Study of the Energy Crossing Between Excited States Affected by the Electronegativity of Substituents for Three 4-Azido-1,8-naphthalimide Derivatives

Rapid detection of H2S is crucial for human physiological health and natural ecosystems. In this study, the fluorescent sensing mechanisms of three 4-azido-1,8-naphthalimide-based fluorescent probes to monitor H2S were theoretically investigated by density functional theory and time-dependent density functional theory. The potential energy curve of the charge transfer (CT) state has a crossover with that of the locally excited (LE) state proved by the constructed linear interpolating internal coordinate pathway. Thus, the transform takes place from the LE state to the CT state causing the fluorescence quenching of the probes from a nonradiative transition process of the CT state. The distance between the Franck-Condon point and the minimal energy conical intersection becomes larger with the increase of the electronegativity of substituents on the 1,8-naphthalimide fluorophore. In addition, the degree of charge separation is closely related to the energy difference between the CT and the LE states which are also essentially affected by the electronegativity of the substituents. Since the electronegativity of the substituents has proved important for the probes, our work lays a certain theoretical foundation for the design and synthesis of more sensitive 4-azido-1,8-naphthalimide-based fluorescent probes.

Activity-Based Fluorescent Probes for Hydrogen Sulfide and Related Reactive Sulfur Species

Hydrogen sulfide (H2S) is not only a well-established toxic gas but also an important small molecule bioregulator in all kingdoms of life. In contemporary biology, H2S is often classified as a "gasotransmitter," meaning that it is an endogenously produced membrane permeable gas that carries out essential cellular processes. Fluorescent probes for H2S and related reactive sulfur species (RSS) detection provide an important cornerstone for investigating the multifaceted roles of these important small molecules in complex biological systems. A now common approach to develop such tools is to develop "activity-based probes" that couple a specific H2S-mediated chemical reaction to a fluorescent output. This Review covers the different types of such probes and also highlights the chemical mechanisms by which each probe type is activated by specific RSS. Common examples include reduction of oxidized nitrogen motifs, disulfide exchange, electrophilic reactions, metal precipitation, and metal coordination. In addition, we also outline complementary activity-based probes for imaging reductant-labile and sulfane sulfur species, including persulfides and polysulfides. For probes highlighted in this Review, we focus on small molecule systems with demonstrated compatibility in cellular systems or related applications. Building from breadth of reported activity-based strategies and application, we also highlight key unmet challenges and future opportunities for advancing activity-based probes for H2S and related RSS.

Small Molecule Optical Probes for Detection of H2S in Water Samples: A Review

Hydrogen sulfide (H2S) is closely linked to not only environmental hazards, but also it affects human health due to its toxic nature and the exposure risks associated with several occupational settings. Therefore, detection of this pollutant in water sources has garnered immense importance in the analytical research arena. Several research groups have devoted great efforts to explore the selective as well as sensitive methods to detect H2S concentrations in water. Recent studies describe different strategies for sensing this ubiquitous gas in real-life water samples. Though many of the designed and developed H2S detection approaches based on the use of organic small molecules facilitate qualitative/quantitative detection of the toxic contaminant in water, optical detection has been acknowledged as one of the best, attributed to the simple, highly sensitive, selective, and good repeatability features of the technique. Therefore, this review is an attempt to offer a general perspective of easy-to-use and fast response optical detection techniques for H2S, fluorimetry and colorimetry, over a wide variety of other instrumental platforms. The review affords a concise summary of the various design strategies adopted by various researchers in constructing small organic molecules as H2S sensors and offers insight into their mechanistic pathways. Moreover, it collates the salient aspects of optical detection techniques and highlights the future scope for prospective exploration in this field based on the limitations of the existing H2S probes.

Biocompatible 7-nitro-2,1,3-benzoxadiazole-embedded naphthalimide for exploring endogenous H2S in living cells

Hydrogen sulfide (H₂S) is a central signaling and antioxidant biomolecule involved in various biological processes. As inappropriate levels of H₂S in the human body are closely related to various diseases, including cancer, a tool capable of detecting H₂S with high selectivity and sensitivity in living systems is urgently required. In this work, we intended to develop a biocompatible and activatable fluorescent molecular probe for detecting H₂S generation in living cells. The 7-nitro-2,1,3-benzoxadiazole-imbedded naphthalimide (1) probe presented here responds specifically to H₂S and produces readily detectable fluorescence at 530 nm. Interestingly, probe 1 exhibited significant fluorescence responses to changes in endogenous H₂S levels as well as high biocompatibility and permeability in living HeLa cells. This allowed for the real-time monitoring of endogenous H₂S generation as an antioxidant defense response in the oxidatively stressed cells.

A chemical tool for blue light-inducible proximity photo-crosslinking in live cells

We developed a proximity photo-crosslinking method (Spotlight) with a 4-azido-N-ethyl-1,8-naphthalimide (AzNP) moiety that can be converted to reactive aryl nitrene species using ambient blue light-emitting diode light. Using an AzNP-conjugated HaloTag ligand (VL1), blue light-induced photo-crosslinked products of various HaloTag-conjugated proteins of interest were detected in subcellular spaces in live cells. Chemical or heat stress-induced dynamic changes in the proteome were also detected, and photo-crosslinking in the mouse brain tissue was enabled. Using Spotlight, we further identified the host interactome of SARS-CoV-2 nucleocapsid (N) protein, which is essential for viral genome assembly. Mass analysis of the VL1-crosslinked product of N-HaloTag in HEK293T cells showed that RNA-binding proteins in stress granules were exclusively enriched in the cross-linked samples. These results tell that our method can reveal the interactome of protein of interest within a short distance in live cells.

Hydrogen sulfide sensing using an aurone-based fluorescent probe

Hydrogen sulfide detection and sensing is an area of interest from both an environmental and a biological perspective. While many methods are currently available, the most sensitive and biologically applicable ones are fluorescence based. In general, these fluorescent probes are based upon large, high-molecular weight, well-characterized fluorescent scaffolds that are synthetically demanding to prepare and difficult to tune and modify. In this study, we have reported a new reduction-based, rationally designed and synthesized turn-on fluorescent probe (Z)-2-(4′-azidobenzylidene)-5-fluorobenzofuran-3(2H)-one (6g) utilizing a low molecular weight aurone fluorophore. During these studies, the modular nature of the synthesis was used to quickly overcome problems with solubility, overlap of excitation of the probe and reduced product, and rate of reaction, resulting in a final compound that is efficient and sensitive for the detection of hydrogen sulfide. The limitation of slow reaction and the reduced fluorescence in a biologically relevent medium was solved by employing cationic surfactant cetyltrimethyl ammonium bromide (CTAB). The probe features a high fluorescence enhancement, fast response (10–30 min), and good sensitivity (1 μm) and selectivity for hydrogen sulfide.

Hydrogen sulfide detection and sensing is an area of interest from both an environmental and a biological perspective.

Synthesis and application of a new chemosensor based on the thiazolylazo-quinazolinone hybrid for detection of F- and S2- in aqueous solutions

A new chemosensor based on the thiazolylazo-quinazolinone hybrid (TAQH) was designed and synthesized for naked-eye sensitive detection of F^- and S^2-in aqueous acetonitrile solution. Spectral characterization of TAQH using FT-IR, ¹H NMR, and ¹³C NMR analysis revealed that the probe TAQH was successfully synthesized using a two steps reaction, including the diazotization-coupling and condensation reactions, respectively. The ion sensing ability of TAQH toward a wide range of anions and metal ions was evaluated by naked-eye detection method and UV-Vis absorption spectroscopy. The chemosensor TAQH displayed a fast and clear color change from yellow to red in the presence of F^- and S^2- ions, enabling easily detect with the naked eye. This clear color change is due to the effective interaction of the basic F^- and S^2- anions with hydroxyl group of chemosensor as a binding site. The experimental data also revealed that the F^- and S^2- ions were sensed by the probe TAQH over a wide pH range from 3 to 8. The results also confirmed that the TAQH has a wide linear detection range for F^- and S^2- ions. From UV-vis titration experiment, the limit of detection (LOD) for F^- and S^2- ions was found to be 3.1 μM and 5.7 μM, respectively. For quantitative measurements, the paper test strips containing TAQH were successfully fabricated and applied to detect F^- and S^2- ions in aqueous solutions. Furthermore, Job's plot based on spectroscopic data showed one-to-one stoichiometry for the interaction of anions with probe TAQH. Therefore, the proposed chemosensor with excellent features like the cost-effective, high sensitively and selectively and short response times can be utilized in any physical and biological conditions.

Fluorescent probe for detecting hydrogen sulfide based on disulfide nucleophilic substitution-addition

In view of the importance of hydrogen sulfide (H₂S) in the organism, a fast, noninvasive method for the detection of H₂S in situ is needed. Fluorescent probes based on disulfide-bond nucleophilic substitution-addition can selectively detect H₂S in vivo, which is very popular because it allows quick response for H₂S, thus it will be a useful tool for monitoring H₂S in the vivo. We developed a dicyanoisopentanone-based H₂S fluorescent probe (EW-H) that used a disulfide group as a self-destructive linker reaction site. Under the nucleophilic substitution of H₂S, the disulfide bond of EW-H was cleaved, and then nucleophilic addition took place intramolecularly to release the fluorophore (at 580 nm). The response to H₂S, EW-H had high sensitivity (86 nM of the detection limit), large Stokes shift (155 nm) and a fast response time. More importantly, the probe was also applied for bioimaging in HepG2 cells, indicating its potential applications in biological organism.

Ratiometric detection and imaging of hydrogen sulfide in mitochondria based on a cyanine/naphthalimide hybrid fluorescent probe

A novel fluorescent probe (L1) for ratiometric detection and imaging of H₂S in mitochondria was developed by combining a H₂S-sensitive naphthalimide fluorophore and a mitochondria targeting cyanine fluorophore.

A hydrogen sulfide triggered self-immolative fluorescent probe for lysosome labeling in live cells

We developed a naphthalimide-based, lysosome-targeting, and self-immolative fluorescent probe for H₂S detection.

Use of azidonaphthalimide carboxylic acids as fluorescent templates with a built-in photoreactive group and a flexible linker simplifies protein labeling studies: applications in selective tagging of HCAII and penicillin binding proteins

A simple design of versatile template-based protein labeling agents has been successfully demonstrated with HCA and PBPs.

Fluorescent Probes for H2S Detection and Quantification

Many diverse, sensitive and structurally novel fluorescent probes have recently been reported for H2S detection. Quantification of H2S requires a selective chemosensor which will react only with H2S against a background of high concentrations of other thiols or reducing agents. Most published probes are able to quantify H2S selectively in a simple in vitro system with the most sensitive probes able to detect H2S at below 100 nM concentrations. A subset of probes also have utility in sensing H2S in living cells, and there are now several with specific sub-cellular localization and a few cases of in vivo applications. Biologists studying H2S now have a wide range of tools to assist them to aid further understanding of the role of H2S in biology.

A novel 3,6-diamino-1,8-naphthalimide derivative as a highly selective fluorescent “turn-on” probe for thiols

A novel 3,6-diamino-1,8-naphthalimide-based fluorescent “turn-on” probe exhibited high selectivity and low detection limit toward Cys.