Fluorogenic ester substrates to assess proteolytic activity

Synthesis and Photophysical Properties of Fluorescein Esters as Potential Organic Semiconductor Materials

Fluorescein (1), a known fluorescent tracer in microscopy with high photophysical properties, was esterified to have fluorescein ethyl ester (2) and O-ethyl-fluorescein ethyl ester (3) in excellent yields. All of them were investigated for the photophysical and electrochemical properties as potential organic semiconductor materials. Absorptions and emission spectra were taken in various solvents, compound 2 showed emission maxima at λ_max = 545 and compound 3 showed λ_max = 550 nm. Optical band gap energy (E_g) was calculated for 1-3 and the values were found in between 2.34 - 2.39 eV. Possibility of shifting emission maxima was studied in various pH (5-9) buffers, and finally the thermal stability was examined using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Increasing of conjugation system of 2 and 3 were studied by HOMO and LUMO distributions of 1-3. Experimental results showed that compounds 2 and 3 have excellent photophysical and electrochemical properties hence can be used as excellent organic semiconductor materials.

A fluorescent τ-probe: quantitative imaging of ultra-trace endogenous hydrogen polysulfide in cells and in vivo

Hydrogen sulfide (H2S) has been recognized as an important endogenous gasotransmitter associated with biological signaling transduction. However, recent biological studies implied that the H2S-related cellular signaling might actually be mediated by hydrogen polysulfides (H2S n , n > 1), not H2S itself. Unraveling such a mystery strongly demanded the quantification of endogenous H2S n in living systems. However, endogenous H2S n has been undetectable thus far, due to its extremely low concentration within cells. Herein, we demonstrated a strategy to detect ultra-trace endogenous H2S nvia a fluorescent τ-probe, through changes of fluorescence lifetime instead of fluorescence intensity. This τ-probe exhibited an ultrasensitive response to H2S n , bringing about the lowest value of the detection limit (2 nM) and a lower limit of quantification (10 nM) to date. With such merits, we quantified and mapped endogenous H2S n within cells and zebrafish. The quantitative information about endogenous H2S n in cells and in vivo may have a significant implication for future research on the role of H2S n in biology. The methodology of the τ-probe established here might provide a general insight into the design and application of any fluorescent probes, beyond the limit of utilizing fluorescence intensity.

Fluorescein hydrazones: A series of novel non-intercalative topoisomerase IIα catalytic inhibitors induce G1 arrest and apoptosis in breast and colon cancer cells

Fluorescein hydrazones (5 and 7) were synthesized in three/four steps with 82-92% yields. All synthesized compounds were evaluated by topoisomerase I (topo I) and topoisomerase IIα (topo IIα)-mediated relaxation and cell viability assays. Among them, most of the compounds showed topo I & IIα inhibitory activity and nineteen compounds showed strong anti-proliferative activity against various cell lines. In brief, 5e inhibited 53% topo IIα (etoposide 29%) at 20 μM and showed excellent antiproliferative activity against DU145 (1.43 ± 0.04 μM), HCT15 (2.4 ± 0.03 μM) and MCF7 (11.4 ± 0.5 μM) cell lines in comparison with adriamycin, etoposide, and camptothecin. Compounds 5e, 5g and 5h were further evaluated to determine their mode of action. Compounds 5e, 5g and 5h functioned as non-intercalative topo IIα catalytic inhibitor with induction of G1 arrest and activation of apoptotic proteins in dose-dependent manner.

A Bright Fluorescent Probe for H2S Enables Analyte-Responsive, 3D Imaging in Live Zebrafish Using Light Sheet Fluorescence Microscopy

Hydrogen sulfide (H2S) is a critical gaseous signaling molecule emerging at the center of a rich field of chemical and biological research. As our understanding of the complexity of physiological H2S in signaling pathways evolves, advanced chemical and technological investigative tools are required to make sense of this interconnectivity. Toward this goal, we have developed an azide-functionalized O-methylrhodol fluorophore, MeRho-Az, which exhibits a rapid >1000-fold fluorescence response when treated with H2S, is selective for H2S over other biological analytes, and has a detection limit of 86 nM. Additionally, the MeRho-Az scaffold is less susceptible to photoactivation than other commonly used azide-based systems, increasing its potential application in imaging experiments. To demonstrate the efficacy of this probe for H2S detection, we demonstrate the ability of MeRho-Az to detect differences in H2S levels in C6 cells and those treated with AOAA, a common inhibitor of enzymatic H2S synthesis. Expanding the use of MeRho-Az to complex and heterogeneous biological settings, we used MeRho-Az in combination with light sheet fluorescence microscopy (LSFM) to visualize H2S in the intestinal tract of live zebrafish. This application provides the first demonstration of analyte-responsive 3D imaging with LSFM, highlighting the utility of combining new probes and live imaging methods for investigating chemical signaling in complex multicellular systems.

A selective phosphine-based fluorescent probe for nitroxyl in living cells

A novel fluorescein-based fluorescent probe for nitroxyl (HNO) based on the reductive Staudinger ligation of HNO with an aromatic phosphine was prepared. This probe reacts with HNO derived from Angeli's salt and 4-bromo Piloty's acid under physiological conditions without interference by other biological redox species. Confocal microscopy demonstrates this probe detects HNO by fluorescence in HeLa cells and mass spectrometric analysis of cell lysates confirms this probe detects HNO following the proposed mechanism.

Fluorescein hydrazones as novel nonintercalative topoisomerase catalytic inhibitors with low DNA toxicity

Fluorescein hydrazones (3a-3l) were synthesized in three steps with 86-91% overall yields. Topo I- and IIα-mediated relaxation and cell viability assay were evaluated. 3d inhibited 47% Topo I (camptothecin, 34%) and 20% Topo II (etoposide 24%) at 20 μM. 3l inhibited 61% Topo II (etoposide 24%) at 20 μM. 3d and 3l were further evaluated to determine their mode of action with diverse methods of kDNA decatenation, DNA-Topo cleavage complex, comet, DNA intercalating/unwinding, and Topo IIα-mediated ATP hydrolysis assays. 3d functioned as a nonintercalative dual inhibitor against the catalytic activities of Topo I and Topo IIα. 3l acted as a Topo IIα specific nonintercalative catalytic inhibitor. 3d activated apoptotic proteins as it increased the level of cleaved capase-3 and cleaved PARP in a dose- and time-dependent manner. The dose- and time-dependent increase of G1 phase population was observed by treatment of 3d along with the increase of p27(kip1) and the decrease of cyclin D1 expression.

An expanded set of fluorogenic sulfatase activity probes

Fluorogenic probes that are activated by an enzymatic transformation are ideally suited for profiling enzyme activities in biological systems. Here, we describe two fluorogenic enzyme probes, 3-O-methylfluorescein-sulfate and resorufin-sulfate, that can be used to detect sulfatases in mycobacterial lysates. Both probes were validated with a set of commercial sulfatases and used to reveal species-specific sulfatase banding patterns in a gel-resolved assay of mycobacterial lysates. The fluorogenic probes described here are suitable for various assays and provide a starting point for creating new sulfatase probes with improved selectivity for mycobacterial sulfatases.

Synthesis of nucleoside 5'-tetraphosphates containing terminal fluorescent labels via activated cyclic trimetaphosphate

2'-Deoxynucleotide 5'-tetraphosphates in which a fluorescent label is attached to the terminal phosphate are used as key reagents in high-throughput DNA sequencing techniques and in single nucleotide polymorphism typing assays. We demonstrate that this class of compounds can be prepared by reacting fluorophores such as 7-hydroxy-4-methylcoumarin, methylfluorescein, fluorescein and resorufin with an activated form of cyclic trimetaphosphate to give intermediate 11. Reaction of 11 with 2'-deoxynucleoside 5'-monophosphates or a nucleoside 5'-monophosphate gave the target compounds in good yield.

Activatable Optical Probes for the Detection of Enzymes

The early detection of many human diseases is crucial if they are to be treated successfully. Therefore, the development of imaging techniques that can facilitate early detection of disease is of high importance. Changes in the levels of enzyme expression are known to occur in many diseases, making their accurate detection at low concentrations an area of considerable active research. Activatable fluorescent probes show immense promise in this area. If properly designed they should exhibit no signal until they interact with their target enzyme, reducing the level of background fluorescence and potentially endowing them with greater sensitivity. The mechanisms of fluorescence changes in activatable probes vary. This review aims to survey the field of activatable probes, focusing on their mechanisms of action as well as illustrating some of the in vitro and in vivo settings in which they have been employed.

Synthesis and utility of fluorogenic acetoxymethyl ethers

Phenolic fluorophores such as fluorescein, Tokyo Green, resorufin, and their derivatives are workhorses of biological science. Acylating the phenolic hydroxyl group(s) in these fluorophores masks their fluorescence. The ensuing ester is a substrate for cellular esterases, which can restore fluorescence. These esters are, however, notoriously unstable to hydrolysis, severely compromising their utility. The acetoxymethyl (AM) group is an esterase-sensitive motif that can mask polar functionalities in small molecules. Here, we report on the use of AM ether groups to mask phenolic fluorophores. The resulting profluorophores have a desirable combination of low background fluorescence, high chemical stability, and high enzymatic reactivity, both in vitro and in cellulo. These simple phenyl ether-based profluorophores could supplement or supplant the use of phenyl esters for imaging biochemical and biological systems.

A palette of fluorescent probes with varying emission colors for imaging hydrogen peroxide signaling in living cells

We present a new family of fluorescent probes with varying emission colors for selectively imaging hydrogen peroxide (H(2)O(2)) generated at physiological cell signaling levels. This structurally homologous series of fluorescein- and rhodol-based reporters relies on a chemospecific boronate-to-phenol switch to respond to H(2)O(2) over a panel of biologically relevant reactive oxygen species (ROS) with tunable excitation and emission maxima and sensitivity to endogenously produced H(2)O(2) signals, as shown by studies in RAW264.7 macrophages during the phagocytic respiratory burst and A431 cells in response to EGF stimulation. We further demonstrate the utility of these reagents in multicolor imaging experiments by using one of the new H(2)O(2)-specific probes, Peroxy Orange 1 (PO1), in conjunction with the green-fluorescent highly reactive oxygen species (hROS) probe, APF. This dual-probe approach allows for selective discrimination between changes in H(2)O(2) and hypochlorous acid (HOCl) levels in live RAW264.7 macrophages. Moreover, when macrophages labeled with both PO1 and APF were stimulated to induce an immune response, we discovered three distinct types of phagosomes: those that generated mainly hROS, those that produced mainly H(2)O(2), and those that possessed both types of ROS. The ability to monitor multiple ROS fluxes simultaneously using a palette of different colored fluorescent probes opens new opportunities to disentangle the complex contributions of oxidation biology to living systems by molecular imaging.