Fluorescent Probe Based on Azobenzene-Cyclopalladium for the Selective Imaging of Endogenous Carbon Monoxide under Hypoxia Conditions

A new mitochondrion targetable fluorescent probe for carbon monoxide-specific detection and live cell imaging

A new mitochondrion targetable molecular probe for carbon monoxide (CO)-specific detection based on palladium-free mediated opening of spirolactam was designed. The turn-on red fluorescence caused by CO enables a safe and powerful method for unravelling the function of CO in biological systems to be established.

Upconversion Nanoprobes for in Vitro and ex Vivo Measurement of Carbon Monoxide

Here, we have developed a new colorimetric and luminescence nanosensor, based on upconversion nanoparticles (UCNPs), for in vitro and ex vivo measurement of carbon monoxide (CO). The nanoprobe has two strong fluorescence emission peaks in the UCNP core to excite fluorophores at 540 and 800 nm. The CO-responsive palladium ion-bounded rhodamine B derivatives (Pd-RBDs) are encapsulated in the mesoporous silica (mSiO₂) shell and the particles outside the cyclodextrin (CD) layer. Reduction of palladium ions by CO results in the release of palladium from the Pd-RBDs, thereby inducing the closure of the spiro ring of the RBD and the accompanying reduction of rhodamine B (RB) absorption at 500-600 nm overlapping with the luminescence spectrum of UCNPs maximized at 540 nm. Therefore, the I₅₄₀/I₈₀₀ ratio of the nanoprobe will increase when CO is present, making it possible to quantitatively measure CO. Besides working in a clean buffer environment with known [CO], this method was evaluated using living cells and tissue sections. Additionally, these probes were also successfully used to investigate the CO-related protective activity of anti-hepatic ischemia-reperfusion injury (HIRI) oligopeptides.

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.

A water-soluble fluorescent probe for detecting creatinine in totally aqueous media and imaging exogenous creatinine in living cells

The design of vigorous tools for creatinine determination is extremely important in the diagnosis and treatment of kidney diseases. In the study, we examine a robust fluorescent turn-on probe (NCP-Pd) for creatinine detection in a completely aqueous solution based on the metal palladium-catalyzed reaction. In the presence of creatinine, the NCP-Pd dissociates and subsequently restores the fluorescence due to elimination of the heavy atom quenching effect and prevention of the photoinduced electron transfer effect. The probe NCP-Pd displays excellent detecting performance with respect to creatinine such as good water solubility, high selectivity, and a low detection limit (0.16 μM). Additionally, in order to ensure its clinical application, this probe is operated in blood serum samples for detecting creatinine and compared with a commercial clinical method. The results indicate an extremely high agreement with the commercial clinical method. Furthermore, the results confirm that the probe NCP-Pd exhibits satisfactory cell permeability and low cytotoxicity and can detect creatinine in L929 and HCT116 cells. The study provides a potential application for detecting creatinine and conducting pathological research on creatinine-involved diseases.

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.

A two-photon fluorescent probe for ratiometric visualization of hypochlorous acid in live cells and animals based on a selenide oxidation/elimination tandem reaction

Utilizing the oxidation/elimination tandem reaction of the α-phenylseleno carbonyl moiety, a two-photon fluorescent probe for ratiometric visualization of hypochlorous acid was developed. Its superior sensing performance and practical applications were well demonstrated.

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.

Mitochondria-targeted near-infrared fluorescent probe for the detection of carbon monoxide in vivo

Carbon monoxide is a critical gasotransmitter in the body and related with mitochondrial respiration. To date, various fluorescent probes for CO have been well proposed, but two main problems remain. One is that most of the probes are not mitochondria-targeting, even if the probes claim to be able to detect CO in living cells. The other is that the probes for CO display excitation and emission within the ultraviolet or visible range, which hinders their applications in vivo. Herein, a hemicyanine-based near-infrared (NIR) fluorescent probe named CyAPC is first synthesized and used to detect mitochondrial CO. The characteristics of probe CyAPC are as follows: (1) The fluorescence emission of the sensing system is at 736 nm belonging to NIR region, which is suitable for bioimaging in vivo. (2) CyAPC, a positively charged molecule, would have a high tendency to localize in mitochondria of cells. (3) The fluorescence change of the probe is attributed to the fact that CO with Pd²⁺ induced cleavage of the allyl formate group from the probe and CyAPC (fluorescence off) is transformed into CyOH (fluorescence on), which is proved by HPLC, MS and DFT calculation. (4) The NIR fluorescent probe is applied for the detection of exogenous and endogenous CO in various biological samples such as cell, tissue and in vivo with satisfactory results.

Evaluating Drug-Induced Liver Injury and Its Remission via Discrimination and Imaging of HClO and H2S with a Two-Photon Fluorescent Probe

Drug-induced liver injury (DILI) has aroused wide concern. Finding new markers or indicators as well as detoxification molecules for DILI is of great significance and good application prospect, which can help develop effective preclinical screening methodology and corresponding treatment protocols. Herein, in this article, DILI caused by antidepressant drugs of duloxetine and fluoxetine and its remission were evaluated by a two-photon fluorescent probe, RPC-1, through discriminating and imaging HClO and H₂S simultaneously. By being applied both in vitro and in vivo, RPC-1 revealed slight up-regulation of HClO and negligible liver damage after administration of either of the two drugs. In contrast, an apparent up-regulation of HClO and obvious liver damage was observed after combined administration of the drugs. Meanwhile, the pretreatment of N-acetyl cysteine (NAC) resulted in the increasing of endogenous H₂S level, which contributed to the remission of DILI. The histological analysis and serological test both gave good consistency with the imaging results. These findings demonstrate that HClO may be an appropriate indicator of DILI, and H₂S plays an important role in the antidotal effect of NAC. We envision that RPC-1 can be used as a powerful tool to predict clinical DILI and study the effect of antidote, as well as explore the molecular mechanisms involved.

Two-photon imaging of formaldehyde in live cells and animals utilizing a lysosome-targetable and acidic pH-activatable fluorescent probe

Lyso-TPFP presents lysosomal targetability and an acidic pH-activatable response toward formaldehyde. Thus, it exclusively visualizes lysosomal formaldehyde and is immune against it in neutral cytosol and other organelles. In addition, two-photon fluorescence imaging endows Lyso-TPFP with the capability of in situ tracking formaldehyde in live cells and animals.

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).

A highly specific and sensitive ratiometric fluorescent probe for carbon monoxide and its bioimaging applications

A simple highly specific and sensitive ratiometric fluorescent probe has been constructed to monitor carbon monoxide levels in living cells.

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.

Carbon Monoxide and Its Controlled Release: Therapeutic Application, Detection, and Development of Carbon Monoxide Releasing Molecules (CORMs)

Carbon monoxide (CO) is attracting increasing attention because of its role as a gasotransmitter with cytoprotective and homeostatic properties. Carbon monoxide releasing molecules (CORMs) are spatially and temporally controlled CO releasers that exhibit superior and more effective pharmaceutical traits than gaseous CO because of their chemistry and structure. Experimental and preclinical research in animal models has shown the therapeutic potential of inhaled CO and CORMs, and the biological effects of CO and CORMs have also been observed in preclinical trials via the genetic modulation of heme oxygenase-1 (HO-1). In this review, we describe the pharmaceutical use of CO and CORMs, methods of detecting CO release, and developments in CORM design and synthesis. Many valuable clinical CORMs formulated using macromolecules and nanomaterials are also described.

Two-photon fluorescent probe for revealing drug-induced hepatotoxicity via mapping fluctuation of peroxynitrite

Drug-induced injury has attracted increasing attention in public health issues. Among them, hepatotoxicity has been regarded as the leading clinical problem caused by drug toxicity. However, owing to the complexity of the involved pathophysiological mechanisms and the lack of noninvasive, straightforward, and real-time tools, drug-induced hepatotoxicity has rarely been predicted satisfactorily. In this paper, by utilizing the reactive species peroxynitrite (ONOO-) as a biomarker, we present a two-photon fluorescent probe, TP-KA, holding rapid response, high specificity and sensitivity towards ONOO-, to investigate drug (acetaminophen and tolcapone)-related liver injury and the remediate effect of N-acetyl cysteine (NAC). With the support of TP-KA, we obtained direct and visual evidence of the upregulation of ONOO- during drug challenge both in live cells and mice, which was accompanied by liver tissue injury and tyrosine nitration. These findings demonstrate that ONOO- is a good and appropriate biomarker of hepatotoxicity, and nitrosative stress may be necessary for acetaminophen and tolcapone to exert their toxicity. Moreover, TP-KA can be employed as a powerful tool to pre-detect drug-induced organism injury and study the effect of antidotes.

Detection and Removal of Endogenous Carbon Monoxide by Selective and Cell-Permeable Hemoprotein Model Complexes

Carbon monoxide (CO) is produced in mammalian cells during heme metabolism and serves as an important signaling messenger. Here we report the bioactive properties of selective CO scavengers, hemoCD1 and its derivative R8-hemoCD1, which have the ability to detect and remove endogenous CO in cells. HemoCD1 is a supramolecular hemoprotein-model complex composed of 5,10,15,20-tetrakis(4-sulfonatophenyl)porphinatoiron(II) and a per-O-methylated β-cyclodextrin dimer having an pyridine linker. We demonstrate that hemoCD1 can be used effectively to quantify endogenous CO in cell lysates by a simple spectrophotometric method. The hemoCD1 assay detected ca. 260 pmol of CO in 10⁶ hepatocytes, which was well-correlated with the amount of intracellular bilirubin, the final breakdown product of heme metabolism. We then covalently attached an octaarginine peptide to a maleimide-appended hemoCD1 to synthesize R8-hemoCD1, a cell-permeable CO scavenger. Indeed, R8-hemoCD1 was taken up by intact cells and captured intracellular CO with high efficiency. Moreover, we revealed that removal of endogenous CO by R8-hemoCD1 in cultured macrophages led to a significant increase (ca. 2.5-fold) in reactive oxygen species production and exacerbation of inflammation after challenge with lipopolysaccharide. Thus, R8-hemoCD1 represents a powerful expedient for exploring specific and still unidentified biological functions of CO in cells.

Biological signaling by carbon monoxide and carbon monoxide-releasing molecules

Carbon monoxide (CO) is continuously produced in mammalian cells during the degradation of heme. It is a stable gaseous molecule that reacts selectively with transition metals in a specific redox state, and these characteristics restrict the interaction of CO with defined biological targets that transduce its signaling activity. Because of the high affinity of CO for ferrous heme, these targets can be grouped into heme-containing proteins, representing a large variety of sensors and enzymes with a series of diverse function in the cell and the organism. Despite this notion, progress in identifying which of these targets are selective for CO has been slow and even the significance of elevated carbonmonoxy hemoglobin, a classical marker used to diagnose CO poisoning, is not well understood. This is also due to the lack of technologies capable of assessing in a comprehensive fashion the distribution and local levels of CO between the blood circulation, the tissue, and the mitochondria, one of the cellular compartments where CO exerts its signaling or detrimental effects. Nevertheless, the use of CO gas and CO-releasing molecules as pharmacological approaches in models of disease has provided new important information about the signaling properties of CO. In this review we will analyze the most salient effects of CO in biology and discuss how the binding of CO with key ferrous hemoproteins serves as a posttranslational modification that regulates important processes as diverse as aerobic metabolism, oxidative stress, and mitochondrial bioenergetics.

Allyl Fluorescein Ethers as Promising Fluorescent Probes for Carbon Monoxide Imaging in Living Cells

Recently, the fluorescent detection of carbon monoxide (CO) in living cells has attracted great attention. However, due to the lack of effective ways to construct fluorescent CO probes, fluorescent detection of CO in living cells is still in its infancy. In this paper, we report for the first time the use of allyl ether as a reaction site for construction of fluorescent CO probes. By this way, two readily available allyl fluorescein ethers were prepared, which were found to be highly selective and sensitive probes for CO in the presence of PdCl₂. These probes have the merits of good stability, good water-solubility, and rapid and distinct colorimetric and remarkable fluorescent turn-on signal changes. Moreover, a very low dose of these two probes can be used to detect and track CO in living cells, indicating that these two probes could be very promising biological tools for CO detection in living systems. Overall, this work provided not only two new promising fluorescent CO probes but also a new way to devise fluorescent CO probes.

Combinatorial Strategy to Identify Fluorescent Probes for Biothiol and Thiophenol Based on Diversified Pyrimidine Moieties and Their Biological Applications

We present a feasible paradigm of developing original fluorescent probes for target biomolecules via combinatorial chemistry. In this developmental program, pyrimidine moieties were investigated and optimized as unique recognition units for thiols for the first time through a parallel synthesis in combination with a rapid screening process. This time-efficient and cost-saving process effectively facilitated the developmental progress and provided detailed structure-reactivity relationships. As a result, Res-Biot and Flu-Pht were identified as optimal fluorescent probes for biothiol and thiophenol, respectively. Their favorable characteristics and superior applicability have been well demonstrated in both chemical and biological contexts. In particular, Res-Biot enables the direct visualization of biothiol fluctuations during oxidative stress and cell apoptosis, indicating its suitability in elucidation of a specific pathophysiological process in both living cells and living animals. Meanwhile, Flu-Pht is competent to visualize thiophenols without the interference from endogenous biothiols in living cells.