Development of a water-soluble near-infrared fluorescent probe for endogenous cysteine imaging

Regulating the photophysical properties of ESIPT-based fluorescent probes by functional group substitution: a DFT/TDDFT study

CONTEXT

In recent years, fluorescent probe technology has received more and more attention. However, the photophysical and photochemical properties of probe molecules still need to be further explored. This paper presents the excited state intramolecular proton transfer (ESIPT) processes and photophysical properties of the probe molecule 4-bromo-2-((E)-((Z)-((5-bromo-1H-indol-2-yl) methylene) hydrazono) methyl) phenol (BHPL) and its four derivatives (BHPL2, BHPL3, BHPL4, and BHPL5). Infrared spectra and geometric structure analyses revealed that introducing the -NH₂ group on the benzene ring with the hydroxyl group will enhance the intramolecular hydrogen bond, which benefits the ESIPT process. Combining their absorption and fluorescence spectra, it can be concluded that BHPL2 and BHPL4 are both excellent probe candidates due to their large Stokes shift. The hole and electron and root mean square displacement analyses manifest that the fluorescence quenching of BHPL4 may be due to the intramolecular charge transfer process. Potential energy curves of BHPL and its four derivatives noted that ESIPT process of the BHPL2 is the most favorable to occur. The frontier molecular orbital and NBO analyses indicated that besides introducing electron-donating groups to reduce the energy gap and enhance fluorescence emission, introducing double electron-withdrawing groups can also achieve this effect, explaining why the energy barrier of ESIPT process for BHPL2 is lower than BHPL5. This work would provide the theoretical basis for designing novel fluorescence probes with more prominent properties.

METHODS

The ground (S₀) and excited (S₁) state structures of all compounds were optimized by density functional theory (DFT) and time-dependent (TDDFT) method, with B3LYP/6-311+G(d,p) level, respectively. The infrared spectra and potential energy curves were simulated at the same theoretical level. The reduced density gradient scatter plots and interaction region indicator isosurfaces were drawn using Multiwfn and VMD programs. The absorption and fluorescence spectra were simulated by the TDDFT/B3PW91/6-311+G(d,p) method. All the calculations in this work are carried out in Gaussian 16 program package.

Discovery of the cysteine dynamics during the development and treatment of diabetic process by fluorescent imaging

Herein, a novel fluorescent probe RhoDCM was developed for monitoring the cysteine (Cys) dynamics. For the first time, the Cys-triggered implement was applied in relatively complete diabetic mice models. The response of RhoDCM towards Cys suggested advantages including practical sensitivity, high selectivity, rapid reaction, and steadiness in various pH and temperature conditions. RhoDCM could basically monitor the intracellular Cys level, both exogenous and endogenous. It could further monitor the glucose level via detecting consumed Cys. Furthermore, the diabetic mice models including the no diabetic control group, the induced model groups by streptozocin (STZ) or alloxan, and the treatment groups induced by STZ and treated with vildagliptin (Vil), dapagliflozin (DA), or metformin (Metf) were constructed. The models were checked by oral glucose tolerance test and significant liver-related serum indexes. Based on the models, the in vivo imaging and penetrating depth fluorescence imaging both indicated that RhoDCM could characterize the status of the development and treatment in the diabetic process via monitoring the Cys dynamics. Consequently, RhoDCM seemed beneficial for inferring the order of severity in the diabetic process and evaluating the potency of therapeutic schedules, which might be informatic for correlated investigations.

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).

Activity-Based Fluorescent Probes Based on Hemicyanine for Biomedical Sensing

In recent years, fluorescent probes, as an analytical tool that can target and rapidly detect analytes, have been increasingly used for applications related to medical treatment, detection, and bioimaging. Researchers are interested in hemicyanine-based fluorescent probes because of their high quantum yield, tunable spectrum characteristics, absorption and emission in the near-infrared (NIR) region, and good photo-stability. The development of these dyes and their derivatives as NIR fluorescent probes for biological applications has advanced significantly in the last ten years. This review introduces processes for making hemicyanine dyes and the methodology for creating functional activity-based fluorescent probes. A variety of hemicyanine-based probes have been systematically developed for the detection of small biomolecules in various illnesses. Finally, the potential drawbacks of hemicyanine-based functional probes, and the prospects for future research and translation into clinical medicine, are also discussed. This study is intended to provide strategies for the development and design of novel fluorescence probes.

Activity-based NIR fluorescent probes based on the versatile hemicyanine scaffold: design strategy, biomedical applications, and outlook

The discovery of a near-infrared (NIR, 650-900 nm) fluorescent chromophore hemicyanine dye with high structural tailorability is of great significance in the field of detection, bioimaging, and medical therapeutic applications. It exhibits many outstanding advantages including absorption and emission in the NIR region, tunable spectral properties, high photostability as well as a large Stokes shift. These properties are superior to those of conventional fluorogens, such as coumarin, fluorescein, naphthalimides, rhodamine, and cyanine. Researchers have made remarkable progress in developing activity-based multifunctional fluorescent probes based on hemicyanine skeletons for monitoring vital biomolecules in living systems through the output of fluorescence/photoacoustic signals, and integration of diagnosis and treatment of diseases using chemotherapy or photothermal/photodynamic therapy or combination therapy. These achievements prompted researchers to develop more smart fluorescent probes using a hemicyanine fluorogen as a template. In this review, we begin by describing the brief history of the discovery of hemicyanine dyes, synthetic approaches, and design strategies for activity-based functional fluorescent probes. Then, many selected hemicyanine-based probes that can detect ions, small biomolecules, overexpressed enzymes and diagnostic reagents for diseases are systematically highlighted. Finally, potential drawbacks and the outlook for future investigation and clinical medicine transformation of hemicyanine-based activatable functional probes are also discussed.

Long wavelength emission fluorescent probe for highly selective detection of cysteine in living cells

We developed a fluorescent probe, named 2-(4-(acryloyloxy) phenyl)-4-(2-carboxyphenyl)-7-(diethylamino) chromenylium (PA-A), for detecting Cys using the -OH protection/deprotection strategy, which can react with Cys to form a red-emitting anthocyanidin derivative fluorophore. The probe has high selectivity to Cys over Hcy and GSH in phosphate buffer solution (PBS, 10 mM, pH = 7.4), high sensitivity, a low detection limit of 4.48 × 10^-8 mol/L, and it can be recognized with the naked eye. Fluorescence imaging experiment of Cys with PA-A at the cellular successfully showed excellent tissue penetration.

Development of a NIR fluorescent probe for highly selective and sensitive detection of cysteine in living cells and in vivo

Cysteine (Cys) plays important physiological roles in the human body, and abnormal Cys concentrations can cause a variety of diseases. Thus, detecting Cys with high selectivity and sensitivity in vivo is important. Near-infrared (NIR) fluorescent probes are widely employed in biological detection because of their excellent optical properties such as minimal damage to biological samples, low background interference and high signal-to-noise ratio. However, few NIR fluorescent probes that can detect Cys over homocysteine (Hcy) and glutathione (GSH) have been reported because of their similar reactivity and structure. In this work, a highly water-soluble NIR probe (CYNA) for detecting Cys whose structure is similar to that of indocyanine green and is based on cyanine skeleton was synthesized and via aromatic nucleophilic substitution-rearrangement (SNAr-rearrangement) to specific recognize the cysteine. The probe showed high selectivity toward Cys and superior biosecurity, excellent biocompatibility and prolonged dynamic imaging. It also has long fluorescence emission wavelength (820 nm), low detection limit (14 nM) and was successfully applied for visualizing Cys in living cells and mice, which has great promise for applications in noninvasive vivo biological imaging and detection.

ICT-modulated NIR water-soluble fluorescent probe with large Stokes shift for selective detection of cysteine in living cells and zebrafish

The fluorescent probes with good water-solubility, long-wavelength emission and large Stokes shift are greatly desirable for in vivo detection. Herein, we designed a novel 1,8-naphthalimide-based near-infrared (NIR) optical and fluorescent probe (NTC) for sensing cysteine (Cys). Using acrylate as the recognition site, the probe demonstrated high selectivity and sensitivity for Cys with a low detection limit (0.093 μM) in aqueous buffer solution due to the excellent water-solubility. Upon the reaction with Cys, the recovery of intramolecular charge transfer (ICT) in the probe led to about 40-fold fluorescence enhancement. Furthermore, the reaction result was investigated by ¹H NMR, and HRMS analyses, and the sensing mechanism was validated by quantum calculations. Finally, NTC was applied to image exogenous and endogenous Cys in HeLa cells and zebrafish selectively, implying that the probe possessed great potential application in biological fluorescence sensing.

A near-infrared phosphorescent iridium(iii) complex for fast and time-resolved detection of cysteine and homocysteine

Thiol-containing amino acids, cysteine (Cys) and homocysteine (Hcy), play crucial roles in the biosystem; their abnormal contents in the cells are linked to many diseases. Herein, we designed and synthesized a novel near-infrared (NIR) phosphorescent iridium(iii) complex-based probe (FNO1) that can detect Cys and Hcy in real-time in the biosystem. Due to the advantages of the iridium complex, the FNO1 probe had excellent chemical stability and photostability, high luminescence efficiency, and long luminescence lifetime. In addition, the probe showed a fast response, high sensitivity, and low cytotoxicity. As verified by high resolution mass spectra (HR-MS) and density functional theory (DFT) calculations, the detection was achieved through the addition of the α,β-unsaturated ketone group in FNO1 by the nucleophilic thiol group in Cys and Hcy. Through time-resolved emission spectroscopy (TRES) and in the presence of a strongly fluorescent dye rhodamine B, the FNO1 probe could detect Cys and Hcy due to its long luminescence lifetime (260/197 ns). Finally, owing to its NIR-emitting properties, the FNO1 probe was successfully applied in the imaging of Cys and Hcy in living cells, zebrafish, and mice.

A novel benzo[a]phenazin-based fluorescence probe for selective detection of cysteine with anti-cancer potency

Among the physiological and pathological sulfur-containing species, cysteine (Cys) is the most typical one which is an important component of the REDOX system in vivo. Monitoring the level of Cys from other competing species seems quite important in pre-clinical diagnosis and therapeutic evaluation. Herein, we developed a selective fluorescent probe, BPCys, for Cys from the benzo[a]phenazin backbone which had the potential of anti-cancer potency. BPCys suggested advantages including high specificity (40 fold over other species), high sensitivity (detection limit: 18 nM), wide pH adaptability (6.0-11.0) and in particular, the anti-cancer effect. Biological assays and in silico simulation hinted the potency of the detecting product on Topoisomerase I/II. In brief, this study raised a practical strategy for monitoring the Cys level in living cells, especially in cancer models with its anti-cancer potential, thus opened the mind of exploring more specific tool for specific applications.

A fluorescent probe based on reversible Michael addition-elimination reaction for the cycle between cysteine and H2O2

Cysteine oxidation by H₂O₂, generating either cysteine sulfenic acid (CSOH) or disulfide (CSSC), is involved in redox homeostasis and signaling. Compared with quantification of the cysteine content, monitoring the cysteine dynamics in real-time, in particular, takes on even greater importance. However, existing fluorescent probes suffer from low specificity or irreversible recognition mechanisms. In the present work, we have successfully developed a reversible fluorescent probe for the cycle between cysteine and H₂O₂ based on the Michael addition-elimination reaction. This probe features a specific and quantitative response to cysteine. The reversible detection was realized repeatedly with the addition of cysteine and H₂O₂ in order. We also demonstrated its usage for monitoring exogenous and endogenous cysteine in living cells. Eventually, this probe was capable of imaging cysteine dynamically in real-time.

Near-infrared turn-on fluorescent probe for discriminative detection of Cys and application in in vivo imaging

Near-infrared (NIR) fluorescent probes are widely employed in biological detection because of their lower damage to biological samples, low background interference, and high signal-to-noise ratio. Herein, a highly water-soluble NIR probe (NIRHA) based on a hemicyanine skeleton and bearing an acrylate moiety was synthesized. The probe showed high selectivity toward cysteine (Cys) over homocysteine (Hcy) and glutathione (GSH). The probe also had low cytotoxicity and was successfully applied in HeLa cells and mouse experiments. Results of bioimaging experiments indicated that the probe was effective for visualizing endogenous Cys in vitro and in vivo.

Near-infrared (NIR) fluorescent probes are widely employed in biological detection because of their lower damage to biological samples, low background interference, and high signal-to-noise ratio.