Protein labeling with fluorogenic probes for no-wash live-cell imaging of proteins

Nitroreductase-Triggered Fluorophore Labeling of Cells and Tissues under Hypoxia

Several reductases, including nitroreductase, are upregulated under hypoxic conditions characterized by an oxygen-deficient microenvironment. Given that hypoxia is a prominent feature of solid tumors, our investigation focused on developing a bioconjugative probe designed for staining tissue under hypoxic conditions, particularly activated by nitroreductase. This probe, developed using our trigger-release-bioconjugation system rooted in the ortho-quinone methide chemistry, exhibited selective activation by nitroreductase and fluorophore labeling within mitochondria and endoplasmic reticulum. As a result, it displayed sustained fluorescence that persisted even after washing steps in cells and tissues. We applied this innovative probe to stain mouse kidney tissue in an acute kidney injury model induced by inadequate oxygen supply. Among various organ tissues examined, only kidney tissue showed significantly higher fluorescence in the injury model compared with the control tissue, as revealed by two-photon microscopic imaging. This research presents a promising avenue for the development of practical staining agents for image-guided tumor surgery.

Bioisostere-conjugated fluorescent probes for live-cell protein imaging without non-specific organelle accumulation

Specific labeling of proteins using membrane-permeable fluorescent probes is a powerful technique for bioimaging. Cationic fluorescent dyes with high fluorescence quantum yield, photostability, and water solubility provide highly useful scaffolds for protein-labeling probes. However, cationic probes generally show undesired accumulation in organelles, which causes a false-positive signal in localization analysis. Herein, we report a design strategy for probes that suppress undesired organelle accumulation using a bioisostere for intracellular protein imaging in living cells. Our design allows the protein labeling probes to possess both membrane permeability and suppress non-specific accumulation and has been shown to use several protein labeling systems, such as PYP-tag and Halo tag systems. We further developed a fluorogenic PYP-tag labeling probe for intracellular proteins and used it to visualize multiple localizations of target proteins in the intracellular system. Our strategy offers a versatile design for undesired accumulation-suppressed probes with cationic dye scaffolds and provides a valuable tool for intracellular protein imaging.

A genetically encoded fluorescent protein sensor for mitochondrial membrane damage detection

Mitochondria are essential cellular organelles; detecting mitochondrial damage is crucial in cellular biology and toxicology. Compared with existing chemical probe detection methods, genetically encoded fluorescent protein sensors can directly indicate cellular and molecular events without involving exogenous reagents. In this study, we introduced a molecular sensor system, MMD-Sensor, for monitoring mitochondrial membrane damage. The sensor consists of two molecular modules. Module I is a fusion structure of the mitochondrial localization sequence (MLS), AIF cleavage site sequence (CSS), nuclear localization sequence (NLS), N-terminus of mNeonGreen and mCherry. Module II is a fusion structure of the C-terminus of mNeonGreen, NLS sequence, and mtagBFP2. Under normal condition, Module I is constrained in the inner mitochondrial membrane anchored by MLS, while Module II is restricted to the nucleus by its NLS fusion component. If the mitochondrial membrane is damaged, CSS is cut from the inner membrane, causing Module I to shift into the nucleus guided by the NLS fusion component. After Module I enters the nucleus, the N- and C-terminus of mNeonGreen meet each other and rebuild its intact 3D structure through fragment complementation and thus generates green fluorescence in the nucleus. Dynamic migration of red fluorescence from mitochondria to the nucleus and generation of green fluorescence in the nucleus indicate mitochondrial membrane damage. Using the MMD-Sensor, mitochondrial membrane damage induced by various reagents, such as uncoupling agents, ATP synthase inhibitors, monovalent cationic carriers, and ROS, in HeLa and 293T cells are directly observed and evaluated.

Bioorthogonal Reactions in Bioimaging

Visualization of biomolecules in their native environment or imaging-aided understanding of more complex biomolecular processes are one of the focus areas of chemical biology research, which requires selective, often site-specific labeling of targets. This challenging task is effectively addressed by bioorthogonal chemistry tools in combination with advanced synthetic biology methods. Today, the smart combination of the elements of the bioorthogonal toolbox allows selective installation of multiple markers to selected targets, enabling multicolor or multimodal imaging of biomolecules. Furthermore, recent developments in bioorthogonally applicable probe design that meet the growing demands of superresolution microscopy enable more complex questions to be addressed. These novel, advanced probes enable highly sensitive, low-background, single- or multiphoton imaging of biological species and events in live organisms at resolutions comparable to the size of the biomolecule of interest. Herein, the latest developments in bioorthogonal fluorescent probe design and labeling schemes will be discussed in the context of in cellulo/in vivo (multicolor and/or superresolved) imaging schemes. The second part focuses on the importance of genetically engineered minimal bioorthogonal tags, with a particular interest in site-specific protein tagging applications to answer biological questions.

Ratiometric fluorescent probes for pH mapping in cellular organelles

The intracellular pH (pHi) in organelles, including mitochondria, endoplasmic reticulum, lysosomes, and nuclei, differs from the cytoplasmic pH, and thus maintaining the pH of these organelles is crucial for cellular homeostasis. Alterations in the intracellular pH (ΔpHi) in organelles lead to the disruption of cell proliferation, ion transportation, cellular homeostasis, and even cell death. Hence, accurately mapping the pH of organelles is crucial. Accordingly, the development of fluorescence imaging probes for targeting specific organelles and monitoring their dynamics at the molecular level has become the forefront of research in the last three decades. Among them, ratiometric fluorescent probes minimize the interference from the excitation wavelength of light, auto-fluorescence from probe concentration, environmental fluctuations, and instrument sensitivity through self-correction compared to monochromatic fluorescent probes, which are known as turn-on/off fluorescent probes. Small-molecular ratiometric fluorescent probes for detecting ΔpHi are challenging yet demanding. To date, sixty-two ratiometric pH probes have been reported for monitoring internal pH alterations in cellular organelles. However, a critical review on organelle-specific ratiometric probes for pH mapping is still lacking. Thus, in the present review, we report the most recent advances in ratiometric pH probes and the previous data on the role of mapping the ΔpHi of cellular organelles. The development strategy, including ratiometric fluorescence with one reference signal (RFRS) and ratiometric fluorescence with two reversible signals (RFRvS), is systematically illustrated. Finally, we emphasize the major challenges in developing ratiometric probes that merit further research in the future.

An “OFF–ON–OFF” fluorescence protein-labeling probe for real-time visualization of the degradation of short-lived proteins in cellular systems

The ability to monitor proteolytic pathways that remove unwanted and damaged proteins from cells is essential for understanding the multiple processes used to maintain cellular homeostasis. In this study, we have developed a new protein-labeling probe that employs an ‘OFF–ON–OFF’ fluorescence switch to enable real-time imaging of the expression (fluorescence ON) and degradation (fluorescence OFF) of PYP-tagged protein constructs in living cells. Fluorescence switching is modulated by intramolecular contact quenching interactions in the unbound probe (fluorescence OFF) being disrupted upon binding to the PYP-tag protein, which turns fluorescence ON. Quenching is then restored when the PYP-tag–probe complex undergoes proteolytic degradation, which results in fluorescence being turned OFF. Optimization of probe structures and PYP-tag mutants has enabled this fast reacting ‘OFF–ON–OFF’ probe to be used to fluorescently image the expression and degradation of short-lived proteins.

An “OFF–ON–OFF” fluorescence probe for real-time imaging of the expression (fluorescence ‘OFF’) and degradation (fluorescence ‘ON’) of short lived PYP-tag proteins in cellular systems.

Fluorescent Benzothiadiazole Derivatives as Fluorescence Imaging Dyes: A Decade of New Generation Probes

The current review describes advances in the use of fluorescent 2,1,3-benzothiadiazole (BTD) derivatives after nearly one decade since the first description of bioimaging experiments using this class of fluorogenic dyes. The review describes the use of BTD-containing fluorophores applied as, inter alia, bioprobes for imaging cell nuclei, mitochondria, lipid droplets, sensors, markers for proteins and related events, biological processes and activities, lysosomes, plasma membranes, multicellular models, and animals. A number of physicochemical and photophysical properties commonly observed for BTD fluorogenic structures are also described.

Deep-Red/Near-Infrared Turn-On Fluorescence Probes for Aldehyde Dehydrogenase 1A1 in Cancer Stem Cells

Accumulating evidence supports that cancer stem cells (CSCs) are responsible for cancer proliferation, metastasis, and therapy resistance; therefore, an effective strategy to identify and isolate CSCs is required urgently. Because of their low invasiveness and high signal/noise ratio, "turn-on" fluorescence probes working in the deep-red/near-infrared (DR/NIR) region are one of the most attractive yet undeveloped tools for CSC detection. Herein, we report DR/NIR turn-on fluorescence probes, CS5-A and CS7-A, targeted to aldehyde dehydrogenase 1A1 as an intracellular CSC marker. In contrast to the conventional "always-on" green-fluorescent ALDEFLUOR, we succeeded in generating high-contrast (signal/noise ratio > 8.3) and wash-free in vitro CSC imaging with the DR probe C5S-A. This probe can facilitate CSC isolation with minimal contamination by autofluorescence from other tissues through fluorescence-activated cell sorting. Furthermore, the NIR absorbance/emission and turn-on properties of C7S-A allow simple and rapid CSC detection in vivo within 15 min.

Design of Large Stokes Shift Fluorescent Proteins Based on Excited State Proton Transfer of an Engineered Photobase

The incredible potential for fluorescent proteins to revolutionize biology has inspired the development of a variety of design strategies to address an equally broad range of photophysical characteristics, depending on potential applications. Of these, fluorescent proteins that simultaneously exhibit high quantum yield, red-shifted emission, and wide separation between excitation and emission wavelengths (Large Stokes Shift, LSS) are rare. The pursuit of LSS systems has led to the formation of a complex, obtained from the marriage of a rationally engineered protein (human cellular retinol binding protein II, hCRBPII) and different fluorogenic molecules, capable of supporting photobase activity. The large increase in basicity upon photoexcitation leads to protonation of the fluorophore in the excited state, dramatically red-shifting its emission, leading to an LSS protein/fluorophore complex. Essential for selective photobase activity is the intimate involvement of the target protein structure and sequence that enables Excited State Proton Transfer (ESPT). The potential power and usefulness of the strategy was demonstrated in live cell imaging of human cell lines.

Challenges facing quantitative large-scale optical super-resolution, and some simple solutions

Optical super-resolution microscopy (SRM) has enabled biologists to visualize cellular structures with near-molecular resolution, giving unprecedented access to details about the amounts, sizes, and spatial distributions of macromolecules in the cell. Precisely quantifying these molecular details requires large datasets of high-quality, reproducible SRM images. In this review, we discuss the unique set of challenges facing quantitative SRM, giving particular attention to the shortcomings of conventional specimen preparation techniques and the necessity for optimal labeling of molecular targets. We further discuss the obstacles to scaling SRM methods, such as lengthy image acquisition and complex SRM data analysis. For each of these challenges, we review the recent advances in the field that circumvent these pitfalls and provide practical advice to biologists for optimizing SRM experiments.

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.

Analysis of sparse molecular distributions in fibrous arrangements based on the distance to the first neighbor in single molecule localization microscopy

Single Molecule Localization Microscopy (SMLM) currently attains a lateral resolution of around 10 nm approaching molecular size. Together with increasingly specific fluorescent labeling, it opens the possibility to quantitatively analyze molecular organization. When the labeling density is high enough, SMLM provides clear images of the molecular organization. However, either due to limited labeling efficiency or due to intrinsically low molecular abundance, SMLM delivers a small set of sparse and highly precise localizations. In this work, we introduce a correlation analysis of molecular locations based on the functional dependence of the complementary cumulative distribution function (CCDF) of the distance to the first neighbor (r₁). We demonstrate that the log(-log(CCDF(r₁))) vs. log(r₁) is characterized by a scaling exponent n that takes extreme values of 2 for a random 2D distribution and 1 for a strictly linear arrangement, and find that n is a robust and sensitive metric to distinguish characteristics of the underlying structure responsible for the molecular distribution, even at a very low labeling density. The method enables the detection of fibrillary organization and the estimation of the diameter of host fibers under conditions where a visual inspection provides no clue.

Engineering of a Red Fluorogenic Protein/Merocyanine Complex for Live-Cell Imaging

A reengineered human cellular retinol binding protein II (hCRBPII), a 15-kDa protein belonging to the intracellular lipid binding protein (iLBP) family, generates a highly fluorescent red pigment through the covalent linkage of a merocyanine aldehyde to an active site lysine residue. The complex exhibits "turn-on" fluorescence, due to a weakly fluorescent aldehyde that "lights up" with subsequent formation of a strongly fluorescent merocyanine dye within the binding pocket of the protein. Cellular penetration of merocyanine is rapid, and fluorophore maturation is nearly instantaneous. The hCRBPII/merocyanine complex displays high quantum yield, low cytotoxicity, specificity in labeling organelles, and compatibility in both cancer cell lines and yeast cells. The hCRBPII/merocyanine tag is brighter than most common red fluorescent proteins.

Modulation of emission and singlet oxygen photosensitisation in live cells utilising bioorthogonal phosphorogenic probes and protein tag technology

We developed a strategy to exploit the bioorthogonal reactivity and phosphorogenic property of iridium(iii) polypyridine nitrone complexes and SNAP-tag protein for the modulation of emission and single oxygen photosensitisation in live cells.

A general strategy to develop cell permeable and fluorogenic probes for multicolour nanoscopy

Live-cell fluorescence nanoscopy is a powerful tool to study cellular biology on a molecular scale, yet its use is held back by the paucity of suitable fluorescent probes. Fluorescent probes based on regular fluorophores usually suffer from a low cell permeability and an unspecific background signal. Here we report a general strategy to transform regular fluorophores into fluorogenic probes with an excellent cell permeability and a low unspecific background signal. Conversion of a carboxyl group found in rhodamines and related fluorophores into an electron-deficient amide does not affect the spectroscopic properties of the fluorophore, but allows us to rationally tune the dynamic equilibrium between two different forms: a fluorescent zwitterion and a non-fluorescent, cell-permeable spirolactam. Furthermore, the equilibrium generally shifts towards the fluorescent form when the probe binds to its cellular targets. The resulting increase in fluorescence can be up to 1,000-fold. Using this simple design principle, we created fluorogenic probes in various colours for different cellular targets for wash-free, multicolour, live-cell nanoscopy.

A SNAP-tag fluorogenic probe mimicking the chromophore of the red fluorescent protein Kaede

Self-labelling protein tags with fluorogenic probes serve as great fluorescence imaging tools to understand key questions of protein dynamics and functions in living cells. In the present study, we report a SNAP-tag fluorogenic probe 4c mimicking the chromophore of the red fluorescent protein Kaede. The molecular rotor properties of 4c were utilized as a fluorogenic probe for SNAP-tag, such that conjugation with SNAPf protein leads to inhibition of twisted intramolecular charge transfer, triggering fluorogenecity. Upon conjugation with SNAPf, 4c exhibited approximately a 90-fold enhancement in fluorescence intensity with fast labelling kinetics (k2 = 15 000 M-1 s-1). Biochemical and spectroscopic studies confirmed that fluorescence requires formation of folded SNAPf·4c covalent conjugate between Cys 145 and 4c. Confocal microscopy and flow cytometry showed that 4c is capable of detecting SNAPf proteins or SNAPf fused with a protein of interest in living cells. This work provides a framework to develop the large family of FP chromophores into fluorogenic probes for self-labelling protein tags.

Live-cell imaging and profiling of c-Jun N-terminal kinases using covalent inhibitor-derived probes

c-Jun N-terminal kinases (JNKs) are involved in critical cellular functions. Herein, small-molecule JNK-targeting probes are reported based on a covalent inhibitor. Together with newly developed two-photon fluorescence Turn-ON reporters and chemoproteomic studies, we showed that some probes may be suitable for live-cell imaging and profiling of JNKs.

Development of Acrylamide-Based Rapid and Multicolor Fluorogenic Probes for High Signal-to-Noise Live Cell Imaging

Protein covalent labeling is dramatically useful for studying protein function in living cells and organisms. In this field, the chemical tag technique combined with fluorogenic probes has emerged as a powerful tool. Herein, a series of TMP tag fluorogenic probes have been developed to span the green to full blue spectral range. These probes feature an acrylamide unit that acts as a linker group to conjugate the fluorophore and the ligand as well as a quencher and a covalent reaction group. After the probes bind to eDHFR:L28C, the acrylamide unit specifically reacts with the thiol group of the L28C residue beside the ligand binding pocket, achieving protein-specific labeling without any liberation of leaving groups. With these probes, multicolor and specific protein labeling with a fast reaction rate ( t_1/2 = 33 s) and dramatic fluorescence enhancement (4000-fold) were obtained. Furthermore, no-wash protein labeling in both living cells and zebrafish was successfully achieved. We expect it may provide a general and highly effective chemical tool for the study of protein function in living cells and organisms.

Fluorogenic probes for super-resolution microscopy

Fluorogenic probes efficiently reduce non-specific background signals, which often results in highly improved signal-to-noise ratios.

Live-Cell Imaging of DNA Methylation Based on Synthetic-Molecule/Protein Hybrid Probe

The epigenetic modification of DNA involves the conversion of cytosine to 5-methylcytosine, also known as DNA methylation. DNA methylation is important in modulating gene expression and thus, regulating genome and cellular functions. Recent studies have shown that aberrations in DNA methylation are associated with various epigenetic disorders or diseases including cancer. This stimulates great interest in the development of methods that can detect and visualize DNA methylation. For instance, fluorescent proteins (FPs) in conjugation with methyl-CpG-binding domain (MBD) have been employed for live-cell imaging of DNA methylation. However, the FP-based approach showed fluorescence signals for both the DNA-bound and -unbound states and thus differentiation between these states is difficult. Synthetic-molecule/protein hybrid probes can provide an alternative to overcome this restriction. In this article, we discuss the synthetic-molecule/protein hybrid probe that we developed recently for live-cell imaging of DNA methylation, which exhibited fluorescence enhancement only after binding to methylated DNA.

Elucidation of the local dynamics of domain-III of human serum albumin over the ps-μs time regime using a new fluorescent label

The ps-μs dynamics of domain-III of human serum albumin (HSA) has been investigated using a new fluorescent marker selectively labeled to the Tyr-411 residue. The location of the marker has been confirmed using Förster resonance energy transfer (FRET) study. Steady state, time-resolved and single molecular level fluorescence techniques have been employed to understand the dynamics within the domain-III of HSA. It is found that solvent reorganization dynamics in domain-III is 1.7 times faster than that in domain-I. The timescale of the local rotational dynamics of domain-III is found to be 2.3 times faster than that of domain-I. Fluorescence correlation spectroscopic experiments reveal that domain-III of HSA has more conformational flexibility than domain-I. Together, the results deliver useful details of the local environment around the domain-III of HSA, which have not been explored earlier, mainly because of a lack of a suitable fluorescent marker for domain-III. The newly synthesized probe serves well as a site specific fluorescent marker for HSA, and can be used for further investigation of the ligand binding properties and enzymatic activity of domain-III of HSA.

A fluorescence nanoscopy marker for corticotropin-releasing hormone type 1 receptor: computer design, synthesis, signaling effects, super-resolved fluorescence imaging, and in situ affinity constant in cells

A new fluorescent marker for CRHR1 shows an antagonist effect and suitability for super resolution fluorescence microscopy.

Illuminating Messengers: An Update and Outlook on RNA Visualization in Bacteria

To be able to visualize the abundance and spatiotemporal features of RNAs in bacterial cells would permit obtaining a pivotal understanding of many mechanisms underlying bacterial cell biology. The first methods that allowed observing single mRNA molecules in individual cells were introduced by Bertrand et al. (1998) and Femino et al. (1998). Since then, a plethora of techniques to image RNA molecules with the aid of fluorescence microscopy has emerged. Many of these approaches are useful for the large eukaryotic cells but their adaptation to study RNA, specifically mRNA molecules, in bacterial cells progressed relatively slow. Here, an overview will be given of fluorescent techniques that can be used to reveal specific RNA molecules inside fixed and living single bacterial cells. It includes a critical evaluation of their caveats as well as potential solutions.

Enabling the Triplet of Tetraphenylethene to Sensitize the Excited State of Europium(III) for Protein Detection and Time-Resolved Luminescence Imaging

A tetraphenylethene (TPE) group that exhibits aggregation-induced emission is incorporated into the ligand of a Eu(III) complex (TPEEu) to sensitize the excited state of Eu(III). In steady-state measurements, TPEEu exhibits weak luminescence when dissolved in aqueous solutions even at a high concentration level, but emits strong fluorescence of TPE and phosphorescence of Eu(III) upon binding with bovine serum albumin. With a delay time of 0.05 ms and a gate time of 1.0 ms in time-resolved measurements, only phosphorescent emission of Eu(III) is observed with a high on/off ratio. Moreover, this probe is successfully used in time-resolved luminescence imaging to eliminate the background signal from biological autofluorescence without a washing process. This work provides a general strategy in designing Ln(III) complexes for detecting a broad range of biological molecules.

Fluorogenic probes reveal a role of GLUT4 N-glycosylation in intracellular trafficking

Glucose transporter 4 (GLUT4) is an N-glycosylated protein that maintains glucose homeostasis by regulating the protein translocation. To date, it has been unclear whether the N-glycan of GLUT4 contributes to its intracellular trafficking. Here, to clarify the role of the N-glycan, we developed fluorogenic probes that label cytoplasmic and plasma-membrane proteins for multicolor imaging of GLUT4 translocation. One of the probes, which is cell impermeant, selectively detected exocytosed GLUT4. Using this probe, we verified the 'log' of the trafficking, in which N-glycan-deficient GLUT4 was transiently translocated to the cell membrane upon insulin stimulation and was rapidly internalized without retention on the cell membrane. The results strongly suggest that the N-glycan functions in the retention of GLUT4 on the cell membrane. This study showed the utility of the fluorogenic probes and indicated that this imaging tool will be applicable for research on various membrane proteins that show dynamic changes in localization.

Fluorescent Rhodamines and Fluorogenic Carbopyronines for Super-Resolution STED Microscopy in Living Cells

A range of bright and photostable rhodamines and carbopyronines with absorption maxima in the range of λ=500–630 nm were prepared, and enabled the specific labeling of cytoskeletal filaments using HaloTag technology followed by staining with 1 μm solutions of the dye–ligand conjugates. The synthesis, photophysical parameters, fluorogenic behavior, and structure–property relationships of the new dyes are discussed. Light microscopy with stimulated emission depletion (STED) provided one‐ and two‐color images of living cells with an optical resolution of 40–60 nm.

Bichromophoric dyes for wavelength shifting of dye-protein fluoromodules

Dye-protein fluoromodules consist of fluorogenic dyes and single chain antibody fragments that form brightly fluorescent noncovalent complexes. This report describes two new bichromophoric dyes that extend the range of wavelengths of excitation or emission of existing fluoromodules. In one case, a fluorogenic thiazole orange (TO) was attached to an energy acceptor dye, Cy5. Upon binding to a protein that recognizes TO, red emission due to efficient energy transfer from TO to Cy5 replaces the green emission observed for monochromophoric TO bound to the same protein. Separately, TO was attached to a coumarin that serves as an energy donor. The same green emission is observed for coumarin-TO and TO bound to a protein, but efficient energy transfer allows violet excitation of coumarin-TO, versus longer wavelength, blue excitation of monochromophoric TO. Both bichromophores exhibit low nanomolar KD values for their respective proteins, >95% energy transfer efficiency and high fluorescence quantum yields.

Two-colour fluorescent imaging in organisms using self-assembled nano-systems of upconverting nanoparticles and molecular switches

The water-insolubility of a potentially versatile photoresponsive 'turn-on' fluorescence probe was overcome by incorporating it into a nano-assembly containing an upconverting nanoparticle wrapped in an amphiphilic polymer. The appeal of the nano-system is not only in the ability to turn "on" and "off" the fluorescence from the organic chromophore using UV and visible light, it is in the fact that the nanoparticle acts as a static probe because it emits red and green light when excited by near infrared light, which is not effected by UV and visible light. This dual-functioning emission behaviour was demonstrated in live organisms.

Design strategy for photoinduced electron transfer-based small-molecule fluorescent probes of biomacromolecules

As the cardinal support of innumerable biological processes, biomacromolecules such as proteins, nucleic acids and polysaccharides are of importance to living systems. The key to understanding biological processes is to realize the role of these biomacromolecules in thte localization, distribution, conformation and interaction with other molecules. With the current development and adaptation of fluorescent technologies in biomedical and pharmaceutical fields, the fluorescence imaging (FLI) approach of using small-molecule fluorescent probes is becoming an up-to-the-minute method for the detection and monitoring of these imperative biomolecules in life sciences. However, conventional small-molecule fluorescent probes may provide undesirable results because of their intrinsic deficiencies such as low signal-to-noise ratio (SNR) and false-positive errors. Recently, small-molecule fluorescent probes with a photoinduced electron transfer (PET) "on/off" switch for biomacromolecules have been thoroughly considered. When recognized by the biomacromolecules, these probes turn on/off the PET switch and change the fluorescence intensity to present a high SNR result. It should be emphasized that these PET-based fluorescent probes could be advantageous for understanding the pathogenesis of various diseases caused by abnormal expression of biomacromolecules. The discussion of this successful strategy involved in this review will be a valuable guide for the further development of new PET-based small-molecule fluorescent probes for biomacromolecules.

Fluorogenic protein labelling: a review of photophysical quench mechanisms and principles of fluorogen design

Fluorescent labelling of specific proteins in complex biological systems remains an important challenge in chemical biology. One promising approach comprises the use of small molecules designed to react specifically with a targeted protein of interest and to increase in fluorescent intensity following this reaction. This kind of fluorogenic reaction generally derives from fluorescence quenching in the unreacted probe that is abrogated over the course of the reaction. Herein, we review the mechanistic principles of three major photophysical quenching mechanisms involving Förster resonance energy transfer (FRET), through-bond energy transfer (TBET), and photoinduced electron transfer (PeT). We then present design principles for novel fluorogenic probes based on an understanding of these quench mechanisms, with emphasis on the emerging utility of density functional theory (DFT) calculations in the design process.

A novel fluoro-chromogenic click reaction for the labelling of proteins and nanoparticles with near-IR theranostic agents

Binding of red-absorbing porphycene isothiocyanates to proteins and nanoparticles leads to near-IR fluorescent and photosensitising conjugates.

Biocompatible flavone-based fluorogenic probes for quick wash-free mitochondrial imaging in living cells

Mitochondria, vital organelles existing in almost all eukaryotic cells, play a crucial role in energy metabolism and apoptosis of aerobic organisms. In this work, we report two new flavone-based fluorescent probes, MC-Mito1 and MC-Mito2, for monitoring mitochondria in living cells. These two probes exhibit remarkably low toxicity, good cell permeability, and high specificity; these probes complement the existing library of mitochondrial imaging agents. The new dyes give nearly no background fluorescence, and their application does not require tedious postwashing after cell staining. The appreciable tolerance of MC-Mito2 encourages a broader range of biological applications for understanding the cell degeneration and apoptosis mechanism.

A general approach for the development of fluorogenic probes suitable for no-wash imaging of kinases in live cells

A general approach is presented for developing small molecule-based fluorogenic probes suitable for no-wash imaging of endogenous kinases in live cells.

A comprehensive study of isomerization and protonation reactions in the photocycle of the photoactive yellow protein

A comprehensive picture of the overall photocycle was obtained to reveal a wide range of structural signals in the photoactive yellow protein.