CO Sense and Release Flavonols: Progress toward the Development of an Analyte Replacement PhotoCORM for Use in Living Cells

Carbon monoxide (CO) is a signaling molecule in humans. Prior research suggests that therapeutic levels of CO can have beneficial effects in treating a variety of physiological and pathological conditions. To facilitate understanding of the role of CO in biology, molecules that enable fluorescence detection of CO in living systems have emerged as an important class of chemical tools. A key unmet challenge in this field is the development of fluorescent analyte replacement probes that replenish the CO that is consumed during detection. Herein, we report the first examples of CO sense and release molecules that involve combining a common CO-sensing motif with a light-triggered CO-releasing flavonol scaffold. A notable advantage of the flavonol-based CO sense and release motif is that it is trackable via fluorescence in both its pre- and postsensing (pre-CO release) forms. In vitro studies revealed that the PdCl₂ and Ru(II)-containing CORM-2 used in the CO sensing step can result in metal coordination to the flavonol, which minimizes the subsequent CO release reactivity. However, CO detection followed by CO release is demonstrated in living cells, indicating that a cellular environment mitigates the flavonol/metal interactions.

Small-molecule fluorescent probes for imaging gaseous signaling molecules: current progress and future implications

Endogenous gaseous signaling molecules including nitric oxide (NO), carbon monoxide (CO) and hydrogen sulfide (H2S) have been demonstrated to perform significant physiological and pharmacological functions and are associated with various diseases in biological systems. In order to obtain a deeper insight into their roles and mechanisms of action, it is desirable to develop novel techniques for effectively detecting gaseous signaling molecules. Small-molecule fluorescent probes have been proven to be a powerful approach for the detection and imaging of biological messengers by virtue of their non-invasiveness, high selectivity, and real-time in situ detection capability. Based on the intrinsic properties of gaseous signaling molecules, numerous fluorescent probes have been constructed to satisfy various demands. In this perspective, we summarize the recent advances in the field of fluorescent probes for the detection of NO, CO and H2S and illustrate the design strategies and application examples of these probes. Moreover, we also emphasize the challenges and development directions of gasotransmitter-responsive fluorescent probes, hoping to provide a general implication for future research.

Nitro reduction-based fluorescent probes for carbon monoxide require reactivity involving a ruthenium carbonyl moiety

Recently, several arylnitro-based fluorescent CO probes have been reported. The design was based on CO's ability to reduce an arylnitro group for fluorescence turn-on. In this work, we assessed the response of three published arylnitro-based fluorescent CO probes, namely COFP, LysoFP-NO2, and NIR-CO toward CO from various sources. We found that only ruthenium-based CO releasing molecules (CO-RMs) were able to turn on the fluorescence while pure CO gas and CO from other sources did not turn-on the probe in the absence of ruthenium. Further experiments with different ruthenium complexes indicate that the reduction of arylnitro group requires the ruthenium carbonyl complex as an essential ingredient. As further confirmation, we also conducted the reduction of the nitro group in a p-nitrobenzamide compound and came to the same conclusion. As such, COFP and related arynitro-based probes are able to sense CORM-2 and CORM-3, but not CO in general. Our findings also indicate the need to use CO from various sources in future assessment of new CO probes.

Development of a new ratiometric probe with near-infrared fluorescence and a large Stokes shift for detection of gasotransmitter CO in living cells

A near-infrared (NIR) ratiometric fluorescent probe, NIR-Ratio-CO, was developed for rapid detection of carbon monoxide (CO) in both solution and living cells through the strategy of Pd⁰-mediated Tsuji-Trost reaction. This probe shows a rapid, highly specific and sensitive detection process for CO, accompanied by colorimetric and distinct ratiometric fluorescence changes at 655 and 592 nm with a large Stokes shift up to 195 nm. The detection limit for CO was measured to be about 61 nM by the fluorescence method. In addition, this probe was successfully applied for ratiometric imaging of both exogenous and endogenous CO in living cells, indicating that it can be used as a novel tool for ratiometric fluorescent detection of CO in living systems.

Synthesis and application of coumarin fluorescence probes

In recent years, the research on fluorescent probes has developed rapidly. Coumarin fluorescent probes have also been one of the hot topics in recent years. For the synthesis and application of coumarin fluorescent probes, great progress has been made. Coumarin fluorescent probes have become more and more widely used in biochemistry, environmental protection, and disease prevention, and have broad prospects. This review introduces the three main light emitting mechanisms (PET, ICT, FRET) of fluorescent probes, and enumerates some probes based on this light emitting mechanism. In terms of the synthesis of coumarin fluorescent probes, the existing substituents on the core of coumarin compounds were modified. Based on the positions of the modified substituents, some of the fluorescent probes reported in the past ten years are listed. Most of the fluorescent probes are formed by modifying the 3 and 7 position substituents on the mother nucleus, and the 4 and 8 position substituents are relatively less modified. In terms of probe applications, the detection and application of coumarin fluorescent probes for Cu²⁺, Hg²⁺, Mg²⁺, Zn²⁺, pH, environmental polarity, and active oxygen and sulfide in the past ten years are mainly introduced.

In recent years, the research on fluorescent probes has developed rapidly.

The development of a hemicyanine-based ratiometric CO fluorescent probe with a long emission wavelength and its applications for imaging COin vitroandin vivo

A novel ratiometric fluorescent probe,Hcy-CO, with long-wavelength emission was developed for visualizing CO in living cells and zebrafish.

Bioapplications of small molecule Aza-BODIPY: from rational structural design to in vivo investigations

This review highlights the empirical design guidelines and photophysical property manipulation of Aza-BODIPY dyes and the latest advances in their bioapplications.

Inorganic Chemistry Approaches to Activity-Based Sensing: From Metal Sensors to Bioorthogonal Metal Chemistry

The complex network of chemical processes that sustain life motivates the development of new synthetic tools to decipher biological mechanisms of action at a molecular level. In this context, fluorescent and related optical probes have emerged as useful chemical reagents for monitoring small-molecule and metal signals in biological systems, enabling visualization of dynamic cellular events with spatial and temporal resolution. In particular, metals occupy a central role in this field as analytes in their own right, while also being leveraged for their unique biocompatible reactivity with small-molecule substrates. This Viewpoint highlights the use of inorganic chemistry principles to develop activity-based sensing platforms mediated by metal reactivity, spanning indicators for metal detection to metal-based reagents for bioorthogonal tracking, and manipulation of small and large biomolecules, illustrating the privileged roles of metals at the interface of chemistry and biology.

A lysosome-targetable fluorescent probe for real-time imaging cysteine under oxidative stress in living cells

As an effective lysosomal biomarker for oxidative stress status, cysteine (Cys) plays an important role in lysosomal proteolysis. Herein, we report the first lysosome-targetable fluorescence probe (MCAB) for Cys-selective detection based on nucleophilic addition reaction of sulfhydryl toward a α, β-unsaturated ketone and demonstrate its application to lysosomal-targetable imaging. MCAB is designed based on a α, β-unsaturated ethanoylcarbazole as the fluorophore and the thiols reaction site, and a methylcarbitol unit as a lysosome-targetable group. Upon reacting with Cys, this probe turns on highly specific fluorescence signals linearly proportional to Cys concentrations over the range of 0-300 μM. MCAB detects Cys with a rapid response time (within 12 min) and low limit of detection (0.38 μM). MCAB is highly selective to Cys over other similar biothiols including homocysteine (Hcy) and glutathione (GSH). Moreover, it also exhibits significant lysosomal-targetable ability, which is ideal for lysosomal Cys-selective imaging. Using MCAB, we have successfully visualized the fluctuation endogenous Cys in lysosomes under oxidative stress status in real-time.

A fluorescent probe for carbon monoxide based on allyl ether rather than allyl ester: A practical strategy to avoid the interference of esterase in cell imaging

Pd⁰-mediated Tsuji-Trost reaction is a practical strategy to design fluorescent probes for carbon monoxide (CO) sensing, and in such reaction CO can reduce Pd²⁺ to Pd⁰ in-situ and remove allyl groups on fluorophores. In most of these probes, esters are commonly used to link allyl on fluorophores. We found that the ester groups could be hydrolyzed by esterase activity of fetal bovine serum (FBS), while FBS is a requisite in cell culture, and the hydrolysis could interfere the Pd⁰-mediated Tsuji-Trost reaction. In this study, we synthesized a fluorescent probe (Cou-CO) using allyl ether as reaction site rather than allyl ester. Cou-CO is non-fluorescence, and could react with CO under the presence of Pd⁰ to form Cou with strong fluorescence, and the maximum excitation and emission wavelengths of Cou are 464 nm and 495 nm respectively. Cou-CO shows excellent selectivity to CO and could avoid the effect of FBS with the limit of detection for CO is 78 nm. Finally, Cou-CO was successfully applied for imaging of CO in living cells.

A near-infrared fluorescent probe for imaging endogenous carbon monoxide in living systems with a large Stokes shift

Near-infrared (NIR) fluorescent probes with a large Stokes shift are very practical tools for bioimaging applications. Carbon monoxide (CO) is a key gaseous signal molecule and its imaging in living systems has attracted great attention in recent years. In this work, a very easy-to-get NIR fluorescent probe with a remarkable large pseudo-Stokes shift (238 nm) for detection of CO was reported. This probe was found to show a rapid NIR fluorescent turn-on response for CO with high selectivity, high sensitivity and a low detection limit (38 nM). Moreover, imaging CO in living cells and animals with this probe was successfully applied with a high signal-to-noise ratio. The results indicate that this probe can be used as a new practical tool for imaging of endogenous CO in living systems.

Mitochondria-Targetable Ratiometric Time-Gated Luminescence Probe for Carbon Monoxide Based on Lanthanide Complexes

As a critical gasotransmitter, carbon monoxide (CO) has been demonstrated to be related with mitochondrial respiration, but the monitoring of CO in mitochondria remains a great challenge. In this work, a unique ratiometric time-gated luminescence (TGL) probe, Mito-NBTTA-Tb³⁺/Eu³⁺, that can specifically respond to mitochondrial CO has been developed. The probe was designed by incorporating a mitochondria-targeting moiety, triphenylphosphonium, into a CO-activatable terpyridine polyacid derivative, 4'-(4-nitrobenzyloxy-2,2':6',2''-terpyridine-6,6''-diyl) bis(methylenenitrilo) tetrakis(acetic acid), for coordinating to Eu³⁺ and Tb³⁺ ions to construct lanthanide complex-based probe for ratiometric TGL detection of CO. Upon reaction with CO, accompanied by the conversion of nitro group to amino group, a 1,6-rearrangement-elimination reaction occurs, which leads to the cleavage of 4-nitrobenzyl group from Mito-NBTTA-Tb³⁺/Eu³⁺, resulting in the significant increase of Tb³⁺ emission at 540 nm and moderate decrease of Eu³⁺ emission at 610 nm. After the reaction, the I₅₄₀/ I₆₁₀ ratio was found to be 48-fold enhanced. This feature allowed Mito-NBTTA-Tb³⁺/Eu³⁺ to be employed as a ratiometric TGL probe for CO detection with the I₅₄₀/ I₆₁₀ ratio as a signal. In addition, the probe showed outstanding mitochondria-localization characteristic, which enabled the probe to be successfully applied to imaging CO within mitochondria of living cells under TGL and ratiometric modes. The application of Mito-NBTTA-Tb³⁺/Eu³⁺ was demonstrated by the visualization and quantitative detection of exogenous and endogenous CO in living cells and mouse liver tissue slices, as well as in living Daphnia magna and mice. All of the results suggested the potential of Mito-NBTTA-Tb³⁺/Eu³⁺ for the quantitative monitoring of CO in vitro and in vivo.

A cell membrane-anchored fluorescent probe for monitoring carbon monoxide release from living cells

Carbon monoxide (CO) acts as an important gasotransmitter in delivering intramolecular and intermolecular signals to regulate a variety of physiological processes. This lipid-soluble gas can freely pass through the cell membrane and then diffuse to adjacent cells acting as a messenger. Although many fluorescent probes have been reported to detect intracellular CO, it is still a challenge to visualize the release behavior of endogenous CO. The main obstacle is the lack of a probe that can anchor onto the cell membrane while having the ability to image CO in real time. In this work, by grafting a polar head onto a long and linear hydrophobic Nile Red molecule, a cell membrane-anchored fluorophore ANR was developed. This design strategy of a cell membrane-anchored probe is simpler than the traditional one of using a long hydrophobic alkyl chain as a membrane-anchoring group, and endows the probe with better water solubility. ANR could rapidly bind to the cell membrane (within 1 min) and displayed a long retention time. ANR was then converted to a CO-responsive fluorescent probe (ANRP) by complexation with palladium based on a metal palladium-catalyzed reaction. ANRP exhibited a fast response to CO with a 25-fold fluorescence enhancement in vitro. The detection limit was calculated to be 0.23 μM, indicating that ANRP is sensitive enough to image endogenous CO. Notably, ANRP showed excellent cell membrane-anchoring ability. With ANRP, the release of CO from HepG2 cells under LPS- and heme-stimulated conditions was visualized and the cell self-protection effect during a drug-induced hepatotoxicity process was also studied. Moreover, ANRP was successfully applied to the detection of intracellular CO in several cell lines and tissues, and the results demonstrated that the liver is the main organ for CO production, and that cancer cells release more CO from their cells than normal cells. ANRP is the first membrane-anchored CO fluorescent probe that has the ability to reveal the relationship between CO release and diseases. It also has prospects for the studying of intercellular signaling functions of CO.

A long-wavelength ultrasensitive colorimetric fluorescent probe for carbon monoxide detection in living cells

A long-wavelength ultrasensitive colorimetric fluorescent probe was developed to track carbon monoxide in living cells.

Strategies for designing organic fluorescent probes for biological imaging of reactive carbonyl species

This review highlights the design strategies of typical organic fluorescent probes for reactive carbonyl species and their application in biological imaging.

Two-photon imaging of 1,4-dithiothreitol (DTT) by a red-emissive fluorescent probe in living cells, tissues and animals

1,4-Dithiothreitol (DTT) is an important small-molecular reducing agent and has extensive applications in biochemistry, peptide/protein chemistry and clinical medicine. The development of effective methods for monitoring DTT is of great importance for its safe use and studying its toxicity to human. In this work, we present a two-photon red-emissive probe for the imaging of DTT in living cells, tissues and animals. The probe employed a two-photon red-emissive xanthene dye as the fluorophore and selected 2,4-dinitrophenylate as the novel recognition site for DTT. In response to DTT, the probe displayed excellent sensitivity and selectivity. The probe was successfully applied to the two-photon imaging of DTT in living cells, and the imaging of DTT in living tissues and animals.

Dual site-controlled two-photon fluorescent probe for the imaging of lysosomal pH in living cells

Abnormal lysosomal pH is closely associated with many diseases, and real-time monitoring of lysosomal pH is important for understanding the lysosome physiological nature. Here, we present a novel lysosome-targeting two-photon fluorescent probe (MP-lys) for monitoring pH changes in living cells. As a dual site-controlled probe, MP-lys employed morpholine and piperazine groups as the lysosome-targeting groups and pH response sites. MP-lys showed rapid, reversible and sensitive fluorescence response to pH. MP-lys possessed lysosome-targeting properties, and could be used for two-photon imaging of chloroquine-induced pH variation in living cells.

A near-infrared fluorescent probe for rapid detection of carbon monoxide in living cells

A near-infrared (NIR) and colorimetric fluorescent probe system was developed for Carbon Monoxide (CO) via a Pd⁰-mediated Tsuji-Trost reaction. In this probe, phenoxide anion formation (DPCO^-) was acted as the signal unit and an allyl carbonate group was used as the recognition unit. This non-fluorescent probe molecule can release the relevant fluorophore after conversion of Pd²⁺ to Pd⁰ by CO. The probe system including probe 1 and Pd²⁺ can be used for "naked-eye" detection of CO, and exhibited high selectivity to CO over various other sensing objects. More importantly, the probe system has great potential for fluorescence imaging of intracellular CO in living cells.

Turn-on and Turn-off Fluorescent Probes for Carbon Monoxide Detection and Blood Carboxyhemoglobin Determination

Water-soluble, carbazole-based two-photon excitable fluorescent probes MPVC-I ("turn-on") and MPVC-II ("turn-off") are rationally designed and synthesized for the selective monitoring of carbon monoxide (CO). Both probes can effectively measure carboxyhemoglobin (HbCO) in the blood of the animals exposed to a CO dose as low as 100 ppm for 10 min. The palladium catalyzed azidocarbonylation reaction was optimized to improve the sensing efficiency.

Palladacycle Based Fluorescence Turn-On Probe for Sensitive Detection of Carbon Monoxide

New selective and sensitive fluorescence probes have always been in great demand for carbon monoxide, an important gasotransmitter molecule, which is involved in critical physiological and pathophysiological processes in the mammalian cardiovascular system. In this work, we synthesized a new palladacycle compound as a fluorescence turn-on probe for selective and quantitative detection of carbon monoxide. The weakly fluorescent probe quickly and selectively reacts with carbon monoxide and releases a highly fluorescent benzimidazole moiety, due to protonolysis of the palladacycle, which greatly enhances the fluorescence intensity. The selective reaction was against interference from other possible coexisting reactive oxygen species, and achieved a detection limit of ∼0.06 μM. Furthermore, the fluorescence turn-on probe was demonstrated with a high cellular uptake rate and was successfully applied for cell imaging of carbon monoxide in living cells.

A New Lysosome-Targetable Turn-On Fluorogenic Probe for Carbon Monoxide Imaging in Living Cells

A lysosome-targetable fluorogenic probe, LysoFP-NO₂, was designed and synthesized based on a naphthalimide fluorophore that can detect selectively carbon monoxide (CO) in HEPES buffer (pH 7.4, 37 °C) through the transformation of the nitro group into an amino-functionalized system in the presence of CO. LysoFP-NO₂ triggered a "turn-on" fluorescence response to CO with a simultaneous increase of fluorescence intensity by more than 75 times. The response is selective over a variety of relevant reactive nitrogen, oxygen, and sulfur species. Also, the probe is an efficient candidate for monitoring changes in intracellular CO in living cells (MCF7), and the fluorescence signals specifically localize in the lysosome compartment.

A naphthalimide-based fluorescence ‘‘turn-on’’ chemosensor for highly selective detection of carbon monoxide: imaging applications in living cells

A naphthalimide-based fluorescence chemosensor, COFP, was designed and synthesized for the detection of carbon monoxide (CO) in HEPES buffer (pH 7.4, 37 °C).

Polysiloxane-based two-photon fluorescent elastomers with superior mechanical and self-healing properties and their application in bioimaging

The first report of the synthesis of novel self-healing elastomers possessing a dynamic dual cross-linked network and exhibiting a unique two-photon fluorescence, which show potential for application in bioimaging.

Nitric oxide activatable photosensitizer accompanying extremely elevated two-photon absorption for efficient fluorescence imaging and photodynamic therapy

A nitric oxide (NO) activatable photosensitizer was constructed for efficient fluorescence imaging and photodynamic therapy.

Elevated nitric oxide (NO) levels perform an important pathological role in various inflammatory diseases. Developing NO-activatable theranostic materials with a two-photon excitation (TPE) feature is highly promising for precision imaging and therapy, but constructing such materials is still a tremendous challenge. Here, we present the first example of a NO-activatable fluorescent photosensitizer (DBB-NO) accompanying extremely NO-elevated two-photon absorption (TPA) for efficient fluorescence imaging and photodynamic therapy (PDT). Upon responding to NO, DBB-NO shows not only a remarkably enhanced fluorescence quantum yield (Φ_F, 0.17% vs. 9.3%) and singlet oxygen quantum yield (Φ_Δ, 1.2% vs. 82%) but also an extremely elevated TPA cross-section (δ, 270 vs. 2800 GM). Simultaneous enhancement of Φ_Δ, Φ_F and δ allows unprecedented two-photon fluorescence brightness (δ × Φ_F = 260.4 GM) and two-photon PDT (TP-PDT) efficiency (δ × Φ_Δ = 2296 GM) which precedes the value for a commercial two-photon photosensitizer by two orders of magnitude. With these merits, the proof-of-concept applications of NO-activatable two-photon fluorescence imaging and TP-PDT in activated macrophages (in which NO is overproduced) were readily realized. This work may open up many opportunities for constructing two-photon theranostic materials with other pathological condition-activatable features for precise theranostics.

Organometallic chemical biology: an organometallic approach to bioconjugation

This review summarizes the history and recent developments of the field of organometallic chemical biology with a particular emphasis on the development of novel bioconjugation approaches. Over the years, numerous transformations have emerged for biomolecule modification with the use of organometallic reagents; these include [3+2] cycloadditions, C–C, C–S, C–N, and C–O bond forming processes, as well as metal-mediated deprotection (“decaging”) reactions. These conceptually new additions to the chemical biology toolkit highlight the potential of organometallic chemistry to make a significant impact in the field of chemical biology by providing further opportunities for the development of chemoselective, site-specific and spatially resolved methods for biomolecule structure and function manipulation. Examples of these transformations, as well as existing challenges and future prospects of this rapidly developing field are highlighted in this review.

A novel dihydrocoumarin under experimental and theoretical characterization

Coumarins are natural and synthetic active ingredients widely applied in diverse types of medicinal treatments, such as cancer, inflammation, infection, and enzyme inhibition (monoamine oxidase B). Dihydrocoumarin compounds are of great interest in organic chemistry due to their structural versatilities and, as part of our investigations concerning the structural characterization of small molecules, this work focuses on crystal structure and spectroscopic characterization of the synthesized and crystallized compound 4-(4-methoxyphenyl)-3,4-dihydro-chromen-2-one (C₁₆H₁₄O₃). Additionally, a theoretical calculation was performed using density functional theory to analyze the sites where nucleophilic or electrophilic attack took place and to examine the molecular electrostatic potential surface. Throughout all of these calculations, both density functional theory and Car-Parrinello molecular dynamics were performed by fully optimized geometry. The spectroscopic analysis indicated the presence of aromatic carbons and hydrogen atoms, and also the carbonyl and methoxy groups that were confirmed by the crystallographic structure. The C₁₆H₁₄O₃ compound has a non-classical intermolecular interaction of type C-H⋅⋅⋅O that drives the molecular arrangement and the crystal packing. Moreover, the main absorbent groups were characterized throughout calculated harmonic vibrational frequencies. Also, natural bond orbital analysis successfully locates the molecular orbital with π-bonding symmetry and the molecular orbital with π* antibonding symmetry. Finally, the gap between highest occupied and lowest unoccupied molecular orbitals implies in a high kinetic stability and low chemical reactivity of title molecule.

Fluorescent probes for imaging formaldehyde in biological systems

Formaldehyde (FA) is a common environmental toxin but is also endogenously produced through a diverse array of essential biological processes, including mitochondrial one-carbon metabolism, metabolite oxidation, and nuclear epigenetic modifications. Its high electrophilicity enables reactivity with a wide variety of biological nucleophiles, which can be beneficial or detrimental to cellular function depending on the context. New methods that enable detection of FA in living systems can help disentangle the signal/stress dichotomy of this simplest reactive carbonyl species (RCS), and fluorescent probes for FA with high selectivity and sensitivity have emerged as promising chemical tools in this regard.