A highly selective FRET-based fluorescent probe for detection of cysteine and homocysteine

Quenching and enhancement mechanisms of a novel Cd-based coordination polymer as a multiresponsive fluorescent sensor for nitrobenzene and aniline

BACKGROUND

Nitroaromatic compounds are inherently hazardous and explosive, so convenient and rapid detection strategies are needed for the sake of human health and the environment. There is an urgent demand for chemical sensing materials that offer high sensitivity, operational simplicity, and recognizability to effectively monitor nitroaromatic residues in industrial wastewater. Despite its importance, the mechanisms underlying fluorescence quenching or enhancement in fluorescent sensing materials have not been extensively researched. The design and synthesis of multiresponsive fluorescent sensing materials have been a great challenge until now.

RESULTS

In this study, a one-dimensional Cd-based fluorescent porous coordination polymer (Cd-CIP-1) was synthesized using 5-(4-cyanobenzyl)isophthalic acid (5-H2CIP) and 4,4'-bis(1-imidazolyl)biphenyl (4,4'-bimp) and used for the selective detection of nitrobenzene in aqueous solution by fluorescence quenching, with a limit of detection of 1.38 × 10-8 mol L-1. The presence of aniline in the Cd-CIP-1 solution leads to the enhancement of fluorescence property. Density functional theory and time-dependent density functional theory calculations were carried out to elucidate the mechanisms of the fluorescence changes. This study revealed that the specific pore size of Cd-CIP-1 facilitates analyte screening and enhances host-guest electron coupling. Furthermore, π-π interactions and hydrogen bond between Cd-CIP-1 and the analytes result in intermolecular orbital overlap and thereby boosting electron transfer efficiency. The different electron flow directions in NB@Cd-CIP-1 and ANI@Cd-CIP-1 lead to fluorescence quenching and enhancement.

SIGNIFICANCE AND NOVELTY

The multiresponsive coordination polymer (Cd-CIP-1) can selectively detect nitrobenzene and recognize aniline in aqueous solutions. The mechanism of fluorescence quenching and enhancement has been thoroughly elucidated through a combination of density functional theory and experimental approaches. This study presents a promising strategy for the practical implementation of a multiresponsive fluorescent chemical sensor.

Isoindolium-Based Allenes: Reactivity Studies and Applications in Fluorescence Temperature Sensing and Cysteine Bioconjugation

The reaction of a series of electron-deficient isoindolium-based allenes with sulfhydryl compounds has been studied, leading to the formation of isoindolium-based vinyl sulfides. The vinyl sulfides generated could be readily converted into the corresponding indanones and amines upon heating at 30-70 °C with good yields up to 61 %. The thermal cleavage reaction of vinyl sulfides was further studied for developing temperature-sensitive systems. Notably, a novel FRET-based fluorescent temperature sensor was designed and synthesized for temperature sensing at 50 °C, giving a 6.5-fold blue fluorescence enhancement. Moreover, chemoselective bioconjugation of cysteine-containing peptides with the isoindolium-based allenes for the construction of multifunctional peptide bioconjugates was investigated. Thermal cleavage of isoindoliums on the modified peptides at 35-70 °C gave indanone bioconjugates with up to >99 % conversion. These results indicated the biocompatibility of this novel temperature-sensitive reaction.

Functionalized quinolizinium-based fluorescent reagents for modification of cysteine-containing peptides and proteins

A series of quinolizinium-based fluorescent reagents were prepared by visible light-mediated gold-catalyzed cis-difunctionalization between quinolinium diazonium salts and electron-deficient alkyne-linked phenylethynyl trimethylsilanes. The electron-deficient alkynyl group of the quinolizinium-based fluorescent reagents underwent nucleophilic addition reaction with the sulfhydryl group on cysteine-containing peptides and proteins. The quinolizinium-based fluorescent reagents were found to function as highly selective reagents for the modification of cysteine-containing peptides and proteins with good to excellent conversions (up to 99%). Moreover, the modified BCArg mutants bearing cationic quinolizinium compounds 1b, 1d, 1e and 1h exhibit comparable activity in enzymatic and cytotoxicity assays to the unmodified one.

A Facile Probe for Fluorescence Turn-on and Simultaneous Naked-Eyes Discrimination of H2S and biothiols (Cys and GSH) and Its Application

Hydrogen sulfide and biothiol molecules such as Cys and GSH acted important roles in many physiological processes. To simultaneously detect and distinguish them was quite necessary by a suitable fluorescent probe. A novel chemosensor 4-(4-(benzo[d]thiazol-2-yl)-2-methoxyphenoxy)-7-nitrobenzo[c][1,2,5]oxadiazole (BMNO) was designed to detect H₂S/Cys/GSH using the combination of nitrobenzofurazan (NBD) and benzothiazole fluorophores linked by a facile ether bond. The probe BMNO was developed for simultaneous identification of H₂S, Cys and GSH. Noticeably, the color changes (from colorless to light purple, light orange and light yellow) of probe BMNO solutions for sensing H₂S, Cys and GSH could be observed by naked eyes, respectively. The probe BMNO exhibited high selectivity and sensitivity for H₂S, Cys and GSH showing distinct optical signal with detection limit as low as 0.15 μM, 0.03 μM and 0.14 μM, respectively. The sensing mechanism was clarified by spectrum analysis and some controlled experiments. In addition, these outstanding properties of probe BMNO enabled its practical applications in detection H₂S in beer, and in cell imaging for Cys and GSH as well.

Click Nucleophilic Conjugate Additions to Activated Alkynes: Exploring Thiol-yne, Amino-yne, and Hydroxyl-yne Reactions from (Bio)Organic to Polymer Chemistry

The 1,4-conjugate addition reaction between activated alkynes or acetylenic Michael acceptors and nucleophiles (i.e., the nucleophilic Michael reaction) is a historically useful organic transformation. Despite its general utility, the efficiency and outcomes can vary widely and are often closely dependent upon specific reaction conditions. Nevertheless, with improvements in reaction design, including catalyst development and an expansion of the substrate scope to feature more electrophilic alkynes, many examples now present with features that are congruent with Click chemistry. Although several nucleophilic species can participate in these conjugate additions, ubiquitous nucleophiles such as thiols, amines, and alcohols are commonly employed and, consequently, among the most well developed. For many years, these conjugate additions were largely relegated to organic chemistry, but in the last few decades their use has expanded into other spheres such as bioorganic chemistry and polymer chemistry. Within these fields, they have been particularly useful for bioconjugation reactions and step-growth polymerizations, respectively, due to their excellent efficiency, orthogonality, and ambient reactivity. The reaction is expected to feature in increasingly divergent application settings as it continues to emerge as a Click reaction.

Fluorescent Probes for Live Cell Thiol Detection

Thiols play vital and irreplaceable roles in the biological system. Abnormality of thiol levels has been linked with various diseases and biological disorders. Thiols are known to distribute unevenly and change dynamically in the biological system. Methods that can determine thiols' concentration and distribution in live cells are in high demand. In the last two decades, fluorescent probes have emerged as a powerful tool for achieving that goal for the simplicity, high sensitivity, and capability of visualizing the analytes in live cells in a non-invasive way. They also enable the determination of intracellular distribution and dynamitic movement of thiols in the intact native environments. This review focuses on some of the major strategies/mechanisms being used for detecting GSH, Cys/Hcy, and other thiols in live cells via fluorescent probes, and how they are applied at the cellular and subcellular levels. The sensing mechanisms (for GSH and Cys/Hcy) and bio-applications of the probes are illustrated followed by a summary of probes for selectively detecting cellular and subcellular thiols.

Fluorescent Imaging for Cysteine Detection In Vivo with High Selectivity

As an essential amino acid, cysteine is involved in various biosynthetic and metabolic processes, such as protein synthesis, hormone synthesis, and redox homeostatic maintenance. Inordinate cysteine levels are often associated with serious diseases. Thus, designing and synthesizing a novel fluorescent probe for determining the concentration of cellular cysteine, which could indirectly monitor the prevalence of these diseases, is essential. We developed a florescence probe P-Cy with good sensitivity for cysteine detection in vivo. P-Cy only exhibited good response toward cysteine but did not show response toward other biothiols, such as homocysteine (Hcy) and glutathione (GSH). In this study, we used P-Cy by successfully imaging cellular endogenous and exogenous cysteine levels. Furthermore, P-Cy was also performed in mice to detect cysteine level, indicating that P-Cy is a powerful tool for cysteine detection in situ.

A specific fluorescent probe for fast detection and cellular imaging of cysteine based on a water-soluble conjugated polymer combined with copper(II)

In pure water system, the specific and rapid detection of cysteine (Cys) is very important and challenging. Herein, a new optical probe was developed for the purpose based on the complex of cupric ion (Cu²⁺) with a water-soluble conjugated polymer, poly[3-(3-N,N-diacetateaminopropoxy)-4-methyl thiophene disodium salts] (PTCO₂). The fluorescence of PTCO₂ in 100% aqueous solution can almost completely extinguished by Cu²⁺ ions due to its intrinsic paramagnetic properties. Among various amino acids, only Cys causes immediately the efficient recovery of the Cu²⁺-quenched fluorescence of PTCO₂ with ~31-folds fluorescence enhancement because of the stronger affinity of Cys to Cu²⁺ leading to the formation of Cu²⁺-Cys complex through Cu-S bond and separation of Cu²⁺ from weak-fluorescent PTCO₂-Cu(II) ensemble and thereby restoring the free PTCO₂ fluorescence. In tris-HCl buffer solution (2 mM, pH 7.4), the intensity of the restored fluorescence is linear with the concentration of Cys, ranging from 0 to 120 μM and the estimated detection limit of Cys is 3.3 × 10^-7 M with the correlation coefficient R = 0.9981. In addition, the PTCO₂-Cu(II) ensemble probe exhibits low cytotoxicity and good membrane penetration, and its application in living cell imaging of Cys has also been explored.

Fluorescein propiolate: a propiolate-decorated fluorescent probe with remarkable selectivity towards cysteine

A fluorescent probe decorated with an alkynyl ester unit (e.g. propiolate) displayed a selective turn-on type fluorescent response towards cysteine.

Two-isophorone fluorophore-based design of a ratiometric fluorescent probe and its application in the sensing of biothiols

A newly designed ratiometric fluorescent probe is applied in the sensing of biothiols.

Bioconjugation with Maleimides: A Useful Tool for Chemical Biology

Maleimide chemistry stands out in the bioconjugation toolbox by virtue of its synthetic accessibility, excellent reactivity, and practicability. The second-generation of clinically approved antibody-drug conjugates (ADC) and much of the current ADC pipeline in clinical trials contain the maleimide linkage. However, thiosuccinimide linkages are now known to be less robust than once thought, and ergo, are correlated with suboptimal pharmacodynamics, pharmacokinetics, and safety profiles in some ADC constructs. Rational design of novel generations of maleimides and maleimide-type reagents have been reported to address the shortcomings of classical maleimides, allowing for the formation of robust bioconjugate linkages. This review highlights the main strategies for rational reagent design that have allowed irreversible bioconjugations in cysteines, reversible labelling strategies and disulfide re-bridging.

Detection of biological thiols based on a colorimetric method

Biological thiols (biothiols), an important kind of functional biomolecules, such as cysteine (Cys) and glutathione (GSH), play vital roles in maintaining the stability of the intracellular environment. In past decades, studies have demonstrated that metabolic disorder of biothiols is related to many serious disease processes and will lead to extreme damage in human and numerous animals. We carried out a series of experiments to detect biothiols in biosamples, including bovine plasma and cell lysates of seven different cell lines based on a simple colorimetric method. In a typical test, the color of the test solution could gradually change from blue to colorless after the addition of biothiols. Based on the color change displayed, experimental results reveal that the percentage of biothiols in the embryonic fibroblast cell line is significantly higher than those in the other six cell lines, which provides the basis for the following biothiols-related study.

Amplified Photodegradation of Cell-Laden Hydrogels via an Addition-Fragmentation Chain Transfer Reaction

Hydrogels crosslinked by allyl-sulfide-containing molecules are presented. By exposure to light in the presence of a photoinitiator and a free monofunctional thiol, photodegradation is achieved. Both the gelation and degradation are cytocompatible and allow for cell encapsulation and subsequent release. The photodegradation kinetics and depths attainable are superior to those of traditional cell-laden photodegradable hydrogels.

A polymer-based probe for specific discrimination of cysteine

A kind of polymer-based turn-on fluorescent probe for specific detection of cysteine with high sensitivity has been developed.

A simple colorimetric and fluorescent probe with high selectivity towards cysteine over homocysteine and glutathione

A novel turn-on fluorescent sensor AQDA with high selective towards cysteine.

Target identification with quantitative activity based protein profiling (ABPP)

As many small bioactive molecules fulfill their functions through interacting with protein targets, the identification of such targets is crucial in understanding their mechanisms of action (MOA) and side effects. With technological advancements in target identification, it has become possible to accurately and comprehensively study the MOA and side effects of small molecules. While small molecules with therapeutic potential were derived solely from nature in the past, the remodeling and synthesis of such molecules have now been made possible. Presently, while some small molecules have seen successful application as drugs, the majority remain undeveloped, requiring further understanding of their MOA and side effects to fully tap into their potential. Given the typical promiscuity of many small molecules and the complexity of the cellular proteome, a high-flux and high-accuracy method is necessary. While affinity chromatography approaches combined with MS have had successes in target identification, limitations associated with nonspecific results remain. To overcome these complications, quantitative chemical proteomics approaches have been developed including metabolic labeling, chemical labeling, and label-free methods. These new approaches are adopted in conjunction with activity-based protein profiling (ABPP), allowing for a rapid process and accurate results. This review will briefly introduce the principles involved in ABPP, then summarize current advances in quantitative chemical proteomics approaches as well as illustrate with examples how ABPP coupled with quantitative chemical proteomics has been used to detect the targets of drugs and other bioactive small molecules including natural products.

A novel displacement-type colorimetric chemosensor for the detection of Cu2+and GSH in aqueous solution

A new selective and sensitive chemosensor1for the sequential detection of Cu²⁺and GSHvianaked-eye has been developed.

Single fluorescent probe for reversibly detecting copper ions and cysteine in a pure water system

In this work, we have engineered a novel fluorescent probe PI, which remarkably can reversible detect copper ion and cysteine in pure water system for the first time.

Orthogonal thiol functionalization at a single atomic center for profiling transthiolation activity of E1 activating enzymes

Transthiolation is a fundamental biological reaction and is utilized by many enzymes involved in the conjugation of ubiquitin and ubiquitin-like proteins. However, tools that enable selective profiling of this activity are lacking. Transthiolation requires cysteine-cysteine juxtaposition; therefore a method that enables irreversible "stapling" of proximal thiols would facilitate the development of novel probes that could be used to profile this activity. Herein, we characterize biocompatible chemistry that enables sequential functionalization of cysteines within proteins at a single atomic center. We use our method to develop a new class of activity-based probe that profiles transthiolation activity of human E1 activating enzymes. We demonstrate use in vitro and in situ and compatibility with competitive activity-based protein profiling. We also use the probe to gain insight into the determinants of transthiolation between E2 and a RING-in-between-RING (RBR) E3 ligase. Furthermore, we anticipate that this method of thiol functionalization will have broad utility by enabling simple redox-stable cross-linking of proximal cysteines in general.

Developments and recent advancements in the field of endogenous amino acid selective bond forming reactions for bioconjugation

Bioconjugation methodologies have proven to play a central enabling role in the recent development of biotherapeutics and chemical biology approaches. Recent endeavours in these fields shed light on unprecedented chemical challenges to attain bioselectivity, biocompatibility, and biostability required by modern applications. In this review the current developments in various techniques of selective bond forming reactions of proteins and peptides were highlighted. The utility of each endogenous amino acid-selective conjugation methodology in the fields of biology and protein science has been surveyed with emphasis on the most relevant among reported transformations; selectivity and practical use have been discussed.

A dual-response BODIPY-based fluorescent probe for the discrimination of glutathione from cystein and homocystein

In situ monitoring of intracellular thiol activity in cell growth and function is highly desirable. However, the discriminative detection of glutathione (GSH) from cysteine (Cys) and homocystein (Hcy) and from common amino acids still remains a challenge due to the similar reactivity of the thiol groups in these amino acids. Here we report a novel strategy for selectively sensing GSH by a dual-response mechanism. Integrating two independent reaction sites with a disulfide linker and a thioether function into a fluorescent BODIPY-based chemsensor can guarantee the synergetic dual-response in an elegant fashion to address the discrimination of GSH. In the first synergetic reaction process, the thiol group in GSH, Cys and Hcy induces disulfide cleavage and subsequent intramolecular cyclization to release the unmasked phenol-based BODIPY (discriminating thiol amino acids from other amino acids). In the second synergetic process, upon the substitution of the thioether with the nucleophilic thiolate to form a sulfenyl-BODIPY, only the amino groups of Cys and Hcy, but not that of GSH, undergo a further intramolecular displacement to yield an amino-substituted BODIPY. In this way, we make full use of the kinetically favorable cyclic transition state in the intramolecular rearrangement, and enable photophysical distinction between sulfenyl- and amino-substituted BODIPY for allowing the discriminative detection of GSH over Cys and Hcy and thiol-lacking amino acids under physiological conditions. Moreover, this probe exhibits a distinguishable ratiometric fluorescence pattern generated from the orange imaging channel to the red channel, which proves the differentiation of GSH from Cys and Hcy in living cells.

Thiol-derivatized minihepcidins retain biological activity

Minihepcidins are small peptides that mimic biological activity of the iron-regulatory hormone hepcidin. Structurally, they contain thiol-free-cysteine residue in position 7 which is crucial for their bioactivity. Nonetheless, free sulfhydryl group is not desirable in pharmaceutical entities as it may lead to dermatological side effects. Moreover free thiol moiety is quite reactive and depending on conditions/reagents may be alkylated and/or oxidized giving various Cys-derivatives: S-alkyl cysteines, sulfoxides, sulfones, disulfides, cysteinesulfinic and cysteic acids. To limit such reactivity and maintain bioactivity of minihepcidin(s) we used thiol-protection strategy based on activated vinyl thioethers. Novel S-protected analogs of physiologically active minihepcidin PR73 were synthesized and tested in vitro showing activity comparable to parental molecule. The most active compound, PR73SH was also tested in vivo showing activity profile analogous to PR73. Collectively, our findings suggest that S-vinyl-derivatization of minihepcidin(s) may be a suitable approach in the development of physiologically active agonists of hepcidin.

Constructing a FRET-based molecular chemodosimeter for cysteine over homocysteine and glutathione by naphthalimide and phenazine derivatives

A chemodosimeter PHSN achieved turn-on fluorescence enhancement at 540 nm and discrimination of cysteine from homocysteine and glutathione in Hela cells.

A phthalimide-based fluorescent probe for thiol detection with a large Stokes shift

A phthalimide-based fluorescent probe for thiols with a large Stokes shift (161 nm) was developed via PET and ESIPT mechanisms. This probe displayed good selectivity and high sensitivity toward thiols. Imaging intracellular thiols was successfully achieved in living cells.

Phosphorescent proteins for bio-imaging and site selective bio-conjugation of peptides and proteins with luminescent cyclometalated iridium(III) complexes

A new bio-conjugation reaction for site selective modification of proteins and peptides with phosphorescent iridium(III) complexes has been developed; the Ir(III)-modified proteins and peptides display long emission lifetimes and large Stoke shifts that can be used for bio-imaging studies.

A near-infrared colorimetric fluorescent chemodosimeter for the detection of glutathione in living cells

A novel near-infrared (NIR) and colorimetric fluorescent molecular probe based on a dicyanomethylene-4H-pyran chromophore for the selective detection of glutathione in living cells has been developed. The fluorescence OFF-ON switch is triggered by cleavage of the 2,4-dinitrobenzensulfonyl (DNBS) unit by the interaction with GSH.

A dual emission fluorescent probe enables simultaneous detection of glutathione and cysteine/homocysteine

Many studies have shown that glutathione (GSH) and cysteine (Cys) / homocysteine (Hcy) levels are interrelated in biological systems. To unravel the complicated biomedical mechanisms by which GSH and Cys/Hcy are involved in various disease states, probes that display distinct signals in response to GSH and Cys/Hcy are highly desirable. In this work, we report a rhodol thioester (1) that responds to GSH and Cys/Hcy with distinct fluorescence emissions in neutral media. Probe 1 reacts with Cys/Hcy to form the corresponding deconjugated spirolactam via a tandem native chemical ligation (NCL) reaction. This intramolecular spirocyclization leads to the "quinone - phenol" transduction of rhodol dyes, and an excited-state intramolecular proton transfer (ESIPT) process between the phenolic hydroxyl proton and the aromatic nitrogen in the benzothiazole unit occurs upon photoexcitation, thus affording 2-(2'-hydroxyphenyl) benzothiazole (HBT) emission (454 nm). In the case of the tripeptide GSH, only transthioesterification takes place removing the intramolecular photo-induced electron transfer (PET) process caused by the electron deficient 4-nitrobenzene moiety giving rise to a large fluorescence enhancement at the rhodol emission band (587 nm). The simultaneous detection of GSH and Cys/Hcy is attributed to the significantly different rates of intramolecular S,N-acyl shift of their corresponding thioester adducts derived from 1. The utility of probe 1 has been demonstrated in various biological systems including serum and cells.

A colorimetric and ratiometric fluorescent probe for distinguishing cysteine from biothiols in water and living cells

A highly selective merocyanine-based fluorescent probe was developed, which can significantly distinguish Cys from Hcy and GSH by their kinetic profiles in water and respond to the intracellular Cys.

A highly sensitive TTF-functionalised probe for the determination of physiological thiols and its application in tumor cells

A tetrathiafulvalene (TTF)-fused piazselenole as a novel redox-active and fluorescent probe for highly sensitive determination of physiological thiols is presented.

Novel pyrazoline-based fluorescent probe for detecting glutathione and its application in cells

A novel compound, 2-(1,5-diphenyl-4,5-dihydro-1H-pyrazol-3-yl)phenyl acrylate (probe L), was designed and synthesized as a highly sensitive and selective fluorescent probe for recognizing and detecting glutathione among cysteine, homocysteine and other amino acids. The structures of related compounds were characterized using IR, NMR and HRMS spectroscopy analysis. The probe is a non-fluorescent compound. On being mixed with glutathione in buffered EtOH:PBS=3:7 solution at pH 7.4, the probe exhibited the blue emission of the pyrazoline at 474 nm and a 83-fold enhancement in fluorescence intensity. This probe is very sensitive and displayed a linear fluorescence off-on response to glutathione with fluorometric detection limit of 8.2 × 10(-8)M. The emission of the probe is pH independent in the physiological pH range. Live-cell imaging of HeLa cells confirmed the cell permeability of the probe and its ability to selectively discriminate GSH from Cys and Hcy in cells. The toxicity of the probe was low in cultured HeLa cells.

Chromogenic and fluorogenic chemosensors and reagents for anions. A comprehensive review of the years 2010-2011

This review focuses on examples reported in the years 2010-2011 dealing with the design of chromogenic and fluorogenic chemosensors or reagents for anions.