Investigating the Photophysical Aspects of a Naphthalene-Based Excited-State Proton Transfer Dye 1-(1H-Benzo[d]imidazol-2-yl)naphthalen-2-ol: pH-Dependent Modulation of Photodynamics

The steady state and time-resolved photophysical behavior of a proton transfer dye 1-(1H-benzo[d]imidazol-2-yl)naphthalen-2-ol (H-BINO) was investigated. The excited state intramolecular proton transfer (ESIPT) reaction in H-BINO is predominant in nonpolar solutions (toluene and DCM) with a lifetime of ∼1.0 ns. However, in polar media (DMF and MeOH), the excited state photodynamics is characterized by a complex equilibrium of emission from the locally excited state (0.1-2.3 ns), the phototautomer (0.5-1.2 ns), and the anionic emission (2.1-5.4 ns). In the solid state, emission from the various aggregated states dictates the photobehavior. Interestingly, the photodynamics in aqueous solution changes starkly as a function of pH with the anionic (2.1 ns) and phototautomeric (0.5-1.0 ns) emissions guiding the photodynamics as the pH of the medium increases. Optimized structural parameters at the proton donor and acceptor sites for the enol and keto forms and the calculated potential energy curve along the proton transfer coordinate at the density functional theory (DFT) level with the B3LYP/6-311G++(d,p) theory support a favorable and barrierless ESIPT process. The current results will surely boost the ongoing research on small molecule emissive materials.

Aggregation-induced emission for the detection of peptide ligases with improving ligation efficiency

BACKGROUND

Monitoring peptide ligase activity is of great significance for biological research, medical diagnosis, and drug discovery. The current methods for the detection of peptide ligases suffer from the limitations of high background signal, elaborate design of substrate, and high reversibility of ligation reaction. In this work, we proposed a simple and sensitive method for ligase detection with reducing ligation reversibility on the basis of aggregation-induced emission (AIE) mechanism.

RESULTS

The peptide probes labeled with AIE luminogens (AIEgens) were water-soluble and emitted weak fluorescence. After ligation reaction, the enzymatic products with AIEgens showed high hydrophobicity and could readily assembly into aggregates, thus lighting up the fluorescence. More interestingly, the formation of aggregates pushed the equilibrium to the generation of the desired ligation products, thus improving the catalytic efficiency by driving the reaction towards completion. The ligation reaction conversion rate (>80 %) is significantly higher than that without blocking the reversibility with additional treatment. With sortase A (SrtA) as the analyte example, the detection limit of this method was found to be 0.01 nM with a linear range of 0-50 nM. The system was applied to evaluate the inhibition efficiency of berberine chloride and quercetin and determine the activity of SrtA in serum, lysate and Staphylococcus aureus with satisfactory results.

SIGNIFICANCE

This study indicated that the ligation efficiency and detection sensitivity can be improved by reducing ligation reversibility through AIE phenomenon. The proposed strategy could be used for the detection of other peptide ligases by adopting sequence-specific peptide substrates.

A simple "turn-on" fluorescent probe capable of recognition cysteine with rapid response and high sensing in living cells and zebrafish

Cysteine (Cys), an essential biological amino acid, participates several crucial functions in various physiological and pathological processes. The sensitive and specific detection of Cys is of great significance for understanding its biological function to disease diagnosis. Herein, we designed and synthesized a simple fluorescence sensor 2-(benzothiophen-2-yl)-4-oxo-4H-chromen-3-yl acrylate (BTCA) composed of a flavonol skeleton as the fluorophore and acrylic ester group as the recognition receptor. Probe BTCA displayed high selectivity and extremely fast response toward Cys in phosphate buffer solution in the presence of other competitive species even Homocysteine (Hcy) and Glutathione (GSH) owing to a specific conjugate addition-cyclization reaction between the acrylate moiety and Cys. The photoluminescence mechanism of probe BTCA toward Cys was modulated by excited state intramolecular proton transfer (ESIPT) process. The sensing property for Cys was studied by UV-Visible, fluorescence spectrophotometric analyses and time-dependent density functional theory (TD-DFT) calculations, those results indicated that probe BTCA possessed excellent sensitivity, higher specificity, dramatically "naked-eye" fluorescence enhancement (30-fold), high anti-interference ability, especially immediate response speed (within 40 s). Additionally, the practicability of sensor BTCA in exogenous and endogenous Cys imaging in living cells and zebrafish was elucidated as well, suggesting that it has remarkedly diagnostic significance in physiological and pathological process.

Real-time imaging of intracellular cysteine level fluctuations during Cu2+ or H2O2 induced redox imbalance using a turn-on fluorescence sensor

Redox balance is a necessary guarantee to maintain the normal physiological activities of organisms. Cysteine (Cys), a critical biological thiol, has the effect of maintaining redox balance in the body. The concentration of intracellular Cys is abnormal under redox imbalance, thereby resulting in multiple diseases. Additionally, studies have revealed that Cu²⁺ can stimulate the body to produce excess reactive oxygen species (ROS, similar to H₂O₂), and the generated ROS will consume reducing substances (such as Cys) in the body, leading to redox imbalance. Thus, finding a simple and effective method to monitor Cys under redox imbalance is pressing. Here, a turn on probe (DDNO) was proposed by connecting SBD-Cl to a red dye (HDM). The probe can specifically recognize Cys with rapid response (180 s) and low detection limit (0.61 μM) through substitution-rearrangement reaction between sulfhydryl and chlorine atom. Bioimaging experiments indicated that the probe has good biocompatibility and cell membrane permeability, which can be applied to monitor the fluctuation of Cys levels in live cells and zebrafish under the redox imbalance induced by Cu²⁺ or H₂O₂.

Probe with large Stokes shift for effective cysteine imaging in living cells

A new fluorescence probe L, which featured with a large Stokes shift (216 nm), was designed for sensitive detection of cysteine (Cys) and a potential sensing mechanism derived from excited state intramolecular proton transfer (ESIPT) was proposed. More importantly, probe L exhibits higher selective to Cys than other amino acid due to its specific cyclization between acrylate group and Cys. Meanwhile, the probe L shows a low detection limit of 8.82 × 10^-8 M, which is enough for detecting Cys in organisms. Furthermore, this probe displays high biocompatibility and can image Cys in living cells.

A turn-on fluorescence probe for cysteine/homocysteine based on the nucleophilic-induced rearrangement of benzothiazole thioether

A fluorescent probe, 4-(benzothiazole-2-ylthio)-7-nitro-2,1,3-benzoxadiazole (TBT-NBD) was developed for cysteine (Cys) and homocysteine (Hcy). The reaction mechanism was based on the Cys/Hcy-induced nucleophilic substitution of benzothiazole thioether then Smiles rearrangement reaction to form corresponding amino-nitrobenzoxadiazole, which emitted yellow-green fluorescence and guaranteed the high selectivity for Cys/Hcy over glutathione (GSH). TBT-NBD could detect Cys/Hcy within 5 min in the presence of high concentration of GSH and other amino acids. Moreover, TBT-NBD had been exploited to identify intracellular Cys/Hcy in living cells in light of its low toxicity.

Exploring the hidden potential of a methoxy substituted HBT derivative as an efficient example of coupling of AIE and ESIPT processes and as an energy harvesting platform

A methoxy substituted HBT derivative 2-(benzo[d]thiazol-2-yl)-6-methoxyphenol (TMP) showed coupling of AIE and ESIPT, underwent FRET with Rhodamine B and detected Sulfide in pure water by ratiometry.

A Hybrid Coumarin-Semifluorescein-Based Fluorescent Probe for the Detection of Cysteine

A new type of turn-on fluorescent probe CF-AC for the detection of Cys was firstly reported. The probe exhibited an excellent response to Cys with high selectively and sensitivity. In the presence of Cys, two fluorescence emission peaks at 525 nm and 650 nm appeared accompanied by the fluorescence color change from blue to red. Morever, the probe had good biocompatibility and could be successfully used for fluorescence imaging of Cys in MCF-7 cells.

A dynamic sugar based bio-inspired, self-healing hydrogel exhibiting ESIPT

A bio-inspired, self-healing chitosan-5-(benzo[d]thiazol-2-yl)-4-hydroxyisophthalaldehyde (CBTHP) fluorescent hydrogel has been developed which exhibits ultrafast ESIPT in both gel and solution phase.

A Simple and Rapid Turn On ESIPT Fluorescent Probe for Colorimetric and Ratiometric Detection of Biothiols in Living Cells

Biothiols, such as cysteine (Cys), homocysteine (Hcy), and glutathione (GSH), play a key role in an extensive range of physiological processes and biological functions. Therefore, the selective and sensitive detection of intracellular thiols is important for revealing cellular function. In this study, ethyl 2-(4-(acryloyloxy)-3-formylphenyl)-4-methylthiazole-5-carboxylate (NL-AC) was designed and synthesized as a colorimetric and ratiometric fluorescent probe that can be utilized to rapidly, sensitively and selectively detect biothiols in physiological media. The fluorescence intensity of this probe using the three target biothiols at a concentration of 20 equiv. of the probe increased by approximately 6~10-fold in comparison to that without the biothiols in aqueous solution. The limits of detection (LOD) for Cys, Hcy and GSH were 0.156, 0.185, and 1.838 μM, respectively. In addition, both ¹H-NMR and MS analyses suggested the mechanism of fluorescence sensing to be excited-state intramolecular proton transfer (ESIPT). The novel colorimetric and ratiometric probe is structurally simple and offers detection within 20 min. Furthermore, this probe can be successfully applied in bioimaging. The results indicate high application potential in analytical chemistry and diagnostics.