Organelle targetable fluorescent probes for hydrogen peroxide

The review summarized organelle target strategies, structures, fluorescence behavior and biological applications of H₂O₂ fluorescent probes with mitochondria, nuclei and lysosomes-targetable ability.     

Organelle targetable fluorescent probes for hydrogen peroxide

Hydrogen peroxide(H_2 O_2), as important products of oxygen metabolism, plays an important role in many biological processes, such as immune responses and cellular signal transduction. However, abnormal production of H_2 O_2 can damage cellular biomolecules, which was closely associated with many diseases.Thus, it is urgent to monitor the level change of H_2 O_2 in living cells, particularly at subcellular levels.Toward this end, a wide variety of H_2 O_2 fluorescent probes have been designed, developed and applied for imaging of H_2 O_2 in subcellular levels. In this review, we highlight the representative cases of H_2 O_2 fluorescent probes with mitochondria, nuclei and lysosomes-targetable ability. The review contains organelle target strategies, structures, fluorescence behavior and biological applications of these probes.     

金属配合物在双光子荧光探针中的应用研究

金属配合物因其优异的光物理性质,如配位结构可调、好的光稳定性、大的斯托克位移、高的荧光量子产率与长的荧光寿命等,在生物成像、分子探针、医学影像等领域中备受关注.与单光子吸收相比,双光子吸收的金属配合物因其具有更加优秀的深度分辨率以及低光损伤性等优点,近些年被广泛应用于生物分子的荧光探针和细胞器染料等.本文综述了近年来具...     

Near‐IR Photochemistry for Biology: Exploiting the Optical Window of Tissue

Photoactive molecules enable much of modern biology and biochemistry—a vast library of fluorescent chromophores is used to track and label cellular structures and macromolecules. However, photochemistry is better known to the synthetic or physical organic chemist as a “light switch” that turns on unusual excited‐state reactivity, isomerization, or dynamic adjustment of structure. This review details a rapidly growing approach to biophotochemistry that uses low‐energy near‐IR wavelengths not only for imaging, but also for close spatial control over chemical switching events in biosystems. Emphasis is placed on topics of biomedical interest: release of gaseous biological messengers, uncaging of drugs, nano‐therapeutics, and modification of biomaterials.     

The Remarkable Effect of Halogen Substitution on the Membrane Transport of Fluorescent Molecules in Living Cells

Small‐molecule‐based fluorescent probes have become important tools in biology for sensing and imaging applications. However, the biological applications of many of the fluorescent molecules are hampered by low cellular uptake and high toxicity. In this paper, we show for the first time that the introduction of halogen atoms enhances the cellular uptake of fluorescent molecules and the nature of halogen atoms plays a crucial role in the plasma membrane transport in mammalian cells. The remarkably higher uptake of iodinated compounds compared to that of their chloro or bromo analogues suggests that the strong halogen bonding ability of iodine atoms may play an important role in the membrane transport. This study provides a novel strategy for the transport of fluorescent molecules across the plasma membrane in living cells.     

Fluorescent Probes for Live Cell Thiol Detection

Thiols play vital and irreplaceable roles in the biological system. Abnormality of thiol levels has been linked with various diseases and biological disorders. Thiols are known to distribute unevenly and change dynamically in the biological system. Methods that can determine thiols’ concentration and distribution in live cells are in high demand. In the last two decades, fluorescent probes have emerged as a powerful tool for achieving that goal for the simplicity, high sensitivity, and capability of visualizing the analytes in live cells in a non-invasive way. They also enable the determination of intracellular distribution and dynamitic movement of thiols in the intact native environments. This review focuses on some of the major strategies/mechanisms being used for detecting GSH, Cys/Hcy, and other thiols in live cells via fluorescent probes, and how they are applied at the cellular and subcellular levels. The sensing mechanisms (for GSH and Cys/Hcy) and bio-applications of the probes are illustrated followed by a summary of probes for selectively detecting cellular and subcellular thiols.     

Imaging methods for elemental, chemical, molecular, and morphological analyses of single cells

Combining elemental, chemical, molecular, and morphological imaging information from individual cells with a lateral resolution well below 1?×?1 μm² is the current technological challenge for investigating the smallest dimensions of living systems. In the race for such analytical performance, several techniques have been successfully developed; some use probes to determine given cellular contents whereas others use possible interactions between cellular matter with light or elements for characterization of contents. Morphological techniques providing information about cell dimensions have, when combined with other techniques, also opened the way to quantitative studies. New analytical opportunities are now being considered in cell biology, combining top-performance imaging techniques, applied to the same biosystem, with microscopy (nm–μm range) techniques providing elemental (micro-X-ray fluorescence, particle-induced X-ray emission, secondary-ion mass spectrometry), chemical (Raman, coherent anti-stokes Raman, Fourier-transform infrared, and near-field), molecular (UV–visible confocal and multiphoton), and morphological (AFM, ellipsometry, X-ray phase contrast, digital holography) information. Dedicated cell-culture methods have been proposed for multimodal imaging in vitro and/or ex vivo. This review shows that in addition to UV–fluorescent techniques, the imaging modalities able to provide interesting information about a cell, with high spatial and time resolution, have grown sufficiently to envisage quantitative analysis of chemical species inside subcellular compartments.     

Near-infrared fluorescent probes: a next-generation tool for protein-labeling applications

The development of near-infrared (NIR) fluorescent probes over the past few decades has changed the way that biomolecules are imaged, and thus represents one of the most rapidly progressing areas of research. Presently, NIR fluorescent probes are routinely used to visualize and understand intracellular activities. The ability to penetrate tissues deeply, reduced photodamage to living organisms, and a high signal-to-noise ratio characterize NIR fluorescent probes as efficient next-generation tools for elucidating various biological events. The coupling of self-labeling protein tags with synthetic fluorescent probes is one of the most promising research areas in chemical biology. Indeed, at present, protein-labeling techniques are not only used to monitor the dynamics and localization of proteins but also play a more diverse role in imaging applications. For instance, one of the dominant technologies employed in the visualization of protein activity and regulation is based on protein tags and their associated NIR fluorescent probes. In this mini-review, we will discuss the development of several NIR fluorescent probes used for various protein-tag systems.

This minireview describes the development of NIR chemical probes for various protein-tag systems.     

Coumarin‐based Fluorescent Probes for Selectively Targeting and Imaging the Endoplasmic Reticulum in Mammalian Cells

Developing improved fluorescent probes for imaging the endoplasmic reticulum (ER) is necessary for structure‐activity studies of this dynamic organelle. Two coumarin‐based compounds with sulfonamide side groups were synthesized and characterized as ER‐targeting probes. Their selectivity to target the ER in HeLa and GM07373 mammalian cells was shown with co‐localization experiments using commercially available probes that localize in the ER, mitochondria, or lysozymes. The hydrophobicity of the coumarin‐based probes was comparable to known probes that partition into the ER membrane. Their cytotoxicity in mammalian cells was low with IC50 values that range from 205 to 252 μm . The fluorescent quantum yields of the coumarin‐based probes when excited with 400 nm light were 0.60, and they have a much narrower emission spectrum (from 435 to 525 nm in methanol) than that of the only commercially available ER probe that is exited with 400 nm light (ER‐Tracker? Blue‐White DPX). Thus, the coumarin‐based probes are more useful for multicolor imaging with yellow and red emitting fluorophores. In addition to the above benefits, ER labeling was achieved with the coumarin‐based probes in both live cells and fixed cells, revealing their versatility for a wide range of cellular imaging applications.     

Subcellular localised small molecule fluorescent probes to image mobile Zn2+

Zn²⁺, as the second most abundant d-block metal in the human body, plays an important role in a wide range of biological processes, and the dysfunction of its homeostasis is related to many diseases, including Type 2 diabetes, Alzheimer''s disease and prostate and breast cancers. Small molecule fluorescent probes, as effective tools for real-time imaging, have been widely used to study Zn²⁺ related processes. However, the failure to control their localisation in cells has limited their utility somewhat, as they are generally incapable of studying individual processes in a specific cellular location. This perspective presents an overview of the recent developments in specific organelle localised small molecule fluorescent Zn²⁺ probes and their application in biological milieu, which could help to extend our understanding of the mechanisms that cells use to respond to dysfunction of zinc homeostasis and its roles in disease initiation and development.

A number of recently developed subcellular localised small molecule fluorescent probes to image mobile Zn²⁺ are reviewed in this perspective.     

“Off–on” red-emitting fluorescent probes with large Stokes shifts for nitric oxide imaging in living cells

Fluorescent probes with larger Stokes shifts in the far-visible and near-infrared spectral region (600–900 nm) are more superior for cellular imaging and biological analysis due to avoiding light scattering interference, reducing autofluorescence from biological sample and encouraging deeper tissue penetration in vivo imaging. In this work, two bis-methoxyphenyl-BODIPY fluorescent probes for the detection of nitric oxide (NO) have been firstly synthesized. Under physiological conditions, these probes can react with NO to form the corresponding triazoles with 250- and 70-fold turn-on fluorescence emitting at 590 and 620 nm, respectively. Moreover, the triazole forms of these probes have large Stokes shifts of 38 nm, in contrast to 10 nm of existing BODIPY probes for NO. Excellent selectivity has been observed against other reactive oxygen/nitrogen species, ascorbic acid and biological matrix. After the evaluation of MTT assay, new fluorescent probes have been successfully applied to fluorescence imaging of NO released from RAW 264.7 macrophages by co-stimulation of lipopolysaccharide and interferon-γ. The experimental results indicate that our fluorescent probes can be powerful candidates for fluorescence imaging of NO due to the low background interference and high detection sensitivity.     

In Cellulo Generation of Fluorescent Probes for Live-Cell Imaging of Cylooxygenase-2

Live-cell imaging with fluorescent probes is an essential tool in chemical biology to visualize the dynamics of biological processes in real-time. Intracellular disease biomarker imaging remains a formidable challenge due to the intrinsic limitations of conventional fluorescent probes and the complex nature of cells. This work reports the in cellulo assembly of a fluorescent probe to image cyclooxygenase-2 (COX-2). We developed celecoxib-azide derivative 14 , possessing favorable biophysical properties and excellent COX-2 selectivity profile. In cellulo strain-promoted fluorogenic click chemistry of COX-2-engaged compound 14 with non/weakly-fluorescent compounds 11 and 17 formed fluorescent probes 15 and 18 for the detection of COX-2 in living cells. Competitive binding studies, biophysical, and comprehensive computational analyses were used to describe protein-ligand interactions. The reported new chemical toolbox enables precise visualization and tracking of COX-2 in live cells with superior sensitivity in the visible range.     

Diversity‐Oriented Approach for Chemical Biology

Synthetic molecules that modulate and probe biological events are critical tools in chemical biology. Utilizing combinatorial and diversity‐oriented synthetic strategies, access to large numbers of small molecules is becoming more and more feasible, and research groups in this field can take advantage of the power of chemical diversity. Since the majority of early studies were focused on the discovery of compounds that perturb protein functions, diversity‐based approaches are often considered as therapeutic lead discovery tactics. However, the diversity‐oriented approach can also be applied to advance distinct aims, such as target protein identification, or the development of imaging probes and sensors. This review provides a personal perspective of the chemical‐diversity‐based approach and how this principle can be adapted to various chemical biology studies.     

“Stainomics”: Identification of mitotracker labeled proteins in mammalian cells

Organelle‐specific cell‐permeable fluorescent dyes are invaluable tools in cell biology as they reveal intracellular dynamics in living cells. Mitrotracker is a family of dyes that strongly label the mitochondrion, a key organelle associated with many crucial cellular functions. Despite the popularity of these dyes, little is known about the molecular mechanism behind their staining specificity. Here, we aimed to identify the protein targets of one member of this dye family, mitotracker red (MTR), by 2DE and MS. MTR bound to cellular proteins covalently, and its fluorescence persisted even after cell lysis, protein solubilization, denaturation, and electrophoresis. This enabled us to display MTR‐labeled proteins by 2DE. The MTR‐specific fluorescent signals on the gel revealed the spots that contained MTR‐conjugated proteins. These spots were analyzed by MS, resulting into the identification of ten proteins. We discovered that one major target is the mitochondrial protein HSP60 and that MTR staining could induce production of HSP60, predisposing cells to heat shock‐like responses. The identification of the molecular targets of biological dyes, or “stainomics,” can help correlate their intracellular staining properties with biochemical affinities. We believe this approach can be applied to a wide range of fluorescent probes.     

Photoswitchable nanoparticles enable high-resolution cell imaging: PULSAR microscopy

Beyond-diffraction-limit optical imaging of cells will reveal biological mechanisms, cellular structures, and physiological processes in nanometer scale. Harnessing the photoswitching properties of spiropyran fluorophores, we achieved nanoresolution fluorescence imaging using photoactuated unimolecular logical switching attained reconstruction (PULSAR) microscopy. The PULSAR microscope successfully resolved nanostructures and subcellular organelles when the photoswitchable nanoparticles containing spiropyran dyes were used as fluorescent probes.     

Functional Near‐Infrared Fluorescent Probes

Near‐infrared (NIR) fluorescent probes have attracted much attention, but despite the availability of various NIR fluorophores, only a few functional NIR probes, that is, probes whose absorption and/or fluorescence spectra change upon specific reaction with biomolecules, have been developed. However, functional probes operating in the NIR range that can be targeted to protons, metal ions, nitric oxide, β‐galactosidase, and cellular stress markers are expected to be effective for fluorescence imaging in vivo. This Focus Review concentrates on these functional NIR probes themselves, not their applications.     

A Theoretical Investigation of Two Typical Two‐Photon pH Fluorescent Probes

Intracellular pH plays an important role in many cellular events, such as cell growth, endocytosis, cell adhesion and so on. Some pH fluorescent probes have been reported, but most of them are one‐photon fluorescent probes, studies about two‐photon fluorescent probes are very rare. In this work, the geometrical structure, electronic structure and one‐photon properties of a series of two‐photon pH fluorescent probes have been theoretically studied by using density functional theory (DFT) method. Their two‐photon absorption (TPA) properties are calculated using the method of ZINDO/sum‐over‐states method. Two types of two‐photon pH fluorescent probes have been investigated by theoretical methods. The mechanisms of the Photoinduced Charge Transfer (PCT) probes and the Photoinduced Electron Transfer (PET) probes are verified specifically. Some designed strategies of good two‐photon pH fluorescent probes are suggested on the basis of the investigated results of two mechanisms. For the PCT probes, substituting a stronger electron‐donating group for the terminal methoxyl group is an advisable choice to increase the TPA cross section. For the PET probes, the TPA cross sections increase upon protonation.     

Recent Progress of Small-Molecule Ratiometric Fluorescent Probes for Peroxynitrite in Biological Systems

Peroxynitrite (ONOO⁻) as a major reactive oxygen species plays important roles in cellular signal transduction and homeostatic regulation. Precise detection of ONOO⁻ in biological systems is vital for exploring its physiological and pathological function. Among numerous detection methods, fluorescence imaging technology using fluorescent probes offers some advantages, including simple operation, high sensitivity and selectivity, as well as real-time and nondestructive detection. In particular, ratiometric fluorescent probes, in which the built-in calibration of the two emission bands prevents interference from the biological environment, have been extensively employed to monitor the fluctuation of bioactive species. In this review, we will discuss small-molecule ratiometric fluorescent probes for ONOO⁻ in live cells or in vivo, which involves chemical structures, response mechanisms, and biological applications. Moreover, the challenges and future prospects of ONOO⁻-responsive ratiometric fluorescent probe are also proposed.     

A new selective ‘turn-on’ small fluorescent cationic probe for recognition of RNA in cells

Fluorescent imaging probes have revolutionised cell biology by monitoring cellular objects. However, the lack of fluorescent probes with high selectivity for RNA has been a drawback. Thus, selective RNA binding for fluorescent sensors is essential. Here, we report the selective fluorescence enhancement upon addition of RNA. By exploiting a selective recognition of small tetra-cationic probe 1 for RNA, we also explain the possible binding mode for RNA. As a membrane-permeant fluorescence probe, 1 provides selective imaging of RNA not only in human neuroblastoma tumour SH-SY5Y cell line used for Parkinson's disease but also in the unicellular green alga cells. Further exploitation could open new opportunities in neurotoxin and cancer biology.     

AIEgens/Nucleic Acid Nanostructures for Bioanalytical Applications

DNA occupies significant roles in life processes, which include encoding the sequences of proteins and accurately transferring genetic information from generation to generation. Recent discoveries have demonstrated that a variety of biological functions are correlated with DNA′s conformational transitions. The non‐B form has attained great attention among the diverse forms of DNA over the past several years. The main reason for this is that a large number of studies have shown that the non‐B form of DNA is associated with gross deletions, inversions, duplications, translocations as well as simple repeating sequences, which therefore causes human diseases. Consequently, the conformational transition of DNA between the B‐form and the non‐B form is important for biology. Conventional fluorescence probes based on the conformational transitions of DNA usually need a fluorophore and a quencher group, which suffers from the complex design of the structure and tedious synthetic procedures. Moreover, conventional fluorescence probes are subject to the aggregation‐caused quenching (ACQ) effect, which limits their application toward imaging and analyte detection. Fluorogens exhibiting aggregation‐induced emission (AIE) have attracted tremendous attention over the past decade. By taking advantage of this unique behavior, plenty of fluorescent switch‐on probes without the incorporation of fluorescent quenchers/fluorophore pairs have been widely developed as biosensors for imaging a variety of analytes. Herein, the recent progress in bioanalytical applications on the basis of aggregation‐induced emission luminogens (AIEgens)/nucleic acid nanostructures are presented and discussed.