal switch of the DSTP. Meanwhile, using the surface-enhanced Raman scattering (SERS) signals of DSTP, the measure of absolute value coupling of reciprocal signals is first used to real-time detection of miRNA. Through simultaneous enhancing the target response signal value and reducing blank value, this work deducted the background effect, and showed high sensitivity and reproducibility. Moreover, the probe shows excellent reversibility and specificity in real quantitative detection of intracellular miRNA. miR-203 was successfully monitored in MCF-7, in accord with the results in vitro as well as in cell lysates. We anticipate that this new dual-signal twinkling and dual-spectrum switch method will be generally useful to image and detect various types of biomolecules in single living cell.

A self-assembled peptide nucleic acid-microRNA nanocomplex for dual modulation of cancer-related microRNAs

Here, we report a new strategy for the construction of a peptide nucleic acid-microRNA nanocomplex with dual function to simultaneously suppress oncogenic microRNAs and upregulate tumor-suppressive microRNAs in target cancer cells.

Fluorogenic "Photoclick" Labeling and Imaging of DNA with Coumarin-Fused Tetrazole in Vivo

Photoclickable fluorogenic probes will enable visualization of specific biomolecules with precise spatiotemporal control in their native environment. However, the fluorogenic tagging of DNA with current photocontrolled clickable probes is still challenging. Herein, we demonstrated the fast (19.5 ± 2.5 M^-1 s^-1) fluorogenic labeling and imaging of DNA in vitro and in vivo with rationally designed coumarin-fused tetrazoles under UV LED photoirradiation. With a water-soluble, nuclear-specific coumarin-fused tetrazole (CTz-SO₃), the metabolically synthesized DNA in cultured cells was effectively labeled and visualized, without fixation, via "photoclick" reaction. Moreover, the photoclickable CTz-SO₃ enabled real-time, spatially controlled imaging of DNA in live zebrafish.

MoS2 quantum dots modified with a labeled molecular beacon as a ratiometric fluorescent gene probe for FRET based detection and imaging of microRNA

A dual-channel ratiometric nanoprobe is described for detection and imaging of microRNA. It was prepared from MoS₂ quantum dots (QDs; with blue emission and excitation/emission peaks at 310/398 nm) which acts as both the gene carrier and as a donor in fluorescence resonance energy transfer (FRET). Molecular beacons containing loops for molecular recognition of microRNA and labeled with carboxyfluorescein (FAM) were covalently attached to the MoS₂ QDs and serve as the FRET acceptor. In the absence of microRNA, the nanoprobe exhibits low FRET efficiency due to the close distance between the FAM tag and the QDs. Hybridization with microRNA enlarges the distance between QD and beacon. This results in an enhancement of the FRET efficiency of the nanoprobe. The ratio of green and blue fluorescence (I₅₂₀/I₃₉₈) increases linearly in the 5 to 150 nM microRNA concentration range in both aqueous solution and diluted artificial cerebrospinal fluid. The limit of detection (LOD) is as low as 0.38 nM and 0.52 nM, respectively. Other features of this nanoprobe include (a) excellent resistance to nuclease-induced false positive signals and (b) the option to use it for distinguishing different cell lines by in-situ imaging of intracellular microRNAs. Graphical abstract Schematic of a dual-channel photoluminescence nanoprobe for the determination of microRNA-21 (miR-21) by monitoring the microRNA-triggered enhancement of the fluorescence resonance energy transfer (FRET) efficiency between MoS₂ QDs and carboxyfluorescein-labeled molecular beacons.