BODIPY-based azamacrocyclic ensemble for selective fluorescence detection and quantification of homocysteine in biological applications

Coumarin-Ensembled Vanadium(V) Compounds and Their Affinity Studies Toward Biological Thiols Probed by Fluorescence Spectroscopy

Fluorescence spectroscopic studies of a pair of new oxido-vanadium(V) compounds with biological thiols, such as homocysteine (Hcy), cysteine (Cys), and glutathione (GSH), have been investigated in this article. Despite notable progress in vanadium-thiol chemistry, no attention has been paid to exploring vanadium-based optical probes to study their interaction with biothiols. For this purpose, two oxido-vanadium(V) compounds, 1 and 2, have been prepared involving a tridentate ONO donor-based luminescent coumarin-derived ligand. Single crystal X-ray diffraction analysis, NMR (¹ H, ¹³ C, and ⁵¹ V) spectroscopy, XPS, and DFT calculations have been used to establish their identities. The vanadium center in these compounds has a distorted octahedral environment. In compound 2, a methanol molecule is coordinated to the vanadium(V) center in the trans position of the terminal oxido moiety. The latter exerts a strong trans-labilizing influence on the coordinating methanol. Both 1 and 2 are weakly fluorescent. Photophysical investigations of the vanadium complexes in aqueous media at physiological pH (7.4) in the presence of various biothiols and amino acids showed significant fluorescence enhancement (83-fold) of the vanadium complexes, specifically with Hcy. The specific affinity of the complexes for Hcy remained unchanged even in the presence of other biothiols and amino acids. Kinetic investigation reveals pseudo-first order behavior of the compound with Hcy. Mechanistic studies have manifested that Hcy-induced reduction triggers the decomplexation of the vanadium compound, followed by hydrolysis and subsequent cyclization. Time-correlated single photon counting suggested that the radiative rate constant (k_r ) of 1 and 2 in the presence of Hcy serves as the prime factor for the fluorescence enhancement of the medium. Compound 1 has been tested efficiently for Hcy measurement in blood plasma rendering it suitable for practical applications.

Fluorescence probe for selectively monitoring biothiols within cells and mouse depression diagnosis

As a global mental disorder, depression is associated with oxidative stress in the brain. Cysteine, a reductive biothiols, regulates the oxidative situation in many biological events including the stress that occurs in the tissues. Exploring the pathology and physiology of depression is still a challenge and always in an urgent need. Thus, developing a new method that could track Cys level without the interferes from other competing substances is of great importance. Herein, we developed a fluorescence probe that could selectively sensing Cys over other biothiols. Besides, we have demonstrated its desirable performance in cellular applications and mouse brain. This work provides a new method for Cys imaging and understanding pathogenesis of depression. We hope the work described here could be used as a potential chemical approach for the diagnosis of Cys associated diseases in clinical applications.

Lysosome-Targeting Fluorescence Sensor for Sequential Detection and Imaging of Cu2+ and Homocysteine in Living Cells

A conjugated polymer-based fluorescence sensor, namely, PTNPy, was constructed on the basis of a polythiophene scaffold coupled with dimethylpyridylamine (DPA) groups in side chains for the consecutive detection and quantification of Cu²⁺ and Hcy in a perfect aqueous medium. A dramatic fluorescence quenching of PTNPy by the addition of Cu²⁺ was observed in Tris-HCl buffer solution (2 mM, pH 7.4), demonstrating a quick (<1 min) and highly selective response to Cu²⁺ with a low limit of detection of 6.79 nM. Subsequently, the Cu²⁺-quenched fluorescence of PTNPy can be completely recovered by homocysteine (Hcy), showing excellent selectivity to Hcy over other competitive species such as cysteine and glutathione. Thanks to the low cytotoxicity and lysosomal targeting ability of PTNPy, it was further applied as an optical sensor for the sequential imaging of Cu²⁺ and Hcy in HeLa cells. More importantly, Hcy concentration was linearly related to the fluorescence intensity of PTNPy in living cells, demonstrating huge potential for real-time monitoring the fluctuation of Hcy levels in living cells.

A unique water soluble probe for measuring the cardiac marker homocysteine and its clinical validation

A series of copper(II) compounds 1-4 were synthesized and developed as fluorogenic probes to measure the cardiac marker homocysteine (Hcy) without any interference from other bioanalytes prevalent in human blood plasma including, cysteine and glutathione. UV-vis and EPR studies have provided confirmatory evidence for reduction-induced-emission-enhancement of the probe, which is responsible for the observed "off-to-on" behaviour towards Hcy. Water solubility, remarkable fluorescence enhancement (55-111 fold), and low detection ability (nearly 2.5 μM) make the probe suitable for clinical testing of cardiac samples. Investigation of 1 against a few reductive interferents testifies its specificity for Hcy. Results from clinical examination of cardiac samples by 1 when combined with the outcome of the reliability testing involving a clinically approved commercial immunoassay kit, validates the prospect of the molecular probe for direct measurement of Hcy in human plasma, which is unprecedented.

Application of a fluorescent H2S probe based on excited-state intramolecular proton transfer for detecting latent mechanism of H2S-induced MCF-7 apoptosis

Background: H₂S is the third gas transmitter affecting the growth, reproduction and survival of cancer cells. However, the H₂S anticancer and antitumor mechanism still needs to be further studied. Methods: Here, FHS-1 was synthesized utilizing excited-state intramolecular proton transfer to detect H₂S in MCF-7 cells, and investigated the effects of varying concentrations NaHS on apoptosis. Results: The study found that FHS-1 detects H₂S levels with high selectivity and pH stability and that H₂S may regulate apoptosis in MCF-7 cells through the p53/mTOR/STAT3 pathway. Conclusion: Researching the influence of H₂S on apoptosis can serve as a theoretical foundation for future research into H₂S-related anticancer medicines, and the H₂S probe can be used as an effective cancer screening tool.

Homocysteine-specific fluorescence detection and quantification for evaluating S-adenosylhomocysteine hydrolase activity

The medium Ks value of copper complex contributed to the specific reduction of Cu2+ by homocysteine and the formation of a stable six-membered ring species.

Colorimetric detection of homocysteine by a pyridylazo dye-based Cu2+ complex via indicator displacement mechanism

A BrPAPS based Cu²⁺ complex has been developed as a colorimetric probe for the selective recognition of homocysteine (Hcy) over cysteine (Cys) and glutathione (GSH) in an aqueous solution via the indicator displacement assay. BrPAPS formed a complex with Cu²⁺ in a 1:1 ratio (BrPAPS-Cu²⁺) accompanied by the color change from yellow to red. Detecting Hcy is based on high affinity of Hcy for Cu²⁺. The addition of Hcy to BrPAPS-Cu²⁺ caused the complex formation of Hcy with Cu²⁺ in a 2:1 stoichiometry, resulting a hypsochromic shift with change back of color from red to yellow by the release of BrPAPS from BrPAPS-Cu²⁺. The absorption response is linear with the Hcy concentration in the range of 0-20 μM with a detection limit of 1.46 μM. Moreover, the detection of Hcy was not significantly affected by other amino acids from the competition experiments. Thus, BrPAPS-Cu²⁺ can be used as a simple probe for Hcy in aqueous solution.

Targeted copper supplementation oriented theranostic for fluorescence and 19F NMR detection of tumors

Alteration of the levels of copper is a promising approach for cancer therapy. Herein, we develop a dual-mode copper vehicle, M985. The biotin-tailed M985 can exert tumor-directed copper supplementation and undergo self-immolative cleavage in living cancerous cells, resulting in the liberation of F542 along with the generation of excess reactive oxygen species. Thus, fluorescence and 19F NMR detection is realized to specifically discriminate cancer cells. F542 acts as a fluorescence reporter and a potent cytotoxic agent, facilitating the visualization of molecular release and distribution, as well as confirming the ER autophagy-induced apoptosis. Therefore, we present a promising dual-mode theranostic M985 for the efficient detection and therapy of cancer.

Concurrent detection and treatment of cyanide-contaminated water using mechanosynthesized receptors

The development of receptors that can detect as well as treat cyanide ions in aqueous samples is indispensable for environmental protection. Herein, we present the bulk solvent-free and instant green synthesis of a series of turn-on fluorimetric probes that can specifically detect the deadly poison cyanide among various anions and metal ions in water. Selective recognition of cyanide by the mechanosynthesized compounds is even observable by the naked eyes, which remained unaffected in the presence of various challenging species. NMR spectroscopic investigation supports the chemodosimetric sensing of cyanide by the receptors. A remarkable 55-83 fold fluorescence enhancement by the probes enabled us to reach a limit of detection (LOD) in the range of 8-26 ppb, well below the permissible limit of cyanide in drinking water. Being minuscule soluble in water, cyanide treatment studies with the ionophores showed greater than 99% reduction in the free cyanide concentration after three consecutive cycles of operation. Furthermore, the compounds can be used as sensitive probes for the estimation of cyanide in human blood serum in physiological conditions. Overall, the results presented in this article will certainly find great use in the area of cyanide pollution with regard to simultaneous sensing and treatment of free cyanide, which is heretofore unprecedented.

Selectively monitoring glutathione in human serum and growth-associated living cells using gold nanoclusters

Glutathione (GSH) plays a variety of vital functions in biological systems. Growth-associated change of GSH level in cells might be critical for cell survival and monitoring of GSH in living cells are of great significance for understanding the dynamic link between GSH and some diseases. In this work, chitason micelles templated gold nanoclusters (CM-Au NCs) emitting red fluorescence were prepared with a simple and rapid method, which shows interesting phenomenon of aggregation induced emission (AIE) affected by the size of the chitosan micelles. The unique CM-Au NCs can be used to develop turn-off fluorescent probe for detecting GSH in human serum and living cells based on the reverse process of AIE of CM-Au NCs, completely different from the principle of aggregation caused quenching (ACQ) effect, which can distinguish GSH from other biothiols (cysteine and homocysteine) and quantitatively detect GSH concentration of human serum in healthy people and cancer patients with high sensitivity. The practical application of fluorescent CM-Au NCs for cellular imaging and detecting GSH level indicates ultra-trace changes of GSH levels in normal and cancer cells could be monitored at different growth stages, which reveals that the levels of GSH in cancer cells was always higher than that of normal cells. Compared with commercial GSH assay kits for detection GSH in human serum and living cells, the proposed method was verified to be accuracy and precision. The results not only reflect the changes of GSH during cell growth at different stages, but also demonstrate the feasibility of reverse process of AIE of CM-Au NCs for detection GSH. This strategy would provide a platform to understand the dynamic link between GSH and disease to clarify the disease mechanism.

Coumarin-Based Small-Molecule Fluorescent Chemosensors

Coumarins are a very large family of compounds containing the unique 2H-chromen-2-one motif, as it is known according to IUPAC nomenclature. Coumarin derivatives are widely found in nature, especially in plants and are constituents of several essential oils. Up to now, thousands of coumarin derivatives have been isolated from nature or produced by chemists. More recently, the coumarin platform has been widely adopted in the design of small-molecule fluorescent chemosensors because of its excellent biocompatibility, strong and stable fluorescence emission, and good structural flexibility. This scaffold has found wide applications in the development of fluorescent chemosensors in the fields of molecular recognition, molecular imaging, bioorganic chemistry, analytical chemistry, materials chemistry, as well as in the biology and medical science communities. This review focuses on the important progress of coumarin-based small-molecule fluorescent chemosensors during the period of 2012-2018. This comprehensive and critical review may facilitate the development of more powerful fluorescent chemosensors for broad and exciting applications in the future.

pH-Dependent Fluorescent Probe That Can Be Tuned for Cysteine or Homocysteine

The very close structural similarities between cysteine and homocysteine present a great challenge to achieve their selective detection using regular fluorescent probes, limiting the biological and pathological studies of these two amino thiols. A coumarin-based fluorescent probe was designed featuring pH-promoted distinct turn-on followed by ratiometric fluorescence responses for Cys and turn-on fluorescence response for Hcy through two different reaction paths. These specific responses demonstrate the activity differences between Cys and Hcy qualitatively for the first time. The probe could also be used for Cys and Hcy imaging in living cells.

Molecular visualizing and quantifying immune-associated peroxynitrite fluxes in phagocytes and mouse inflammation model

Reactions of peroxynitrite (ONOO^-) with biomolecules can lead to cytotoxic and cytoprotective events. Due to the difficulty of directly and unambiguously measuring its levels, most of the beneficial effects associated with ONOO^- in vivo remain controversial or poorly characterized. Recently, optical imaging has served as a powerful noninvasive approach to studying ONOO^- in living systems. However, ratiometric probes for ONOO^- are currently lacking. Herein, we report the design, synthesis, and biological evaluation of F₄₈₂, a novel fluorescence indicator that relies on ONOO^--induced diene oxidation. The remarkable sensitivity, selectivity, and photostability of F₄₈₂ enabled us to visualize basal ONOO^- in immune-stimulated phagocyte cells and quantify its generation in phagosomes by high-throughput flow cytometry analysis. With the aid of in vivo ONOO^- imaging in a mouse inflammation model assisted by F₄₈₂, we envision that F₄₈₂ will find widespread applications in the study of the ONOO^- biology associated with physiological and pathological processes in vitro and in vivo.