A selective fluorescent probe for carbon monoxide imaging in living cells

A genetically encoded fluorescent probe is capable of selectively detecting carbon monoxide inside living cell. The probe, named COSer (CO sensor), consists of a circularly permuted yellow fluorescent protein (cpYFP) inserted into the regulatory domain of the bacterial CO-sensing protein, CooA, which gives the probe its selective CO-binding property.     

A highly selective, cell-permeable fluorescent nanoprobe for ratiometric detection and imaging of peroxynitrite in living cells

Peroxynitrite (ONOO(-)) is a highly reactive species implicated in the pathology of numerous diseases and there is currently great interest in developing fluorescent probes that can selectively detect ONOO(-) in living cells. Herein, a polymeric micelle-based and cell-penetrating peptide-coated fluorescent nanoprobe that incorporates ONOO(-) indicator dye and reference dye for the ratiometric detection and imaging of ONOO(-) has been developed. The nanoprobe effectively avoids the influences from enzymatic reaction and high-concentration ·OH and ClO(-). The improved ONOO(-) selectivity of the nanoprobe is achieved by a delicate complementarity of properties between the nanomatrix and the embedded molecular probe (BzSe-Cy). This nanoprobe also has other attractive properties, such as good water solubility, photostability, biocompatibility, and near-infrared excitation and emission. Fluorescence imaging experiments by confocal microscopy show that this nanoprobe is capable of visualizing ONOO(-) produced in living cells and it exhibits very low toxicity and good membrane permeability. We anticipate that this technique will be a potential tool for the precise pathological understanding and diagnosis of ONOO(-)-related human diseases.     

Long‐Wavelength Fluorescent Reporters for Monitoring Protein Kinase Activity

In vivo optical imaging must contend with the limitations imposed by the optical window of tissue (600–1000 nm). Although a wide array of fluorophores are available that are visualized in the red and near‐IR region of the spectrum, with the exception of proteases, there are few long wavelength probes for enzymes. This situation poses a particular challenge for studying the intracellular biochemistry of erythrocytes, the high hemoglobin content of which optically obscures subcellular monitoring at wavelengths less than 600 nm. To address this, tunable fluorescent reporters for protein kinase activity were developed. The probing wavelength is preprogrammed by using readily available fluorophores, thereby enabling detection within the optical window of tissue, specifically in the far‐red and near‐IR region. These agents were used to monitor endogenous cAMP‐dependent protein kinase activity in erythrocyte lysates and in intact erythrocytes when using a light‐activatable reporter.     

Photocaged DNAzymes as a General Method for Sensing Metal Ions in Living Cells

DNAzymes, which are sequences of DNA with catalytic activity, have been demonstrated as a potential platform for sensing a wide range of metal ions. Despite their significant promise, cellular sensing using DNAzymes has however been difficult, mainly because of the “always‐on” mode of first‐generation DNAzyme sensors. To overcome this limitation, a photoactivatable (or photocaged) DNAzyme was designed and synthesized, and its application in sensing Zn^II in living cells was demonstrated. In this design, the adenosine ribonucleotide at the scissile position of the 8–17 DNAzyme was replaced by 2′‐O‐nitrobenzyl adenosine, rendering the DNAzyme inactive and thus allowing its delivery into cells intact, protected from nonspecific degradation within cells. Irradiation at 365 nm restored DNAzyme activity, thus allowing the temporal control over the sensing activity of the DNAzyme for metal ions. The same strategy was also applied to the GR‐5 DNAzyme for the detection of Pb^II, thus demonstrating the possible scope of the method.     

Repetitive reversible labeling of proteins at polyhistidine sequences for single-molecule imaging in live cells.

Sensitive live-cell fluorescence microscopy and single-molecule imaging are severely limited by rapid photobleaching of fluorescent probes. Herein, we show how to circumvent this problem using a novel, generic labeling strategy. Small nickel-nitrilotriacetate fluorescent probes are reversibly bound to oligohistidine sequences of exposed proteins on cell surfaces, permitting selective observation of the proteins by fluorescence microscopy. Photobleached probes are removed by washing and replaced by new fluorophores, thus enabling repetitive acquisition of single-molecule trajectories on the same cell and allowing variation of experimental conditions between acquisitions. This method offers free choice of fluorophores while being minimally perturbing. The strength of the method is demonstrated by labeling engineered polyhistidine sequences of the serotonin-gated 5-HT(3) receptor on the surface of live mammalian cells. Single-molecule microscopy reveals pronounced heterogeneous mobility patterns of the 5-HT(3) receptor. After activating the receptor with serotonin, the number of immobile receptors increases substantially, which might be important for receptor regulation at synapses.     

A tunable ratiometric pH sensor based on carbon nanodots for the quantitative measurement of the intracellular pH of whole cells

The whole picture: Carbon nanodots labeled with two fluorescent dyes have been developed as a tunable ratiometric pH sensor to measure intracellular pH. The nanosensor shows good biocompatibility and cellular dispersibility. Quantitative determinations on intact HeLa cells and pH fluctuations associated with oxidative stress were performed.     

A Two‐Dimensional Coordination Compound as a Zinc Ion Selective Luminescent Probe for Biological Applications

A 2D coordination compound {[Cu₂(HL)(N₃)]?ClO₄}_∞ ( 1 ; H₃L=2,6‐bis(hydroxyethyliminoethyl)‐4‐methyl phenol) was synthesized and characterized by single‐crystal X‐ray diffraction to be a polymer in the crystalline state. Each [Cu₂(HL)(N₃)]⁺ species is connected to its adjacent unit by a bridging alkoxide oxygen atom of the ligand to form a helical propagation along the crystallographic a axis. The adjacent helical frameworks are connected by a ligand alcoholic oxygen atom along the crystallographic b axis to produce pleated 2D sheets. In solution, 1 dissociates into [Cu₂(HL)₂(H₃L)]?2H₂O ( 2 ); the monomer displays high selectivity for Zn²⁺ and can be used in HEPES buffer (pH 7.4) as a zinc ion selective luminescent probe for biological application. The system shows a nearly 19‐fold Zn²⁺‐selective chelation‐enhanced fluorescence response in the working buffer. Application of 2 to cultured living cells (B16F10 mouse melanoma and A375 human melanoma) and rat hippocampal slices was also studied by fluorescence microscopy.     

Single-Sided Competitive Axial Coordination of G-Quadruplex/Hemin as Molecular Switch for Imaging Intracellular Nitric Oxide

Axial coordination is a crucial biological process to regulate biomolecules’ functions in natural enzymes. However, it is a great challenge to determine the single or dual axial interaction between the metal center of enzymes and the ligand. In this work, a controllable axial coordination system was developed based on G-quadruplex/hemin complex by designing a series of fluorescent derivatives. The mechanism on axial coordination of G-quadruplex/hemin with coumarin-imidazole ligands was proposed to be single-sided, and led to fluorescence quenching of ligands. Upon addition of nitric oxide, the fluorescence of ligands was recovered through competitive axial coordination, providing a “signal on” strategy for signal transduction. More significantly, the fluorescent imaging of intracellular nitric oxide was achieved after conjugating with gold nanoparticles. Also, the proposed protocol provided a smart strategy to monitor the relationship between nitric oxide and p53 protein activity in living cells.     

Two-photon fluorescent probes for long-term imaging of calcium waves in live tissue

2-Acetyl-6-(dimethylamino)naphthalene-derived two-photon fluorescent Ca2+ probes (ACa1-ACa3) are reported. They can be excited by a 780 nm laser beam, show 23-50-fold enhancement in one- and two-photon excited fluorescence in response to Ca2+, emit fourfold stronger two-photon excited fluorescence than Oregon Green 488 BAPTA-1 upon complexation with Ca2+, and can selectively detect intracellular free Ca2+ ions in live cells and living tissues with minimum interference from other metal ions and membrane-bound probes. Moreover, these probes are capable of monitoring calcium waves at a depth of 120-170 microm in live tissues for 1100-4000 s using two-photon microscopy with no artifacts of photobleaching.     

A Mix‐and‐Read Fluorescence Strategy for the Switch‐On Probing of Kinase Activity Based on an Aptameric‐Peptide/Graphene‐Oxide Platform

Protein kinase plays a vital role in regulating signal‐transduction pathways and its simple and quick detection is highly desirable because traditional kinase assays typically rely on a time‐consuming kinase‐phosphorylation process (ca. 1 h). Herein, we report a new and rapid fluorescence‐based sensing platform for probing the activity of protein kinase that is based on the super‐quenching capacity of graphene oxide (GO) nanosheets and specific recognition of the aptameric peptide (FITC‐IP₂₀). On the GO/peptide platform, the fluorescence quenching of FITC‐IP₂₀ that is adsorbed onto GO can be restored by selective binding of active protein kinase to the aptameric peptide, thereby resulting in the fast switch‐on detection of kinase activity (ca. 15 min). The feasibility of this method has been demonstrated by the sensitive measurement of the activity of cAMP‐dependent protein kinase (PKA), with a detection limit of 0.053 mU μL^?1. This assay technique was also successfully applied to the detection of kinase activation in cell lysate.     

An Activity‐Based Probe for Studying Crosslinking in Live Bacteria

Penicillin‐binding proteins (PBPs) catalyze the crosslinking of peptidoglycan (PG), an essential process for bacterial growth and survival, and a common antibiotic target. Yet, despite its importance, little is known about the spatiotemporal aspects of crosslinking—largely because of a lack of experimental tools for studying the reaction in live bacteria. Here we introduce such a tool: an activity‐based probe that enables visualization and relative quantitation of crosslinking in vivo. In Staphylococcus aureus, we show that fluorescent mimics of the natural substrate of PBPs (PG stem peptide) are covalently incorporated into the cell wall, installing fluorophores in place of natural crosslinks. These fluorescent stem peptide mimics (FSPMs) are selectively recognized by a single PBP in S. aureus: PBP4. Thus, we were able to use FSPM pulse‐labeling to localize PBP4 activity in live cells, showing that it is recruited to the septum in a manner dependent on wall teichoic acid.