An ESIPT-based fluorescent probe for sensitive and selective detection of Cys/Hcy over GSH with a red emission and a large Stokes shift

An ESIPT-based fluorescent probe (Probe 1) using acrylate as recognition group for the selective and sensitive detection of cysteine/homocysteine (Cys/Hcy) has been developed. In the presence of Cys/Hcy, this probe was transformed into 1,3-bis(bispyridin-2ylimino)isoindolin-4-ol (dye 4) which displayed red fluorescence with a large Stokes shift (217 nm) when excited. The detection limits are as low as 5.4 nM and 7.0 nM for Cys and Hcy respectively (based on S/N = 3). Importantly, this probe has been successfully demonstrated for the detection of intracellular Cys/Hcy in living cells.     

A highly selective colorimetric probe based on 2,2′,2″-trisindolylmethene for cysteine/homocysteine

The interaction and colorimetric sensing properties of receptor 1, tris(3-methylindole-2-yl)methene as the perchlorate salt, with amino acids in aqueous MeCN at neutral pH were investigated using UV-vis spectroscopic techniques. Specifically, receptor 1 behaves as a colorimetric probe for selective and sensitive detection of cysteine (Cys)/homocysteine (Hcy) based on the nucleophilic addition reaction between the sulphydryl group of Cys/Hcy and the meso carbon-carbon double bond of receptor 1, leading to clear color change from violet to colorless. A more quantitative determination for Cys/Hcy was preliminary performed by flow injection analysis (FIA) coupled with spectrophotometry. The selective binding ability of receptor 1 toward Cys/Hcy has also been evaluated by electrochemical techniques.     

Rational Design of an “OFF–ON” Phosphorescent Chemodosimeter Based on an Iridium(III) Complex and Its Application for Time‐Resolved Luminescent Detection and Bioimaging of Cysteine and Homocysteine

Biothiols, such as cysteine (Cys) and homocysteine (Hcy), play very crucial roles in biological systems. Abnormal levels of these biothiols are often associated with many types of diseases. Therefore, the detection of Cys (or Hcy) is of great importance. In this work, we have synthesized an excellent “OFF‐ON” phosphorescent chemodosimeter 1 for sensing Cys and Hcy with high selectivity and naked‐eye detection based on an Ir^III complex containing a 2,4‐dinitrobenzenesulfonyl (DNBS) group within its ligand. The “OFF‐ON” phosphorescent response can be assigned to the electron‐transfer process from Ir^III center and C^N ligands to the DNBS group as the strong electron‐acceptor, which can quench the phosphorescence of probe 1 completely. The DNBS group can be cleaved by thiols of Cys or Hcy, and both the ³M LCT and ³LC states are responsible for the excited‐state properties of the reaction product of probe 1 and Cys (or Hcy). Thus, the phosphorescence is switched on. Based on these results, a general principle for designing “OFF‐ON” phosphorescent chemodosimeters based on heavy‐metal complexes has been provided. Importantly, utilizing the long emission‐lifetime of phosphorescence signal, the time‐resolved luminescent assay of 1 in sensing Cys was realized successfully, which can eliminate the interference from the short‐lived background fluorescence and improve the signal‐to‐noise ratio. As far as we know, this is the first report about the time‐resolved luminescent detection of biothiols. Finally, probe 1 has been used successfully for bioimaging the changes of Cys/Hcy concentration in living cells.     

A dual-modal probe for NIR fluorogenic and ratiometric photoacoustic imaging of Cys/Hcy in vivo

Biothiols, such as cysteine(Cys) and homocysteine(Hcy), play vital roles in biological homeostasis and are closely related to various pathological and physiological processes in the living systems. Therefore, the in vivo detection of biothiols is of great importance for early diagnosis of diseases and assessment of disease progression. In this work, we developed a near-infrared(NIR) fluorescence and photoacoustic dual-modal molecular probe(NIR-S) that can be specifically activated by Cys or Hcy. The aryl-thioether substituted cyanine probe can undergo nucleophilic substitution and Smiles rearrangement reaction, resulting in specific turn-on NIR fluorescence and ratiometric photoacoustic responses for Hcy/Cys. Thus, NIR-S not only realizes the specific NIR fluorescence and photoacoustic dual mode imaging to detect Hcy/Cys in solution, but also can be applied to living cells and mice to detect Hcy/Cys. This work provided a practical tool to detect Hcy/Cys levels in vivo, which would be beneficial for the early diagnosis and progress of diseases.     

A colorimetric,ratiometric and water-soluble fluorescent probe for simultaneously sensing glutathione and cysteine/homocysteine

A chlorinated coumarin-aldehyde was developed as a colorimetric and ratiometric fluorescent probe for distinguishing glutathione (GSH), cystenine (Cys) and homocysteine (Hcy). The GSH-induced substitution-cyclization and Cys/Hcy-induced substitution-rearrangement cascades lead to the corresponding thiol-coumarin-iminium cation and amino-coumarin-aldehyde with distinct photophysical properties. The probe can be used to simultaneously detect GSH and Cys/Hcy by visual determination based on distinct different colors – red and pale-yellow in PBS buffer solution by two reaction sites. From the linear relationship of fluorescence intensity and biothiols concentrations, it was determined that the limits of detection for GSH, Hcy and Cys are 0.08, 0.09 and 0.18 μM, respectively. Furthermore, the probe was successfully used in living cell imaging with low cell toxicity.     

Benzothiazole‐Pyimidine‐Based BF2 Complex for Selective Detection of Cysteine

Due to the similar structure and reactivity of cysteine (Cys), homocysteine (Hcy) and glutathione (GSH), the simultaneous discrimination of Cys over Hcy and GSH by a single fluorescent sensor is still a great challenge. In this work, a benzothiazole‐pyimidine‐based boron difluoride complex ( BPB ) was developed as a new fluorescent sensor for Cys. The sensor exhibits a highly selective “turn‐on” response to cysteine over Hcy, GSH and other amino acids in aqueous solution at physiological pH. The observed pseudo‐first‐order rate constant for the reaction of BPB with Cys was calculated to be about 0.062 min⁻¹. The detection limit of this sensor for Cys was determined to be 332 nm, and bioimaging of exogenous Cys by this sensor was successfully applied in living cells, thus indicating that this sensor holds great potential for biological applications.     

Detection of homocysteine and cysteine

At elevated levels, homocysteine (Hcy, 1) is a risk factor for cardiovascular diseases, Alzheimer's disease, neural tube defects, and osteoporosis. Both 1 and cysteine (Cys, 3) are linked to neurotoxicity. The biochemical mechanisms by which 1 and 3 are involved in disease states are relatively unclear. Herein, we describe simple methods for detecting either Hcy or Cys in the visible spectral region with the highest selectivity reported to date without using biochemical techniques or preparative separations. Simple methods and readily available reagents allow for the detection of Cys and Hcy in the range of their physiologically relevant levels. New HPLC postcolumn detection methods for biological thiols are reported. The potential biomedical relevance of the chemical mechanisms involved in the detection of 1 is described.     

A dual-response naphthofluorescein-based fluorescent probe for multiple-channel imaging of cysteine/homocysteine in living cells

A naphthofluorescein-based fluorescent probe with two independent reaction sites (nitro-2,1,3-benzoxadiazole and acrylate moiety) was developed. Integrating these two reaction sites into a single molecule not only can guarantee the selective detection of Cys/Hcy in an elegant fashion, but also can enable Cys/Hcy detection in a multiple-channel responsive manner.     

New Chemodosimetric Reagents as Ratiometric Probes for Cysteine and Homocysteine and Possible Detection in Living Cells and in Blood Plasma

In this work, we have rationally designed and synthesized two new reagents ( L₁ and L₂ ), each bearing a pendant aldehyde functionality. This aldehyde group can take part in cyclization reactions with β‐ or γ‐amino thiols to yield the corresponding thiazolidine and thiazinane derivatives, respectively. The intramolecular charge‐transfer (ICT) bands of these thiazolidine and thiazinane derivatives are distinctly different from those of the molecular probes ( L₁ and L₂ ). Such changes could serve as a potential platform for using L₁ and L₂ as new colorimetric/fluorogenic as well as ratiometric sensors for cysteine (Cys) and homocysteine (Hcy) under physiological conditions. Both reagents proved to be specific towards Cys and Hcy even in the presence of various amino acids, glucose, and DNA. Importantly, these two chemodosimetric reagents could be used for the quantitative detection of Cys present in blood plasma by using a pre‐column HPLC technique. Such examples are not common in contemporary literature. MTT assay studies have revealed that these probes have low cytotoxicity. Confocal laser scanning micrographs of cells demonstrated that these probes could penetrate cell membranes and could be used to detect intracellular Cys/Hcy present within living cells. Thus, the results presented in this article not only demonstrate the efficiency and specificity of two ratiometric chemodosimeter molecules for the quantitative detection of Cys and Hcy, but also provide a strategy for developing reagents for analysis of these vital amino acids in biological samples.     

水溶性半胱氨酸和高半胱氨酸探针的合成及应用

以对硝基甲苯和变色酸为原料,经硝化、重氮化反应合成了水溶性偶氮化合物(1),其结构经1H NMR和元素分析表征.利用UV-vis滴定法研究了在缓冲溶液(pH 7.4)中1对半胱氨酸(Cys)和高半胱氨酸(Hcy)的识别能力.结果表明:1与Cys或Hcy作用后引起吸收光谱蓝移～69 nm,能够用于Cys和Hcy的裸眼识别.     

Highly selective fluorescent sensor for homocysteine and cysteine

A new fluorescent sensor (1) based on a coumarin was synthesized for the selective detection of homocysteine (Hcy) and cysteine (Cys). The chemosensor has shown a selective response to Hcy or Cys over other various amino acids via a ring formation of thiazinane or thiazolidine. When Hcy or Cys was added, the fluorescent intensity of 1 was completely quenched through a photo-induced electron transfer with the sensitivity of sub-millimolar concentration.     

Discriminatory Detection of Cysteine and Homocysteine Based on Dialdehyde‐Functionalized Aggregation‐Induced Emission Fluorophores

We demonstrate a concept‐proof work of using fluorescence (FL) “turn‐on” probes for the discriminatory detection of cysteine (Cys) over homocysteine (Hcy). The fluorogens are provided with aggregation‐induced emission (AIE) characteristic and functionalized with two aldehyde‐groups (DMTPS‐ALD and TPE‐ALD). All the detections were carried out in a biocompatible medium (10 mM HEPES buffer and DMSO, pH 7.4). In principle, the formation of thiazinane/thiazolidine through the chemical reaction of aldehydes on the probe molecules and the residue of Cys/Hcy determines the selective recognition of Cys and Hcy over other amino acids and glucose. The FL responses originate from the AIE property of thiazinane/thiazolidine resultants, which have low solubility and precipitate (aggregate) in the detection medium. The discrimination between Cys and Hcy comes from the difference in reaction kinetics of TPE‐ALD/DMTPS‐ALD with Cys and Hcy, thereby the FL responses show different time courses and intensity enhancement. It is worth noting that TPE‐ALD outshined the other two probes in performance with fast response, a high FL enhancement up to 16‐fold, high sensitivity, and good specificity and selectivity. Moreover, its FL response threshold at 250 μM is very close to the lower limit of the normal level of Cys in human plasma, which implies that TPE‐ALD could be applied as a potential indicator of Cys deficiency.     

Near-Infrared mitochondria-targeted fluorescent probe for cysteine based on difluoroboron curcuminoid derivatives

A highly selective dual-channel NIR fl uorescent probe (DFB1) based on curcuminoid difl uoroboron is developed for discrimination Cys over GSH, Hcy and other amino acids in mitochondria of living cells.     

一种钌(Ⅱ)配合物荧光探针用于半胱氨酸和高半胱氨酸的检测

为了检测半胱氨酸和高半胱氨酸,本文合成了一种基于钌(Ⅱ)配合物的荧光探针。 结果表明,该探针可实现对半胱氨酸和高半胱氨酸的较好的灵敏性和选择性检测。 在优化的实验条件下,5~35 μmol/L浓度区间,探针的荧光强度与半胱氨酸和高半胱氨酸浓度呈良好的线性关系。 其检测限分别为0.60和0.78 μmol/L。 该研究为基于钌(Ⅱ)配合物的荧光探针定量检测生物活性分子提供了一种有用的方法。     

α-Acrylaldehyde 3-Pyrrolyl BODIPY as a Specific Optical Sensor for Cysteine and Homocysteine

A new fluorophore, α-acrylaldehyde 3-pyrrolyl BODIPY was synthesized by treating 3-pyrrolyl BODIPY with a mixture of 3-(dimethylamino) acrolein and POCl₃ under Vilsmeier–Haack reaction conditions. The X-ray structure revealed that the fluorophore was almost planar, and the appended pyrrole was in the same plane with a small deviation from the mean plane. We investigated the potential use of α-acrylaldehyde 3-pyrrolyl BODIPY for sensing thiol containing amino acids such as cysteine/homocysteine (Cys/Hcy). Our studies showed that the α-acrylaldehyde- 3-pyrrolyl BODIPY was found to be useful for exclusive sensing of Cys/Hcy and to exhibit different optical signaling responses to Cys and Hcy at physiological pH in aq. CH₃CN (1 : 1 v/v, PBS) medium. The enhancement in optical properties for Cys and quenching in same properties for Hcy was attributed to different binding modes of Cys/Hcy with α-acrylaldehyde 3-pyrrolyl BODIPY.     

Dual‐Emission Channels for Simultaneous Sensing of Cysteine and Homocysteine in Living Cells

The development of a fluorescent probe to distinguish between cysteine (Cys) and homocysteine (Hcy) is always a challenge owing to their structural similarity, and the simultaneous detection of Cys and Hcy by utilizing different emission channels is especially difficult. In this work, we designed and synthesized a new fluorescent probe to differentiate between Cys and Hcy on the basis of a coumarin derivative with a chlorine atom and an α,β‐unsaturated aldehyde. Cys and Hcy induced different cascade reactions with the probe, which led to different products with distinct photophysical properties. The nonfluorescent probe responded to Cys and emitted strong blue fluorescence, whereas it reacted with Hcy and generated yellow fluorescence without interference from glutathione. In addition, the probe was successfully applied to distinguish between Cys and Hcy in living cells.     

A CuII-Salicylidene Glycinato Complex for the Selective Fluorometric Detection of Homocysteine over 20 Proteinogenic Amino Acids

Homocysteine (Hcy) is a sulfur-containing α-amino acid that differs by one methylene (CH₂) subunit from homologous cysteine (Cys). Elevated levels of Hcy are diagnostic markers of cardiovascular disease and other medical conditions. We present a new Cu^II-salicylidene glycinato complex 1 for the selective fluorometric detection of Hcy in water. In the presence of this analyte, the non-fluorescent copper-complex demetallates and disassembles into its building blocks. This process liberates a 3-chloro-5-sulfosalicylaldehyde signaling unit and is accompanied by a 51-fold turn-on fluorescence at 485 nm (λ_ex=350 nm). Out of twenty proteinogenic amino acids, only histidine (12-fold turn-on fluorescence) and Cys (8-fold turn-on fluorescence) trigger some disassembly of probe 1 . In comparison with important pioneering work on the detection of biothiols, this study strikingly demonstrates that structural modifications of chelate core structures steer substrate selectivity of metal-based probes. Importantly, probe 1 has proven suitable for the detection of Hcy in artificial urine.     

Simultaneous determination of homocysteine,methionine and cysteine in maternal plasma after delivery by HPLC‐fluorescence detection with DBD‐F as a label

An HPLC‐fluorescence (FL) method for determination of sulfur‐containing amino acids such as homocysteine (Hcy), methionine (Met) and cysteine (Cys) in human plasma was developed. The sulfur‐containing amino acids were labeled with 4‐(N,N‐dimethylaminosulfonyl)‐7‐fluoro‐2,1,3‐benzoxadiazole (DBD‐F). Calibration curves in the range of 1–100 µm (Hcy and Met) and 5–500 µm (Cys) indicated good linearities (r ≥ 0.998). The limits of detection at a signal‐to‐noise ratio of 3 were 0.13 (Hcy), 0.02 (Met) and 0.11 µm (Cys), respectively. Acceptable results for accuracy and precision of intra‐ and inter‐day measurements were obtained. The results of Hcy and Cys obtained by the proposed method indicated good correlations with the conventional method (r > 0.911, n = 20). Furthermore, the method was applied to determination of the sulfur‐containing amino acids in maternal plasma (n = 200) after delivery. The concentrations of Hcy, Met and Cys as a median (inter quartile range, Q₁ and Q₃) were 5.37 (3.32–7.79) μm , 25.20 (20.10–31.06) μm and 147.25 (102.81–189.31) μm , respectively. Copyright © 2012 John Wiley & Sons, Ltd.     

Sensing for intracellular thiols by water-insoluble two-photon fluorescent probe incorporating nanogel

A novel “turn-on” two-photon fluorescent probe containing a π-conjugated triarylboron luminogen and a maleimide moiety DMDP-M based on the photo-induced electron transfer (PET) mechanism for biothiol detection was designed and synthesized. By simply loading the hydrophobic DMDP-M on a cross-linked Pluronic^® F127 nanogel (CL-F127), a probing system DMDP-M/CL-F127 was established, which shows quick response, high selectivity and sensitivity to cysteine (Cys), homocysteine (Hcy) and glutathione (GSH) in aqueous phase. The DMDP-M/CL-F127 system presented the fastest response to Cys with a rate constant of 0.56 min⁻¹, and the detection limit to Cys was calculated to be as low as 0.18 μM. The DMDP-M/CL-F127 system has been successfully applied to the fluorescence imaging of biothiols in NIH/3T3 fibroblasts either with single-photon or two-photon excitation because of its high biocompatibility and cell-membrane permeability. The present work provides a general, simple and efficient strategy for the application of hydrophobic molecules to sensing biothiols in aqueous phase, and a novel sensing system for intracellular biothiols fitted for both single-photon and two-photon fluorescence imaging.     

Rational Design of an Ultrasensitive and Highly Selective Chemodosimeter by a Dual Quenching Mechanism for Cysteine Based on a Facile Michael‐Transcyclization Cascade Reaction

Differentiation of biologically important thiols, such as cysteine (Cys), homocysteine (Hcy), and glutathione (GSH) is still a challenging task. Herein, we present a novel fluorescent chemodosimeter capable of selectively detecting Cys over other biothiols including Hcy and GSH and other amino acids by a facile thiol‐Michael addition/transcyclization rearrangement cascade click process. The unique transcyclization step is critical for the selectivity as a result of the kinetically favorable formation of a six‐membered ring with the Cys Michael adduct. Moreover, the probe adopts a distinctive dual quenching mechanism—photoinduced electron transfer (PET) and photoinduced intramolecular charge transfer (ICT) to deliver a drastic turn‐on fluorescence response only at the Cys‐selective transcylization step. The judicious selection of strong electron‐withdrawing naphthalimide fluorophore with maleimide group enhances the electrophilicity and thus reactivity for the cascade process leading to fast detection and ultrasensitivity with a detection limit of 2.0 nm (S/N=3). The probe has demonstrated its practical utility potential in Cys imaging in live cells.