Nucleophilicity of cysteine and related biothiols and the development of fluorogenic probes and other applications

Multiple-State Control over Supramolecular Chirality through Dynamic Chemistry Mediated Molecular Engineering

Dynamic chemistry utilizing both covalent and noncovalent bonds provides valid protocols in manipulating properties of self-assemblies and functions. Here we employ dynamic chemistry to realize multiple-route control over supramolecular chirality up to five states. N-protected fluorinated phenylalanine in the carboxylate state self-assembled into achiral nanoparticles ascribed to the amphiphilicity. Protonation promoted one-dimensional growth into helices with shrunk hydrophilicity, which in the presence of disulfide pyridine undergo chirality inversion promoted by the hydrogen bonding-directed coassembly. Further interacting with the water-soluble reductant cleavages the disulfide bond to initiate the rearrangement of coassemblies with a chirality inversion as well. Finally, by tuning the pH environments, aromatic nucleophilic substitution reaction between reduced products and perfluorinated phenylalanine occurs, giving distinct chiral nanoarchitectures with emerged luminescence and circularly polarized luminescence. We thus realized a particular five-state control by combining dynamic chemistry at one chiral compound, which greatly enriches the toolbox in fabricating responsive chiroptical materials.

N-Aryl-DABCO Salts as an Unprecedented Sensing Platform for the Detection of Thiols and Selenols

Quaternary N-aryl-DABCO salts were introduced for the first time as a highly selective sensing platform for thiols and selenols. By employing this platform, a highly sensitive coumarin based "off-on" fluorescent probe was designed and synthesized. The probe possesses a good solubility in water, low background fluorescence, and, most importantly, demonstrates high selectivity to aryl thiols and selenols over their aliphatic counterparts and other common nucleophiles. A dramatic increase in fluorescence intensity is achieved through the selective cleavage of the quaternized DABCO-ring, yielding a piperazine derivatives with a high fluorescence quantum yield (~72 %). Moreover, stability of the probe to the most used reducing agents DTT and TCEP was demonstrated. The limits of detection for p-thiocresol and phenyl selenide were evaluated to be 22 nM and 6 nM, respectively.

Detection and discrimination of glutathione among biological thiols based on oxalyl dihydrazide decorated sulfur nanodots

The quantitative detection and discrimination of glutathione (GSH) were achieved based on oxalyl dihydrazide (ODH) decorated sulfur nanodots. ODH resulted in the aggregation and fluorescence quenching of the sulfur nanodots, and GSH selectively triggered fluorescence recovery through forming stronger hydrogen bonds with ODH than other biological thiols.

Long-term stable and efficient degradation of ornidazole with minimized by-product formation by a biological sulfidogenic process based on elemental sulfur

Conventional biological treatment processes cannot efficiently and completely degrade nitroimidazole antibiotics, due to the formation of highly antibacterial and carcinogenic nitroreduction by-products. This study investigated the removal of a typical nitroimidazole antibiotic (ornidazole) during wastewater treatment by a biological sulfidogenic process based on elemental sulfur (S0-BSP). Efficient and stable ornidazole degradation and organic carbon mineralization were simultaneously achieved by the S0-BSP in a 798-day bench-scale trial. Over 99.8 % of ornidazole (200‒500 μg/L) was removed with the removal rates of up to 0.59 g/(m3·d). Meanwhile, the efficiencies of organic carbon mineralization and sulfide production were hardly impacted by the dosed ornidazole, and their rates were maintained at 0.15 kg C/(m3·d) and 0.49 kg S/(m3·d), respectively. The genera associated with ornidazole degradation were identified (e.g., Sedimentibacter, Trichococcus, and Longilinea), and their abundances increased significantly. Microbial degradation of ornidazole proceeded by several functional genes, such as dehalogenases, cysteine synthase, and dioxygenases, mainly through dechlorination, denitration, N-heterocyclic ring cleavage, and oxidation. More importantly, the nucleophilic substitution of nitro group mediated by in-situ formed reducing sulfur species (e.g., sulfide, polysulfides, and cysteine hydropolysulfides), instead of nitroreduction, enhanced the complete ornidazole degradation and minimized the formation of carcinogenic and antibacterial nitroreduction by-products. The findings suggest that S0-BSP can be a promising approach to treat wastewater containing multiple contaminants, such as emerging organic pollutants, organic carbon, nitrate, and heavy metals.

Blue-light-driven photoactivity of L-cysteine-modified graphene quantum dots and their antibacterial effects

The widespread abuse of traditional antibiotics has led to a global rise in antibiotic-resistant bacteria, which give in return unprecedented health risks. Therefore, there is a large and urgent need for the development of new, smart antibacterial agents able to efficiently kill or inhibit bacterial growth. In this study, we investigated the antibacterial activity of S, N-doped Graphene Quantum Dots (GQDs) as a light-triggered antibacterial agent. Gamma irradiation was employed as a tool to achieve one-step modification of GQDs in the presence of L-cysteine amino acid as a source of heteroatoms. X-ray Photoelectron Spectroscopy (XPS), nuclear magnetic resonance (NMR), and zeta potential measurements provided the necessary data to clarify the structure of modified dots and verify the introduction of both S- and N-atoms in GQDs structure, but also severe changes in the aromatic, sp2 domains. Namely, γ-irradiation caused a bonding of S atoms in 1.14 at.% mainly as thiol groups, and N in 1.81 at.% as amino groups, but sp2 contribution in GQD structure was lowered from 63.00 to 4.86 at.%, as measured in dots irradiated at a dose of 200 kGy. Fluorescence quenching measurements showed that L-cysteine-modified dots are able to bind to human serum albumin. The antibacterial activity of GQDs combined with 1 and 6 h of blue light (470 nm) irradiation was tested against 8 bacterial strains. GQD-cys-25 sample provided the best results, with minimum inhibitory concentration (MIC) as low as 125 μg/mL against S. aureus, E. faecalis, and E. coli after only 1 h of blue light exposure.

Applications of Pyrrole and Pyridine-based Heterocycles in Cancer Diagnosis and Treatment

BACKGROUND

The escalation of cancer worldwide is one of the major causes of economy burden and loss of human resources. According to the American Cancer Society, there will be 1,958,310 new cancer cases and 609,820 projected cancer deaths in 2023 in the United States. It is projected that by 2040, the burden of global cancer is expected to rise to 29.5 million per year, causing a death toll of 16.4 million. The hemostasis regulation by cellular protein synthesis and their targeted degradation is required for normal cell growth. The imbalance in hemostasis causes unbridled growth in cells and results in cancer. The DNA of cells needs to be targeted by chemotherapeutic agents for cancer treatment, but at the same time, their efficacy and toxicity also need to be considered for successful treatment.

OBJECTIVE

The objective of this study is to review the published work on pyrrole and pyridine, which have been prominent in the diagnosis and possess anticancer activity, to obtain some novel lead molecules of improved cancer therapeutic.

METHODS

A literature search was carried out using different search engines, like Sci-finder, Elsevier, ScienceDirect, RSC etc., for small molecules based on pyrrole and pyridine helpful in diagnosis and inducing apoptosis in cancer cells. The research findings on the application of these compounds from 2018-2023 were reviewed on a variety of cell lines, such as breast cancer, liver cancer, epithelial cancer, etc. Results: In this review, the published small molecules, pyrrole and pyridine and their derivatives, which have roles in the diagnosis and treatment of cancers, were discussed to provide some insight into the structural features responsible for diagnosis and treatment. The analogues with the chromeno-furo-pyridine skeleton showed the highest anticancer activity against breast cancer. The compound 5-amino-N-(1-(pyridin-4- yl)ethylidene)-1H-pyrazole-4-carbohydrazides was highly potent against HEPG2 cancer cell. Redaporfin is used for the treatment of cholangiocarcinoma, biliary tract cancer, cisplatin-resistant head and neck squamous cell carcinoma, and pigmentation melanoma, and it is in clinical trials for phase II. These structural features present a high potential for designing novel anticancer agents for diagnosis and drug development.

CONCLUSION

Therefore, the N- and C-substituted pyrrole and pyridine-based novel privileged small Nheterocyclic scaffolds are potential molecules used in the diagnosis and treatment of cancer. This review discusses the reports on the synthesis of such molecules during 2018-2023. The review mainly discusses various diagnostic techniques for cancer, which employ pyrrole and pyridine heterocyclic scaffolds. Furthermore, the anticancer activity of N- and C-substituted pyrrole and pyridine-based scaffolds has been described, which works against different cancer cell lines, such as MCF-7, A549, A2780, HepG2, MDA-MB-231, K562, HT- 29, Caco-2 cells, Hela, Huh-7, WSU-DLCL2, HCT-116, HBL-100, H23, HCC827, SKOV3, etc. This review will help the researchers to obtain a critical insight into the structural aspects of pyrrole and pyridine-based scaffolds useful in cancer diagnosis as well as treatment and design pathways to develop novel drugs in the future.

Water-Soluble Dual-Channel Fluorescent Probe for Sensitive Detection of Biothiols In Vitro and In Vivo

Benefiting from high spatiotemporal resolution, deep tissue penetration, and excellent sensitivity, fluorescence imaging technology has been widely applied in cancer diagnosis and treatment. In recent years, a large number of fluorescent probes for monitoring the levels of endogenous biothiols have been reported, which have significant implications for cancer diagnosis and treatment. However, most probes still suffer from poor biological compatibility and easy attachment by the environment. This work presents the development of a water-soluble dual-channel fluorescent probe, named MAL-NBD, for sensitively detecting biothiols. Nonfluorescent MAL-NBD is transformed into fluorescent groups MAL and NBD-SR/NR through nucleophilic substitution by biologically active thiols, producing dual-channel fluorescence signals for precise detection of biologically active thiols. Taking advantage of the excellent biocompatibility and low biotoxicity, MAL-NBD is successfully used for imaging HeLa cancer cells and zebrafish larvae, promoting its potential application for the precise detection of biological thiols involved in physiological and pathological processes.

Bioimaging and Sensing Thiols In Vivo and in Tumor Tissues Based on a Near-Infrared Fluorescent Probe with Large Stokes Shift

The well-known small-molecule biothiols have been used to maintain the normal metabolism of peroxy radicals, forming protein structures, resisting cell apoptosis, regulating metabolism, and protecting the homeostasis of cells in the organism. A large amount of research has found that abnormal levels of the above biothiols can cause some adverse diseases, such as changes in hair pigmentation, a slower growth rate, delayed response, excessive sleep and skin diseases. In order to further investigate the exact intracellular molecular mechanism of biothiols, it is imperative to explore effective strategies for real-time biothiol detection in living systems. In this work, a new near-infrared (NIR) emission fluorescence probe (probe 1) for sensitive and selective detection of biothiols was devised by combining dicyanoisophorone derivatives with the dinitrobenzenesulfonyl (DNBS) group. As expected, probe 1 could specifically detect biothiols (Cys, Hcy and GSH) through the dinitrobenzenesulfonyl group to form dye 2, which works as a signaling molecule for sensing biothiols in real samples. Surprisingly, probe 1 showed superior sensing characteristics and low-limit detection towards biothiols (36.0 nM for Cys, 39.0 nM for Hcy and 48.0 nM for GSH) with a large Stokes shift (134 nm). Additionally, the function of probe 1 as a platform for detecting biothiols was confirmed by confocal fluorescence imaging of biothiols in MCF-7 cells and zebrafish. More importantly, the capability of probe 1 in vivo has been further evaluated by imaging the overexpressed biothiols in tumor tissue. It is reasonable to believe that probe 1 can provide a valuable method to explore the relationship between biothiols and the genesis of tumor.

Manipulating the monomer-dimer transformation of a heptamethine cyanine ligand: near infrared chromogenic recognition of biothiols

A novel absorbance recovery method has been developed for the determination of biothiols with a near-infrared reagent. This method employs a two-reagent system composed of cation heptamethine cyanine (CyL) and Hg²⁺. The absorbance of CyL, with a maximum peak at 760 nm, was decreased due to addition of Hg²⁺, but recovered when biothiols were added. Under optimal conditions, the reciprocal extent of recovered absorbance was proportional to the concentration of biothiols. The calibration curves are linear over the range of (0.3-7.0) × 10^-6 M for cysteine, (1.0-10.0) × 10^-6 M for homocysteine and (1.0-9.0) × 10^-6 M for glutathione. Because of the specific affinity of Hg²⁺for biothiols, there is minimal interference from other amino acids. This method has been successfully applied to the determination of homocysteine in human urine samples with satisfactory results.

Review on Carbon Dot-Based Fluorescent Detection of Biothiols

Biothiols, such as cysteine (Cys), homocysteine (Hcy), and glutathione (GSH), play a vital role in gene expression, maintaining redox homeostasis, reducing damages caused by free radicals/toxins, etc. Likewise, abnormal levels of biothiols can lead to severe diseases, such as Alzheimer's disease (AD), neurotoxicity, hair depigmentation, liver/skin damage, etc. To quantify the biothiols in a biological system, numerous low-toxic probes, such as fluorescent quantum dots, emissive organic probes, composited nanomaterials, etc., have been reported with real-time applications. Among these fluorescent probes, carbon-dots (CDs) have become attractive for biothiols quantification because of advantages of easy synthesis, nano-size, crystalline properties, low-toxicity, and real-time applicability. A CDs-based biothiols assay can be achieved by fluorescent "Turn-On" and "Turn-Off" responses via direct binding, metal complex-mediated detection, composite enhanced interaction, reaction-based reports, and so forth. To date, the availability of a review focused on fluorescent CDs-based biothiols detection with information on recent trends, mechanistic aspects, linear ranges, LODs, and real applications is lacking, which allows us to deliver this comprehensive review. This review delivers valuable information on reported carbon-dots-based biothiols assays, the underlying mechanism, their applications, probe/CDs selection, sensory requirement, merits, limitations, and future scopes.

Construction of a red emission fluorescent probe for selectively detection of cysteine in living cells

Cysteine (Cys) is a vital amino acid in the body, and its abnormal expression level is associated with many diseases. In this study, a novel fluorescent probe ACHB was synthesized, showing high selectivity, anti-interference ability and achieving accurate detection of cysteine. Different from most previous off-on probes, ACHB showed an on-off fluorescence response to Cys. Acrylic ester was used as a recognizer while green fluorescence protein (GFP) chromophore derivative 4-hydroxybenzylidene-imidazolinone (HBI) was used as the fluorophore. The addition of Cys leads to the hydrolysis of the red-emitting probe (613 nm), releasing a precursor with a lower fluorescent signal and showing an on-off spectral signal, which was ideal for obtaining sensitive detection with high specificity. Furthermore, the probe was successfully applied for simultaneous determination of cysteine (Cys) in living cells and biological sample (mouse serum). In conclusion, probe ACHB is a promising tool to display the intracellular cysteine concentration level, providing a good visualization method for clinical diagnosis and scientific basic research.

Dual Response Site Fluorescent Probe for Highly Sensitive Detection of Cys/Hcy and GSH In Vivo through Two Different Emission Channels

Much research has demonstrated that metabolic imbalances of biothiols are closely associated with the emergence of different types of disease. In view of the significant effect of biothiols, quantitative evaluation and discrimination of intracellular Cys/Hcy and GSH in complex biological environments is very important. In this study, probe CDS-NBD, synthesized by attaching 2,4-dinitrobenzenesulfonate (DNBS, site 1) and nitrobenzoxadiazole (NBD, site 2) as the highly sensitive and selective dual response site for thiols onto the coumarin derivative 7-hydroxycoumarin-4-acetic acid, exhibited large separation of the emission wavelengths, fast response, notable fluorescence enhancement, excellent sensitivity and selectivity to Cys/Hcy and GSH over other biological species. Additionally, CDS-NBD could make a distinction between two different fluorescent signals, GSH (an obvious blue fluorescence) and Cys/Hcy (a mixed blue-green fluorescence). Further study on imaging of Cys/Hcy and GSH in vivo by employing probe CDS-NBD could also be successfully achieved.

Simultaneous Discrimination of Cys/Hcy and GSH With Simple Fluorescent Probe Under a Single-Wavelength Excitation and its Application in Living Cells, Tumor Tissues, and Zebrafish

Owing to the important physiological sits of biothiols (Cys, Hcy, and GSH), developing accurate detection methods capable of qualitative and quantitative analysis of biothiols in living systems is needed for understanding the biological profile of biothiols. In this work, we have designed and synthesized a 4′-hydroxy-[1,1′-biphenyl]-4-carbonitrile modified with NBD group-based fluorescent probe, BPN-NBD, for sensitive detection of Cys/Hcy and GSH by dual emission signals via a single-wavelength excitation. BPN-NBD exhibited an obvious blue fluorescence (λmaxem = 475 nm) upon the treatment with GSH and reacted with Cys/Hcy to give a mixed blue-green fluorescence (λmaxem = 475 and 545 nm). Meanwhile, BPN-NDB performed sufficient selectivity, rapid detection (150 s), high sensitivity (0.011 µM for Cys, 0.015 µM for Hcy, and 0.003 µM for GSH) and could work via a single-wavelength excitation to analytes and had the ability to image Cys/Hcy from GSH in living MCF-7 cells, tumor tissues, and zebrafish by exhibiting different fluorescence signals. Overall, this work provided a powerful tool for thiols visualization in biological and medical applications.

A novel electrochemical conducting polymer sensor for the rapid, selective and sensitive detection of biothiols

A rapid, selective and sensitive, novel conducting-polymer sensing platform for the detection and analysis of biothiols.

Rational design of a dual-reactive probe for imaging the biogenesis of both H2S and GSH from L-Cys rather than D-Cys in live cells

Biothiols and their interconversion are involved in cellular redox homestasis as well as many physiological processes. Here, a dual-reactive dual-quenching fluorescent probe was rationally developed based on thiolysis reactions of...

Coumarin-based fluorescent probe with 4-phenylselenium as the active site for multi-channel discrimination of biothiols

Biological mercaptans, also known as biothiols, play their own roles in a number of important physiological processes, and the abnormal levels of biothiols are closely associated with a variety of...

Nanoparticle-based colorimetric sensors to detect neurodegenerative disease biomarkers

Neurodegenerative disorders (NDDs) are progressive, incurable health conditions that primarily affect brain cells, and result in loss of brain mass and impaired function. Current sensing technologies for NDD detection are limited by high cost, long sample preparation, and/or require skilled personnel. To overcome these limitations, optical sensors, specifically colorimetric sensors, have garnered increasing attention towards the development of a cost-effective, simple, and rapid alternative approach. In this review, we evaluate colorimetric sensing strategies of NDD biomarkers (e.g. proteins, neurotransmitters, bio-thiols, and sulfide), address the limitations and challenges of optical sensor technologies, and provide our outlook on the future of this field.

Native Chemical Ligation-Based Fluorescent Probes for Cysteine and Aminopeptidase N Using meso-thioester-BODIPY

meso-Carboxyl-BODIPY responds to small electronic changes resulting from acyl substitution reactions with a marked change in fluorescence. Herein, the minute changes that accompany the thioester to amide conversion encountered in native chemical ligation (NCL) are exploited in the construction of fluorescent "turn-on" probes. Two fluorogenic probes, 1 a and 4, derived from a meso-thioester-BODIPY scaffold, were designed for the selective detection of cysteine (1 a) and aminopeptidase N (4), respectively. The aromatic (1 a) and aliphatic (4) thioesters of meso-carboxyl-BODIPY are nonfluorescent. However, specific analyte-induced conversion to the meso-amide derivative caused significant spectral changes and a dramatic fluorescence enhancement. Probe 1 a exhibited a large fluorescence "turn-on" response with high selectivity toward cysteine via a tandem NCL reaction. Probe 4 was successfully applied to the monitoring and imaging of endogenous aminopeptidase N in live cancer cells.

Structure modulation on fluorescent probes for biothiols and the reversible imaging of glutathione in living cells

The detection of small molecular biothiols (cysteine, homocysteine and glutathione) is of great importance, as they involve in a series of physiological and pathological processes and are associated with many diseases. To realize the real-time monitoring of a specific biothiol, a rapid and reversible probe is required. Therefore, three probes, namely, o-MNPy, m-MNPy and p-MNPy, with pyridine substituted α, β-unsaturated ketone as the recognition site, were reported here, and the reactivity of the recognition site was finely tuned by the connection mode of the pyridine unit. To single out the optimal one, the response performances of three probes toward each biothiol were systemically studied, taking the differences of the intracellular contents of three biothiols into account during the evaluation. Biothiols reacted with the probes through Michael addition, and results showed that the slight structural variations could affect the performances of the probes obviously. p-MNPy with the pyridine unit connected to the recognition site through the para-position of the nitrogen atom, revealed the best sensing ability among the three probes. It demonstrated rapid response, good selectivity and sensitivity, excellent pH adaptability to Cys and GSH, and displayed reversible detection toward GSH. Finally, p-MNPy was successfully applied to track the GSH fluctuations under the oxidative stress stimulated by H₂O₂ in living cells.

A reversible fluorescent probe for GSH was obtained through structure modulation, by which the intracellular GSH fluctuation was imaged.

A novel near-infrared fluorescent probe based on isophorone for the bioassay of endogenous cysteine

A dicyanoisophorone/acrylate-combined probe (DDP) was synthesized and designed as a near-infrared (NIR) fluorescent sensor for the rapid identification of Cys over Hcy and GSH in aqueous solution with a large Stokes shift (143 nm). The detection limit of Cys was 1.23 μM, which was lower than that of the intracellular Cys concentration. DDP was cell membrane-permeable and had been successfully applied to the detection of intracellular Cys in HeLa cells. The detection mechanism was determined by ¹H NMR titration, MS and DFT calculations.