A Smart Molecule for Selective Sensing of Nitric Oxide: Conversion of NO to HSNO; Relevance of Biological HSNO Formation

Comprehensive insights into fluorescent probes for the determination nitric oxide for diseases diagnosis

Nitric oxide (NO) plays an important role in multiple physiological processes of the body involved in regulation, such as cardiovascular relaxation, neural homeostasis, and immune regulation, etc. The real-time monitoring of NO is of great significance in the investigation of related disease mechanisms and the evaluation of pharmacodynamics. Fluorescent probes are considered as a highly promising approach for pharmaceutical analysis and bioimaging due to their non-invasive character, real-time detection, and high sensitivity. However, there are still some challenges in the determination of biological nitric oxide with fluorescent probes, such as low anti-interference ability, poor function modifiability, and low organ specificity. Therefore, it would be beneficial to develop a new generation of fluorescent probes for real-time bioimaging of NO in vivo after this systematic summary.

Sulfur-based fluorescent probes for biological analysis: A review

The widespread adoption of small-molecule fluorescence detection methodologies in scientific research and industrial contexts can be ascribed to their inherent merits, including elevated sensitivity, exceptional selectivity, real-time detection capabilities, and non-destructive characteristics. In recent years, there has been a growing focus on small-molecule fluorescent probes engineered with sulfur elements, aiming to detect a diverse array of biologically active species. This review presents a comprehensive survey of sulfur-based fluorescent probes published from 2017 to 2023. The diverse repertoire of recognition sites, including but not limited to N, N-dimethylthiocarbamyl, disulfides, thioether, sulfonyls and sulfoxides, thiourea, thioester, thioacetal and thioketal, sulfhydryl, phenothiazine, thioamide, and others, inherent in these sulfur-based probes markedly amplifies their capacity for detecting a broad spectrum of analytes, such as metal ions, reactive oxygen species, reactive sulfur species, reactive nitrogen species, proteins, and beyond. Owing to the individual disparities in the molecular structures of the probes, analogous recognition units may be employed to discern diverse substrates. Subsequent to this classification, the review provides a concise summary and introduction to the design and biological applications of these probe molecules. Lastly, drawing upon a synthesis of published works, the review engages in a discussion regarding the merits and drawbacks of these fluorescent probes, offering guidance for future endeavors.

N-Nitrosation Based Fluorescence Turn-On Nitric Oxide Probe: Kinetic and Cell Imaging Studies

Nitric oxide (NO) is a ubiquitous messenger molecule playing a key role in various physiological and pathological processes. However, producing a selective turn-on fluorescence response to NO is a challenging task due to (a) the very short half-life of NO (typically in the range of 0.1-10 s) in the biological milieu and (b) false positive responses to reactive carbonyl species (RCS) (e.g., dehydroascorbic acid and methylglyoxal etc.) and some other reactive oxygen/nitrogen species (ROS/RNS), especially with o-phenylenediamine (OPD) based fluorosensors. To avoid these limitations, NO sensors should be designed in such a way that they react spontaneously with NO to give turn-on response within the time frame of t1/2 (typically in the range of 0.1-10 s) of NO and λem in the visible wavelength along with good cell permeability to achieve biocompatibility. With these views in mind, a N-nitrosation based fluorescent sensor, NDAQ, has been developed that is highly selective to NO with ∼27-fold fluorescence enhancement at λem = 542 nm with high sensitivity (LOD = 7 ± 0.4 nM) and shorter response time, eliminating the interference of other reactive species (RCS/ROS/RNS). Furthermore, all the photophysical studies with NDAQ have been performed in 98% aqueous medium at physiological pH, indicating its good stability under physiological conditions. The kinetic assay illustrates the second-order dependency with respect to NO concentration and first-order dependency with respect to NDAQ concentration. The biological studies reveal the successful application of the probe to track both endogenous and exogenous NO in living organisms.

Triphenylamine-embedded copper(II) complex as a "turn-on" fluorescent probe for the detection of nitric oxide in living animals

Nitric oxide (NO) is one of three major signaling molecules, which is involved in a large amount of physiological and pathological processes in biological systems. Furthermore, more and more evidence indicates that NO levels are closely associated with several aspects of human health. Accordingly, it is of great significance to develop a convenient and reliable detection method for NO in biological systems. In this work, a novel triphenylamine-embedded copper(II) complex (NZ-Cu²⁺) has been developed to be used as a fluorescence probe for the detection of NO in living animals. The proposed sensing mechanism of NZ-Cu²⁺ towards NO has been confirmed by high-resolution mass spectrometry, spectroscopic titration and density functional theory calculation. NO induced the conversion of paramagnetic Cu²⁺ to diamagnetic Cu⁺, which blocked the photoinduced electron transfer process of NZ-Cu²⁺, resulting in a remarkable enhancement of the emission spectra. The NZ-Cu²⁺ probe possesses several advantages including high selectivity, low detection limit (12.9 nM), long emission wavelength (640 nm), large Stokes shift (201 nm), fast response time (60 s) and low cytotoxicity. More importantly, NZ-Cu²⁺ has been successfully applied to detect NO in vivo by fluorescence imaging.

A novel dual-channel fluorescent probe for selectively and sensitively imaging endogenous nitric oxide in living cells and zebrafish

Nitric oxide (NO) plays various physiological and pathological roles in lots of biological processes. It is crucial to detect NO sensitively and selectively in vivo and in vitro as homeostasis of NO is closely related to various diseases. Herein, a novel dual-channel fluorescent dye (ENNH₂) based on dicarboxyimide anthracene was developed as a highly sensitive and selective probe to detect NO in living systems using the dual-channel fluorescence. ENNH₂ can emit bright red fluorescence due to the intramolecular charge transfer (ICT) from the amino group at the 6-position of 1,2-dicarboxyimide anthracene to the conjugated aromatic ring, and the ICT is effectively inhibited by the reductive deamination of the amino in the presence of NO to obtain the remarkable strong green emission with the excellent sensitivity (5.52 nM). Promisingly, ENNH₂ exhibits an excellent performance in endogenous NO dual-channel fluorescence imaging of RAW 264.7 cells and zebrafish.

An N-nitrosation reaction-based fluorescent probe for detecting nitric oxide in living cells and inflammatory zebrafish

Nitric oxide (NO), an essential biological messenger molecule, participates in various physiological and pathological processes. The sensitive and specific detection of NO is of great significance for understanding the biological function of NO. Here, we synthesized a fluorescent probe (Rho-NO) for highly selective detection of NO both in vitro and in vivo. The high selectivity of Rho-NO is attributed to the fact that NO is easily replaced by electron donor amino group to form N-nitrosation products, causing rhodamine spiro ring open and fluorescence emit. Rho-NO showed a good linear response to NO (0-100 µM) with a low detection limit (0.06 µM). Importantly, it exhibited excellent specificity for NO detection in human serum and was also applied for imaging NO in living cells and inflammatory model of zebrafish. This work proves the potential of Rho-NO in pathological research and disease diagnosis.

A coumarin embedded highly sensitive nitric oxide fluorescent sensor: kinetic assay and bio-imaging applications

Fluorescence spectroscopy is a significant bio-analytical technique for specific detection of nitric oxide (NO) and for broadcasting the in vitro and in vivo biological activities of this gasotransmitter. Herein, a benzo-coumarin embedded smart molecular probe (BCM) is employed for NO sensing through detailed fluorescence studies in purely aqueous medium. All the spectroscopic analysis and literature reports clearly validate the mechanistic insight of this sensing strategy i.e., the initial formation of 1,2,3,4-oxatriazole on treatment of the probe with NO which finally converted to its carboxylic acid derivative. This oxatriazole formation results in a drastic enhancement in fluoroscence intensity due to the photoinduced electron transfer (PET) effect. The kinetic investigation unveils the second and first-order dependency on [NO] and [BCM] respectively. The very low detection limit (16 nM), high fluorescence enhancement (123 fold) in aqueous medium and good formation constant (Kf = (4.33 ± 0.48) × 104 M-1) along with pH invariability, non-cytotoxicity, biocompatibility and cell permeability make this probe a very effective one for tracking NO intracellularly.

Recent progress on the organic and metal complex-based fluorescent probes for monitoring nitric oxide in living biological systems

Nitric oxide (NO) is an important gaseous signaling molecule related to various human diseases. To investigate the biological functions of NO, many strategies have been developed for real-time monitoring the NO levels in biological systems. Among these strategies, fluorescent probes are considered to be one of the most efficient and applicable methods owing to their excellent sensitivity and selectivity, high spatiotemporal resolution, noninvasiveness, and experimental convenience. Therefore, great efforts have been paid to the design, synthesis, and fluorescence investigation of novel NO fluorescent probes in the past several years. However, few of them exhibit practical applications owing to the low concentration, short half-life, and rapid diffusion characteristics of NO in biological systems. Rational design of NO fluorescent probes with excellent selectivity and sensitivity, low cytotoxicity, long-lived fluorescent emission, and low background interference is still a challenge for scientists all over the word. To provide spatial-temporal information, this article focuses on the progress made in the organic and metal complex-based NO fluorescent probes during the past five years. The key structural elements and sensing mechanisms of NO fluorescent probes are discussed. Some novel ratiometric, luminescence, and photoacoustic probes with low background interference and deep tissue penetrating ability are mentioned. All these probes have been used for imaging exogenous and endogenous NO in cells and animal models. More importantly, this article also describes the development of multi-functional NO fluorescent probes, such as organelle targeting probes, dual-analysis probes, and probe-drug conjugates, which will inspire the design of various functional fluorescent probes.

A near-infrared fluorescent probe for observing thionitrous acid-mediated hydrogen polysulfides formation and fluctuation in cells and in vivo under hypoxia stress

Hydrogen polysulfides (H₂S_n, n>1) as important intracellular reactive sulfur species (RSS) are believe to be responsible for cellular redox regulation. Lots of researches about H₂S_n focusing on their formation, detection and bio-function in signalling regulation are spring up but with poor understanding, especially for biosynthesis and bio-function remain complicated and confusing. Recent studies reveal that thionitrous acid (HSNO) as potential intermediate linked signalling molecules of nitrogenous and sulphureous during biotic redox regulation. However, there are limited evidences for supporting the interrelation and bioeffect between HSNO and H₂S_n. Herein, we have successfully designed a near-infrared (NIR) fluorescent probe ((2-fluoro-5-nitrobenzoyl)oxy)-Benzo[e]cyanine (BCy-FN) for detection H₂S_n and for the first time observing HSNO-mediated H₂S_n generation in cells and in vivo. The probe is harvested from fluorophore BCy-Keto and 2-fluoro-5-nitrobenzoic acid in one step, featuring mitochondria localization. The unique Enol-Keto tautomerization of fluorophore enables the probe becomes more sensitive and has powerful application. Hypoxia model has been constructed and powerfully interpreted the pretreatment of HSNO for zebrafish hypoxia process effectively improves H₂S_n levels and defends the hypoxia induced brain damage. We believe the present studies will help environmentalist and biologist for better understanding of biosynthesis and bio-function in HSNO-mediated H₂S_n formation process under hypoxia stress.

An Update on Thiol Signaling: S-Nitrosothiols, Hydrogen Sulfide and a Putative Role for Thionitrous Acid

Long considered vital to antioxidant defenses, thiol chemistry has more recently been recognized to be of fundamental importance to cell signaling. S-nitrosothiols—such as S-nitrosoglutathione (GSNO)—and hydrogen sulfide (H₂S) are physiologic signaling thiols that are regulated enzymatically. Current evidence suggests that they modify target protein function primarily through post-translational modifications. GSNO is made by NOS and other metalloproteins; H₂S by metabolism of cysteine, homocysteine and cystathionine precursors. GSNO generally acts independently of NO generation and has a variety of gene regulatory, immune modulator, vascular, respiratory and neuronal effects. Some of this physiology is shared with H₂S, though the mechanisms differ. Recent evidence also suggests that molecules resulting from reactions between GSNO and H₂S, such as thionitrous acid (HSNO), could also have a role in physiology. Taken together, these data suggest important new potential targets for thiol-based drug development.

Rapid and sensitive detection of nitric oxide by a BODIPY-based fluorescent probe in live cells: glutathione effects

Glutathione effects on the sensing reaction toward nitric oxide in live cells.

Saying NO to H2S: A Story of HNO, HSNO, and SSNO. Inorg Chem 2019;58:4039-4051. [PMID: 30883105 DOI: 10.1021/acs.inorgchem.8b02592]

Interactions between small inorganic molecules are fundamental to the understanding of basic reaction mechanisms and some of the initial processes of chemical evolution that preceded organic molecules and led to the origin of life. The kinetics of these processes are suitable for the fast generation of a variety of new chemical entities and the propagation of a cascade of chemical reactions, a property that is ideal for signaling purposes even in biological systems. NO and H₂S are such molecules that are nowadays recognized as biological gasotransmitters involved in the regulation of physiological functions through protein modifications such as S-nitrosothiol, disulfide, and persulfide formations. In this Viewpoint, we review the current understanding of interactions of NO (and organic and metal nitrosyl species) with H₂S, in both chemical and biochemical contexts. Through the formation of HNO, (H)SNO (and its isomers), (H)SSNO, and polysulfides, these two gasotransmitters initiate reaction networks with significant roles in cell signaling. The chemical reactivities and biological effects of these nitrogen and sulfur species are still unresolved, and, thus, a cross-talk between all of them represents a challenging interdisciplinary field that awaits exciting new findings. We tackle some of the intriguing and open questions and provide perspectives for future research directions.

Turn-On Fluorescence Probe for Nitric Oxide Detection and Bioimaging in Live Cells and Zebrafish

An effective bioanalytical method for rapid, sensitive, specific, and in situ sensing of nitric oxide (NO) is the key for further unveiling the biological functions of this gasotransmitter molecule in vitro and in vivo. In this contribution, a new fluorescence probe for sensing and imaging of NO in live systems was developed. The probe, FP-NO, was designed by exploring a novel sensing mechanism, i.e., the rotation of the N-N single bond of a coumarin derivative. FP-NO was prepared by incorporating a recognition unit, thiosemicarbazide moiety into a coumarin fluorophore. The weakly fluorescent FP-NO quickly and selectively reacts with NO to form a highly fluorescent product, FP-P. Such an enhancement of fluorescence emission allows NO detection with high sensitivity. The detection limit was 47.6 nM. The reaction mechanism was validated by HRMS titration analysis and the "OFF-ON" fluorescence response mechanism was rationalized by theoretical computation. FP-NO is biocompatible and live cell membrane permeable. The feasibility of FP-NO as the fluorescence probe for imaging and flow cytometry analysis of exogenous NO in MCF-7 cells and exogenous NO production in inflamed J774A.1 macrophage cells was then evaluated. Visualization of exogenous and endogenous NO production in live zebrafish was then achieved, implying the potential application of FP-NO in the studies of the NO roles in live organisms.

A smart molecular probe for selective recognition of nitric oxide in 100% aqueous solution with cell imaging application and DFT studies

A simple, least-cytotoxic as well as an efficient fluorescent sensor HqEN480 recognizes NO in 100% aqueous solution with cell imaging application.

BODIPY-based hydrazine as a fluorescent probe for sensitive and selective detection of nitric oxide: a new strategy

A novel fluorescent probe 8-HB was developed with a BODIPY as a fluorophore and 8-substituted hydrazine as a reactive site for sensitive and selective detection of nitric oxide (NO), generating major fluorescent dehydrazinated BODIPY and minor non-fluorescent azide BODIPY.

Rhodamine scaffolds as real time chemosensors for selective detection of bisulfite in aqueous medium

Rhodamine and its derivatives have been widely used in designing fluorescent ‘turn on’ cation sensors, while very few rhodamine based fluorescent probes have been reported to date for the detection of anions in water.

Selective sensing of nitric oxide by a 9,10-phenanthroquinone-pyridoxal based fluorophore

In this article, we have designed and synthesized a new, convenient and efficient phenanthroquinone-pyridoxal based fluorogenic probe PQPY, highly suitable for the selective and sensitive detection of nitric oxide in an aerated aqueous (7 : 3/H2O : MeCN) medium at pH 7.0 (10 mM HEPES buffer). Upon addition of nitric oxide, this probe exhibits emission in the green region (λem = 505 nm) which is ascribed to ICT (intramolecular charge transfer) from the phenanthroquinone moiety to the imidazole -N-N[double bond, length as m-dash]O fragment. The apparent formation constant, Kf, of the NO product of the ligand is (1.00 ± 0.2) × 105 M-1 and the LOD is 78 nM. The substantial enhancement of the life-time of the ligand (τ0 = 2.68 ns) occurs due to binding with nitric oxide (τ0 = 3.96 ns). This probe is low cytotoxicity, cell permeable and suitable for living cell imaging application.

Design of a Pyrene Scaffold Multifunctional Material: Real-Time Turn-On Chemosensor for Nitric Oxide, AIEE Behavior, and Detection of TNP Explosive

A dual-emission pyrene-based new fluorescent probe (N-(4-nitro-phenyl)-N'-pyren-1-ylmethyl-ene-ethane-1,2-diamine (PyDA-NP)) displays green fluorescence for nitric oxide (NO) sensing, whereas it exhibits blue emission in the aggregated state. The mechanism of nitric oxide (NO/NO+) sensing is based on N-nitrosation of aromatic secondary amine, which was not interfered by reactive oxygen species and reactive nitrogen species. The aggregation-induced enhancement of emission (AIEE) behaviors of the PyDA-NP could be attributed to the restriction of intramolecular rotation and vibration, resulting in rigidity enhancement of the molecules. The AIEE behavior of the probe was well established from fluorescence, dynamic light scattering, scanning electron microscopy, transmission electron microscopy, X-ray diffraction, optical fluorescence microscopy, and time-resolved photoluminescence studies. In a H2O/CH3CN binary mixture (8:2 v/v), the probe showed maximum aggregation with extensive (833-fold) increases in fluorescence intensity and high quantum yield (0.79). The aggregated state of the probe was further applied for the detection of nitroexplosives. It displayed efficient sensing of 2,4,6-trinitrophenol (TNP), corroborating mainly the charge-transfer process from pyrene to a highly electron-deficient TNP moiety. Furthermore, for the on-site practical application of the proposed analytical system, a contact-mode analysis was performed.

Chemical Biology of H2S Signaling through Persulfidation

Signaling by H2S is proposed to occur via persulfidation, a posttranslational modification of cysteine residues (RSH) to persulfides (RSSH). Persulfidation provides a framework for understanding the physiological and pharmacological effects of H2S. Due to the inherent instability of persulfides, their chemistry is understudied. In this review, we discuss the biologically relevant chemistry of H2S and the enzymatic routes for its production and oxidation. We cover the chemical biology of persulfides and the chemical probes for detecting them. We conclude by discussing the roles ascribed to protein persulfidation in cell signaling pathways.

A rhodamine-based turn-on nitric oxide sensor in aqueous medium with endogenous cell imaging: an unusual formation of nitrosohydroxylamine

The sensor L3 selectively recognizes NO in purely aqueous medium with an unusual formation of nitrosohydroxylamine with a turn-on fluorescence response which might be suitable for in vivo application.

Fundamental chemistry of binary S,N and ternary S,N,O anions: analogues of sulfur oxides and N,O anions

The fundamental chemistry and significance of S,N and S,N,O anions and their conjugate acids in a variety of settings are discussed.

A ratiometric mitochondria-targeting two-photon fluorescent probe for imaging of nitric oxide in vivo

A two-photon ratiometric fluorescent probe (Mito-N) has been developed for monitoring mitochondrial nitric oxide (NO) in vivo.