Near-Infrared Probes for Biothiols (Cysteine, Homocysteine, and Glutathione): A Comprehensive Review

Biothiols (cysteine, homocysteine, and glutathione) are an important class of compounds with a free thiol group. These biothiols plays an important role in several metabolic processes in living bodies when present in optimum concentration. Researchers have developed several probes for the detection and quantification of biothiols that can absorb in UV, visible, and near-infrared (NIR) regions of the electromagnetic spectrum. Among them, NIR organic probes have attracted significant attention due to their application in in vivo and in vitro imaging. In this review, we have summarized probes for these biothiols, which could work in the NIR region, and discussed their sensing mechanism and potential applications. Along with focusing on the pros and cons of the reported probes we have classified them according to the fluorophore used and summarized their photophysical and sensing properties (emission, response time, limit of detection).

Colorimetric detection of homocysteine by a pyridylazo dye-based Cu2+ complex via indicator displacement mechanism

A BrPAPS based Cu²⁺ complex has been developed as a colorimetric probe for the selective recognition of homocysteine (Hcy) over cysteine (Cys) and glutathione (GSH) in an aqueous solution via the indicator displacement assay. BrPAPS formed a complex with Cu²⁺ in a 1:1 ratio (BrPAPS-Cu²⁺) accompanied by the color change from yellow to red. Detecting Hcy is based on high affinity of Hcy for Cu²⁺. The addition of Hcy to BrPAPS-Cu²⁺ caused the complex formation of Hcy with Cu²⁺ in a 2:1 stoichiometry, resulting a hypsochromic shift with change back of color from red to yellow by the release of BrPAPS from BrPAPS-Cu²⁺. The absorption response is linear with the Hcy concentration in the range of 0-20 μM with a detection limit of 1.46 μM. Moreover, the detection of Hcy was not significantly affected by other amino acids from the competition experiments. Thus, BrPAPS-Cu²⁺ can be used as a simple probe for Hcy in aqueous solution.

Copolymer-Based Fluorescence Nanosensor for In Situ Imaging of Homocysteine in the Liver and Kidney of Diabetic Mice

Homocysteine (Hcy) is one of the important biomarkers of clinical diagnosis, which is closely related to the occurrence and development of many diseases. Current analysis methods have difficulties in detecting Hcy in cells and living organisms. As a powerful technique, fluorescence methods combined the laser confocal imaging technology can achieve real-time visual tracking in cells and in vivo. Herein, we establish a conjugated copolymer-based fluorescence nanosensor (DPA-PFNP-Cu(II)) using the connected 2,7-dibromofluorene and 4,7-bis (2-bromothiophen-5-yl)-2-1-3-benzothiadiazole as the main chain. The competitive coordination between Hcy and Cu(II) allows the fluorescence of the polymer off to on. Finally, the nanosensor is applied for in situ imaging of Hcy levels in the kidney and liver of diabetic mice and is found that Hcy levels were positively correlated with the degree of diabetes. Notably, the depth of tissue penetration of the nanosensor enables Hcy detection of the liver and kidney through in vivo imaging without damage. Two-photon imaging and in vivo imaging achieve consistent results, which correct each other, improving the accuracy of the test result. The present works provide a new imaging technique for studying the occurrence and development of diabetes and screening of new drugs for treatment at the living level.

BODIPY-Based Fluorescent Probes for Biothiols

Fluorescent probes for biothiols have aroused increasing interest owing to their potential to enable better understanding of the diverse physiological and pathological processes related to the biothiol species. BODIPY fluorophores exhibit excellent optical properties, which can be readily tailored by introducing diverse functional units at various positions of the BODIPY core. In the present review, the development of fluorescent probes based on BODIPYs for the detection of biothiols are systematically summarized, with emphasis on the preferable detection of individual biothiols, as well as simultaneous discrimination among cysteine (Cys), homocysteine (Hcy), reduced glutathione (GSH). In addition, organelle-targeting probes for biothiols are also highlighted. The general design principles, various recognition mechanisms, and biological applications are elaboratively discussed, which could provide a useful reference to researchers worldwide interested in this area.

Fluorescent probe for sensitive discrimination of Hcy and Cys/GSH in living cells via dual-emission

Abnormal levels of Cys, Hcy and GSH are associated with various diseases, thus monitoring biothiols is of great significance. In this work, a dual-emission responsive near-infrared fluorescent probe NIR-NBD for detecting Hcy and Cys/GSH was developed based on the conjugation of a dicyanoisophorone based fluorophore (NIR-OH) and 7-nitrobenzofurazan (NBD). To our surprise, the addition of Hcy induced significant fluorescence enhancement at both 549 and 697 nm; while Cys/GSH resulted in major fluorescence emission at 697 nm. The detection limit was determined to be 33.2 nM for Cys, 33.5 nM for Hcy, and 34.4 nM for GSH. Therefore, the probe can be used for discriminative detection of Hcy and Cys/GSH. Moreover, fluorescence imaging of HeLa cells indicated that the probe was cell membrane permeable and could be used for visualizing Hcy and Cys/GSH in living cells.

A ratiometric fluorescent BODIPY-based probe for rapid and highly sensitive detection of cysteine in human plasma

Biological thiols, especially low molecular weight thiols, including cysteine (Cys), homocysteine (Hcy) and glutathione (GSH), play a pivotal role in physiological and pathological systems. Thus, the detection of biothiols is highly important for early diagnosis of diseases and evaluation of disease progression. Herein, we developed a highly selective and sensitive ratiometric fluorescent 8-Cl BODIPY-based probe with high fluorescence quantum yields. The probe displayed a sensitive response to Cys and Hcy over other biothiols, which can be visualized colorimetrically and/or fluorescently. The probe was successfully applied to detect Cys in human plasma, demonstrating its great value for practical application in biological sample analysis.

A dual-response fluorescent probe for the discrimination of cysteine from glutathione and homocysteine

A highly selective and sensitive turn-on fluorescent BODIPY-based probe for the simultaneous and selective detection of Cys and Hcy/GSH from dual emission channels was developed. The spatial steric hindrance of the methyl groups at 1- and 7-positions in BODIPY skeleton prevented intramolecular displacement of sulfur with amino group of Hcy but not of Cys. GSH molecular skeleton is larger and amino is far away from sulfydryl group, and the product of the reaction of probe with GSH canstay in thiol phase. Therefore, the probe was successfully applied to the detection of Cys from GSH/Hcy. The confocal microscopy experiments implied that this probe is a promising candidate for imaging of Cys and Hcy/GSH in Hela cells.

Coumarin-based fluorescent ‘AND’ logic gate probes for the detection of homocysteine and a chosen biological analyte

With this research we set out to develop a number of coumarin-based ‘AND’ logic fluorescence probes that were capable of detecting a chosen analyte in the presence of HCys. Probe JEG-CAB was constructed by attaching the ONOO⁻ reactive unit, benzyl boronate ester, to a HCys/Cys reactive fluorescent probe, CAH. Similarly, the core unit CAH was functionalised with the nitroreductase (NTR) reactive p-nitrobenzyl unit to produce probe JEG-CAN. Both, JEG-CAB and JEG-CAN exhibited a significant fluorescence increase when exposed to either HCys and ONOO⁻ (JEG-CAB) or HCys and NTR (JEG-CAN) thus demonstrating their effectiveness to function as AND logic gates for HCys and a chosen analyte.

With this research we set out to develop of a number of coumarin-based ‘AND’ logic fluorescence probes that were capable of detecting a chosen analyte in the presence of HCys.

A series of BODIPY-based probes for the detection of cysteine and homocysteine in living cells

Biothiols, such as glutathione (GSH), homocysteine (Hcy) and cysteine (Cys), are important biomarkers and play crucial roles in many physiological processes. Thus, the detection of biothiols is highly important for early diagnosis of diseases and evaluation of disease progression. Herein, new types of BODIPY-based fluorescent probes (probe 1, probe 2 and probe 3) capable of cysteine (Cys)/homocysteine (Hcy) sensing with high selectivity over other amino acids were developed. In addition, we further studied the influence of different electronegativity substituents on these probes to sensing Cys/Hcy. Ultimately, we concluded that the electron withdrawing group on probe 1 can accelerate the probe response to Cys/Hcy, and probe 1 was successfully applied for selective imaging Cys/Hcy in living cells.

BODIPY-based turn-on fluorescent probes for cysteine and homocysteine

Cysteine (Cys), homocysteine (Hcy), and glutathione (GSH) are interconnected and play essential roles in many biological processes. It is significant importance to detect these thiols for investigating their functions in cells and disease diagnosis. In this work, we have designed and synthesized two novel BODIPY-based turn-on fluorescent probes (BDP-Ph and BDP-R-Ph) carrying 4-methoxythiophenol moiety at meso position as good leaving group for highly selective detection of Cys and Hcy. Furthermore, the probes have been successfully applied to detect intracellular Cys and Hcy by fluorescent imaging in living cells.

Fluorescent chemodosimeter for quantification of cystathionine-γ-synthase activity in plant extracts and imaging of endogenous biothiols

A new reagent for quantification of CgS in plant extracts using a generalized methodology suitable for recognition of homocysteine (Hcy) with luminescence ON response.

Synaptic Effects of Dopamine Breakdown and Their Relation to Schizophrenia-Linked Working Memory Deficits

Working memory is the ability to hold information "online" over a time delay in order to perform a task. This kind of memory is encoded in the brain by persistent neural activity that outlasts the presentation of a stimulus. Patients with schizophrenia perform poorly in working memory tasks that require the brief memory of a target location in space. This deficit indicates that persistent neural activity related to spatial locations may be impaired in the disease. At the circuit level, many studies have shown that NMDA receptors and the dopamine system are involved in both schizophrenia pathology and working memory-related persistent activity. In this Hypothesis and Theory article, we examine the possible connection between NMDA receptors, the dopamine system, and schizophrenia-linked working memory deficits. In particular, we focus on the dopamine breakdown product homocysteine (HCY), which is consistently elevated in schizophrenia patients. Our previous studies have shown that HCY strongly reduces the desensitization of NMDA currents. Here, we show that HCY likely affects NMDA receptors in brain regions that support working memory; this is because these areas favor dopamine breakdown over transport to clear dopamine from synapses. Finally, within the context of two NMDA-based computational models of working memory, we suggest a mechanism by which HCY could give rise to the working memory deficits observed in schizophrenia patients.

Recent Progress in Chemosensors Using Aldehyde-bearing Fluorophores for the Detection of Specific Analytes and their Bioimaging

In recent years, aldehyde-appended fluorescence probes have attracted increasing attention. Fluorescent biological imaging includes many modern applications for cell and tissue imaging in biomedical research. Meanwhile, the nucleophilic mechanism is a very simple and convenient procedure for the preparation of aldehyde-sensing probes. This tutorial review focuses on aldehyde-bearing chemosensors based on nucleophilic addition mechanism with biological applications.

HBT-based turn-on fluorescent probe for discrimination of homocysteine from glutathione/cysteine and its bioimaging applications

In this work, we firstly reported a new type of turn-on fluorescent probe HBTI for Hcy over GSH/Cys based on ESIPT and heavy atom effect strategy.

A mitochondria-targeted turn-on fluorescent probe for the detection of glutathione in living cells

A novel turn-on red fluorescent BODIPY-based probe (Probe 1) for the detection of glutathione was developed. Such a probe carries a para-dinitrophenoxy benzyl pyridinium moiety at the meso position of a BODIPY dye as self-immolative linker. Probe 1 responds selectively to glutathione with the detection limit of 109nM over other amino acids, common metal ions, reactive oxygen species, reactive nitrogen species, and reactive sulfur species. A novel electrostatic interaction to modulate the SNAr attack of glutathione was believed to play significant role for the observed selective response to glutathione. The cleavage of dinitrophenyl ether by glutathione leads to the production of para-hydroxybenzyl moiety which is able to self-immolate through an intramolecular 1,4-elimination reaction to release the fluorescent BODIPY dye. The low toxic probe has been successfully used to detect mitochondrial glutathione in living cells.

Fluorescein Tri-Aldehyde Promotes the Selective Detection of Homocysteine

Elevated homocysteine levels are a well-known independent risk factor for cardiovascular disease. To date, relatively few selective fluorescent probes for homocysteine detection have been reported. The lack of sensing reagents and remaining challenges largely derive from issues of sensitivity and/or selectivity. For example, homocysteine is a structural homologue of the more abundant (ca, 20-25 fold) aminothiol cysteine, differing only by an additional methylene group side chain. Fluorescein tri-aldehyde, described herein, has been designed and synthesized as a sensitive and selective fluorophore for the detection of homocysteine in human plasma samples. It responds to analytes selectively via a photoinduced electron transfer (PET) inhibition process that is modulated by predictable analyte-dye product hybridization and ionization states. Mulliken population analysis of fluorescein tri-aldehyde and its reaction products reveals that the characteristic formation of multiple cationic of homocysteine-derived heterocycles leads to enhanced relative negative charge build up on the proximal phenolate oxygen of the fluorophore as a contributing factor to selective emission enhancement.

A near-infrared BODIPY-based fluorescent probe for the detection of hydrogen sulfide in fetal bovine serum and living cells

A near-infrared (NIR) “off–on” fluorescent probe was developed for the detection of H₂S. The new probe possesses a highly selective and sensitive response to H₂S. The probe has low toxicity and was successfully used to detect H₂S in biological serum samples and living cells.

Highly selective colorimetric and fluorescent BODIPY dyes for sensing of cysteine and/or homocysteine

Selective “naked eye” and fluorescent sensing of biothiols by visible-light excitable α- and β-formyl BODIPYs1and3was developed.

Long-wavelength analyte-sensitive luminescent probes and optical (bio)sensors

Long-wavelength luminescent probes and sensors become increasingly popular. They offer the advantage of lower levels of autofluorescence in most biological probes. Due to high penetration depth and low scattering of red and NIR light such probes potentially enable in vivo measurements in tissues and some of them have already reached a high level of reliability required for such applications. This review focuses on the recent progress in development and application of long-wavelength analyte-sensitive probes which can operate both reversibly and irreversibly. Photophysical properties, sensing mechanisms, advantages and limitations of individual probes are discussed.

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.

Design strategies of fluorescent probes for selective detection among biothiols

Simple thiol derivatives, such as cysteine (Cys), homocysteine (Hcy), and glutathione (GSH), play key roles in biological processes, and the fluorescent probes to detect such thiols in vivo selectively with high sensitivity and fast response times are critical for understanding their numerous functions. However, the similar structures and reactivities of these thiols pose considerable challenges to the development of such probes. This review focuses on various strategies for the design of fluorescent probes for the selective detection of biothiols. We classify the fluorescent probes for discrimination among biothiols according to reaction types between the probes and thiols such as cyclization with aldehydes, conjugate addition-cyclization with acrylates, native chemical ligation, and aromatic substitution-rearrangement.

Synthesis of mono-(p-dimethylamino)styryl-containing BOPHY dye for a turn-on pH sensor

Mono-substitutional BOPHY 3a with a (p-dimethylamino)styryl group in the α-position was confirmed to be synthesized by the Knoevenagel-type condensation. Dimethylamino-containing BOPHY 3a can be used as a pH probe.

A styryl-containing aza-BODIPY as a near-infrared dye

A novel styryl-containing pyrrole was developed by the reaction of E-4-phenylbut-3-en-2-one with 3-phenyl-2H-azirine in the presence of LDA. Utilizing this pyrrole, asymmetric styryl-containing aza-BODIPYs have been prepared in the NIR region as a result of the extension of conjugation.