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Curcumin-based “Enhanced SNAr” Promoted Ultrafast Fluorescent Probe for Thiophenols Detection in Aqueous Solution and in Living Cells Yongkang Yue, Fangjun Huo, Yongbin Zhang, Jianbin Chao, Caixia Yin, and Ramon Martinez-Mañez Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b02520 • Publication Date (Web): 03 Oct 2016 Downloaded from http://pubs.acs.org on October 3, 2016
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Analytical Chemistry
Curcumin-based “Enhanced SNAr” Promoted Ultrafast Fluorescent Probe for Thiophenols Detection in Aqueous Solution and in Living Cells Yongkang Yue,† Fangjun Huo,‡ Yongbin Zhang,‡ Jianbin Chao,‡ Caixia Yin,†,* Ramón MartínezMáñez,ǁ,§,* †
Key Laboratory of Chemical Biology and Molecular Engineering of Ministry of Education, Key Laboratory of Materials for Energy Conversion and Storage of Shanxi Province, Institute of Molecular Science, Shanxi University, Taiyuan 030-006, China. *E-mail:
[email protected]. Tel/Fax: +86-351-7011022 ‡ Research Institute of Applied Chemistry, Shanxi University, Taiyuan 030-006, China ǁ Instituto Interuniversitario de Investigación de Reconocimiento Molecular y Desarrollo Tecnológico, (IDM), Universitat Politécnica de València, Universitat de València. Camino de Vera s/n, 46022-Valencia, Spain. *E-mail:
[email protected]. Tel/Fax: +34 963877343 § CIBER de Bioingenierίa, Biomateriales y Nanotecnologίa (CIBER-BBN), Spain ABSTRACT: We report herein a highly selective and sensitive turn-on fluorescent probe (compound 1) with a fast response time (less than 2 min) for thiophenol detection based on an “enhanced SNAr” reaction between thiophenols and a sulfonyl-ester moiety covalently attach to curcumin. Reaction of 1 in Hepes−MeOH (1:1, v/v, pH 7.4) in the presence of 4-methylthiophenol (MTP) resulted in a remarkable enhancement of the fluorescence. A linear response in the presence of MTP of the relative fluorescent intensity (F − F0) of 1 at 536 nm in the 0−40 µM MTP concentration range was found. A limit of detection (LOD) for the detection of MTP of 26 nM, based on the definition by IUPAC (CDL = 3 Sb/m), was calculated. Probe 1 was applied to monitor and imaging exogenous MTP in live cells and to the detection of MTP in real water samples.
Thiols, including aliphatic thiols and thiophenols, are important compounds in both the chemical industry and in biological systems. Although aliphatic thiols, (e.g. cysteine, homocysteine and glutathione), are molecules that show important roles in many physiological processes,1−5 thiophenols are highly toxic and are widely used in the preparation of polymers, pesticides and certain pharmaceuticals.6 Median lethal dose (LC50) values of thiophenols for fish have been reported at concentrations of ca. 0.01−0.4 mM.7,8 Exposure to thiophenol as liquid and vapor has been reported to cause serious health problems, including central nervous system damage, increased respiration, muscle weakness, hind limb paralysis and coma, and even death.9 As a serious pollutant species, thiophenols have been added to the priority lists of pollutants by the United States Environment Protection Agency (EPA waste code P014).10 In this context, the development of simple fast techniques for the selective detection of thiophenols is of importance in the fields such as chemistry, biology and environmental sciences. Fluorescent probes for the detection of thiophenols were first reported by Lin’s and coworkers through the thiolysis of dinitrophenyl ethers. This simple procedure has been used to detect thiophenols in water, soil and living cells.11 Wang’s group developed a probe which was able to signal thiophenols via a nucleophilic aromatic substitution (SNAr) mechanism with the release of SO2 and a fluorophore, which resulted in the revival of emission.12 In this scenario, several fluorescent
probes, showing relatively high sensitivity and selectivity, have been recently reported based on these general mechanisms.13−23 However, all reported probes have shown a slow response, with reaction times usually within the 20-30 min range, and, as far as we know, the fastest reported probe able to detect thiophenols takes over 5 minutes. This logy response influences accuracy when detecting thiophenols in practical applications because of the oxidative activity of thiophenol derivatives.24 Moreover, a slow response hinders the real-time visualization of thiophenols in both in vitro and in vivo studies. Thus designing fluorescent probes with a fast response, high sensitivity and selectivity toward thiophenols is still a challenge. Most thiophenols probes reported to date follow strategies based on the use of a SNAr reaction of thiophenols with a sulfamide moiety or a phenolic ether group (RNH-SO2R’, RO-R’). In this context we envisioned that the use of a sulfonylester (RO-SO2R’) would activate the reaction with thiophenols to provide a faster response. In order to complete our design we selected curcumin as a satisfactory fluorophore for its eminent optical properties and biocompatibility.25,26 By combining curcumin and a sulfonyl-ester moiety we prepared probe 1 (see Scheme 1). Benefited by the stronger SNAr activity of thiophenols, which was promoted by the lower pKa value of thiophenols compared with aliphatic thiols,12,13,15 probe 1 displayed a high sensitivity and selectivity response to thiophenols with a very fast response (ca. 100s). We also demon-
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strated that probe 1 can be used for the detection of thiophenols in living cells. Scheme 1. Design and Synthesis of Probe 1 for Thiophenols.
EXPERIMENTAL SECTION Materials and Chemicals. All chemicals were purchased from commercial suppliers and used without further purification. All solvents were purified prior to use. Distilled water was used after passing through a water ultra-purification system. TLC analysis was performed using precoated silica plates. Hitachi F−7000 fluorescence spectrophotometer was employed to measure fluorescence spectra. Shanhai Huamei Experiment Instrument Plants, China provided a PO-120 quartz cuvette (10 mm). 1H NMR and 13C NMR experiments were performed with a BRUKER AVANCE III HD 600 MHz and 151 MHz NMR spectrometer, respectively (Bruker, Billerica, MA). Coupling constants (J values) are reported in hertz. HR MS determinations were carried out on an AB SCIEX TripleTOF 5600 Instruments. The cell imaging experiment was measured using a Leica DMi8 fluorescence inversion microscope system. Synthesis of Probe 1. Curcumin (0.368 g, 1 mmol), trimethylamine (3 equiv.) was dissolved in 20 mL chloroform. After refluxed for 5 min, 2,4-dinitrobenzenesulfonyl chloride (2 equiv.) in 5 mL chloroform was added dropwise to the system and the mixture was refluxed for 5 hours. After cooling to room temperature, the mixture was filtrated, washed with 20 mL cold MeOH, and dried under reduce pressure. The crude product was purified on a silica gel column using ethyl acetate/petroleum ether (1:1) to afford 0.331 g (0.4 mmol, 40 %) probe 1 as yellow solid. 1H NMR (600 MHz, DMSOd6): δ 9.09 (s, 2H), 8.66 (d, J = 8.6 Hz, 2H), 8.30 (d, J = 8.8 Hz, 2H), 7.64 (d, J = 15.8 Hz, 2H), 7.54 (s, 2H), 7.36 (d, J = 8.3 Hz, 2H), 7.29 (d, J = 8.5 Hz, 2H), 7.04 (d, J = 16.0 Hz, 2H), 6.19 (s, 1H), 3.59 (s, 6H); 1C NMR (151 MHz, DMSO-d6): δ 183.5, 151.7, 151.6, 148.2, 139.9, 138.9, 136.3, 133.4, 132.9, 127.8, 126.4, 124.6, 121.9, 121.2, 113.4, 102.6, 56.4. HR MS [M-H]−: m/z Calcd 827.0454, Found 827.0468. Cell Imaging and Cytotoxicity Assay. The HepG2 cells were grown in Dulbecco’s Modified Eagle’s medium supplemented with 12% Fetal Bovine Serum and 1% antibiotics at 37 °C in humidified environment of 5% CO2.27-28 Cells were plated on 6-well plate and allowed to adhere for 24 h. Before the experiments, cells were washed with PBS 3 times. Some of
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the HepG2 were treated with 30 µM of probe 1 (THF stock solution) in culture media for 30 min at 37 °C and washed 3 times with PBS. Meanwhile, another portion of HepG2 cells were incubated with 1 (30 µM THF stock solution) for 30 min at 37 °C with 30 µM of 4-methylthiophenol (MTP) added for 10 min, 20 min, and 30 min respectively. Cell imaging was then carried out after washing cells with PBS buffer. Cytotoxicity Experiments. HepG2 cells were seeded in 96well plates and cultured at 37 °C (5% CO2) for 24 h. After washing with PBS, different concentrations of probe 1 (0, 2.5, 5, 10, 25, and 50 µM) in culture medium (without serum) were added to the wells and incubated for 5 or 10 h. Subsequently, CCK-8 (10 % in serum free culture medium) was added to each well which was washed with PBS two times, and the plate was incubated for another 1 h. Optical densities at 450 nm were then measured. MTP Detection in Real Samples. Water samples were collected from Linde Lake and rainwater in Shanxi University. After filtration, the samples were spiked with different amounts of MTP (3 µM, 10 µM, 20 µM).17, 29 Upon the addition of 1 mL water sample to 2 mL probe 1 in Hepes-MeOH buffer (1:3, pH 7.4), the resulting 3 mL mixture was used for further fluorescent measurement. The concentration of MTP was determined with probe 1 using a calibration curve. Each concentration was determined three times.
RESULTS AND DISCUSSION Design of Probe 1 and the Proposed Detection Mechanism. In this work, we utilized a sulfonyl-ester moiety to synthesize a novel “enhanced SNAr” based fluorescent probe for thiophenols detection. Benefited from the weaker electron donating effect of the sulfonyl-ester moiety compared with the reported sulfamide and phenolic ether moieties, probe 1 featured ultrafast responses towards thiophenols while maintained high sensitivity and selectivity properties. The proposed detection mechanism of probe 1 for the detection of thiophenols was displayed in Scheme 2. For probe 1, the strong electron withdrawing moiety quenched the fluorescent emission. However, the sulfydryl of of thiophenols induced SNAr process eliminated the 2,4-dinitrobenzenesulfonyl moiety to produce curcumin and released the fluorescent emission. To further confirm the detection mechanism, we performed 1H NMR studies in DMSO-d6 upon reaction of 1 with 4-methylthiophenol (MTP). Compared with the 1H NMR spectrum of 1, the methyl ether proton at 3.59 ppm disappeared and a new signal at 3.84 ppm emerged, which belonged to the methyl ether proton of curcumin upon the addition of MTP (Figure S6). Moreover a new signal at 2.42 ppm in the reaction mixture was assigned to the methyl proton of compound 3. Finally, a peak appeared at 369.1328, which corresponded to [curcumin + H]+ (HR MS [curcumin + H]+: m/z Calcd 369.1333, Found 369.1328) in the HR MS experiments, which further proved the above-mentioned mechanism (Figure S6). By combining the UV-vis spectral (Figure S2) and these signal changes, the detection mechanism was verified as the desulfonylation of 1 in the presence of thiophenols via a SNAr reaction.
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Scheme 2. Proposed Detection Mechanism of MTP using Probe 1. O O
O S O
NO2
Ar SN
O
OH
SO2
NO2 SH O
pH = 7.4
O +
O2N
NO2
HO
HO
S NO2
O O S
NO2
O
OH
r
O probe 1
NA
O
S
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Analytical Chemistry
curcumin
3
Sensing Properties of Probe 1 towards MTP. In a first step we tested the sensing behavior of probe 1 to MTP, as a representative thiophenol, in Hepes buffer−MeOH (1:1, v/v, pH 7.4) at 25°C. As it can be seen in Figure 1a, 1 (50 µM) was not fluorescent (λex = 404 nm). However, addition of MTP induced the appearance of a significant fluorescent emission at 536 nm with a 20-fold fluorescent intensity enhancement when the concentration of MTP reached 80 µM. UV–vis spectral changes were also studied (Figure S1). Probe 1 in Hepes−MeOH (1:1, v/v, pH 7.4) showed a broad peak at 395 nm, whereas new peaks emerged at 358 nm and 429 nm upon the addition of MTP. When the UV–vis spectrum of probe 1 after reaction with MTP was compared to that of curcumin (Figure S2) in Hepes buffer−MeOH (1:1, v/v, pH 7.4), a complete match was observed. This strongly suggests that the reaction shown in Scheme 2 took place. In order to further investigate the sensitivity of 1 to MTP, titration experiments of 1 using certain concentrations of MTP (0−40 µM) in Hepes−MeOH (1:1, v/v, pH 7.4) were carried out. A linear response of the fluorescent intensity (F − F0) at 536 nm within the 0−40 µM MTP concentration range was found (see Figure 1b), which indicates that probe 1 may be potentially used to quantitatively detect MTP. The limit of detection (LOD), based on the definition by IUPAC (CDL = 3 Sb/m), was 26 nM (from 10 blank solutions).30−33 This demonstrates that probe 1 is highly sensitive for thiophenols when compared with other thiophenols chemosensors (Table S1).
Figure 1. (a) Changes in the emission of 1 (50 µM) upon addition of MTP (90 µM) in Hepes buffer−MeOH (1:1, v/v, pH 7.4) at 25 °C (the insert shows the emission intensity of 1 as a function of the concentration of MTP). (b) F-F0 changes of 1 (50 µM) in the presence of MTP (0-40 µM concentration range) in Hepes buffer−MeOH (1:1, v/v, pH 7.4) at 25 °C. λex = 404 nm, λem = 536 nm, slit: 10 nm/5 nm. In order to evaluate selectivity, the response of probe 1 (50 µM) to other analytes, including thiophenol, 2aminobenzenethiol, phenol, aniline, Cys, Hcy, GSH, mercaptoethanol, NaHS, NaHSO3, NaCN, NaSCN, NaCl, NaBr and H2O2 was also tested in Hepes−MeOH (1:1, v/v, pH 7.4). These studies showed a selective response of 1 to thiophenols (i.e. MTP, thiophenol and 2-aminobenzenethiol). Not even 40 equiv. of other species induced any emission modulation (Figure 2a). Competition experiments were also carried out and probe 1 was used to detect MTP in the presence of other analytes. As it can be seen in Figure 2b, none of the potential competing species interfered with MTP detection, which indicated that 1 is highly selectivity toward thiophenols.
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Figure 2. (a) Fluorescent response of 1 (50 µM) in the presence of various analytes including MTP, thiophenol, 2aminobenzenethiol, phenol, aniline, Cys, Hcy, GSH, mercaptoethanol, NaHS, NaHSO3, NaCN, NaSCN, NaCl, NaBr and H2O2 (50 µM for MTP, thiophenol, and 2aminobenzenethiol and 2000 µM for other species) in Hepes buffer−MeOH (1:1, v/v, pH 7.4). (b) Competing responses of 1 (50 µM) to various substances (50 µM for MTP and 2000 µM for other species) in Hepes buffer−MeOH (1:1, v/v, pH 7.4). (1) blank, (2) MTP, (3) phenol, (4) aniline, (5) Cys, (6) Hcy, (7) GSH, (8) mercaptoethanol, (9) NaHS, (10) NaHSO3, (11) NaCN, (12) NaSCN. λex= 404 nm, λem= 536 nm, slit: 10 nm/5 nm. The optimum pH for the system was investigated using solution with pH values between 2.0 and 13.0. As shown in Figure 3a, probe 1 displayed distinct turn-on fluorescent responses to MTP within the 6.0−11.0 range, which further demonstrated the wide potential applicability of probe 1 in different media (details in Figure S3). The kinetic analysis of 1 in the presence of 100 equiv. of MTP evidenced that the reaction ended in 100 s (Figure 3b), which was the fastest fluorescent probe for thiophenols reported so far (see Table S1). The pseudo-first-order rate constant was calculated to be 0.12 s−1. This fast distinct response in fluorescent emission suggests that 1 could be used as a fluorescent probe for real-time MTP monitoring.
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Figure 3. (a) Fluorescent intensity of 1 (50 µM) with the addition of MTP (40 µM) in the solution of MeOH and Hepes buffer at various pH values (λex= 404 nm, λem= 536 nm). (b) Kinetic analysis of probe 1 towards MTP (50 µM 1 with 100 equiv. of MTP) in Hepes buffer−MeOH (1:1, v/v, pH 7.4) at 25 °C (Inset: linear fitting of pseudo-first order reaction). Cellular Imaging. Cytotoxicity experiments demonstrated negligible toxicity of probe 1 towards HepG2 cells after 10 h culturing even at concentrations up to 50 µM (Figure S4). Promoted by the excellent optical properties and low toxicity, probe 1 was further used for MTP detection in HepG2 cells. As shown in Figure 4a1, the HepG2 cells displayed no fluorescence when incubated with 30 µM of 1 for 30 min at 37 °C. Conversely, the further addition of MTP (30 µM) induced the appearance of significant fluorescent emission (Figure 4a2, 4a3 and 4a4), which was clearly observable after only 10 min. Further emission enhancement was observed according to time, which could be caused by slow MTP permeation in cells. These results indicated that 1 can be applied to the straightforward visualization of thiophenols in living cells.
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Analytical Chemistry 1 is a suitable probe for both the environmental and biological fluorescent detection of the highly toxic thiophenols species.
ASSOCIATED CONTENT Supporting Information Structure characterizations of 1, pH dependent spectra and data for investigation of the sensing mechanism. This material is available free of charge on the ACS Publications website.
AUTHOR INFORMATION Figure 4. Fluorescence imaging of HepG2 cells: (a1) fluorescence image of HepG2 cells after treated with 1 (30 µM) after 30 min; (a2) (a3) (a4) fluorescence images of HepG2 cells preincubated with 1 (30 µM) and further tretaed with MTP (30 µM) for 10 min, 20min, and 30 min respectively; (b1-b4) bright fields of a1-a4. MTP Detection in Water Samples. Finally, to value the potential utility of probe 1 to determine MTP in a real scenario, experiments were carried out to detect MTP in water samples. In a typical experiment filtrated water samples from Linde Lake and rainwater (Shanxi University) were spiked with 2016 2016different concentrations of MTP (3 µM, 10 µM, 20 µM) and mixed with probe 1 in Hepes-MeOH buffer (see Experimental section for details). The concentration of MTP in the resulting mixture was determined using a calibration curve similar to that shown in Figure 1b. Recoveries ranged from 94% to 103% which demonstrated the applicability of probe 1 for MTP detection in real water samples. Table 1. Detection of MTP in Water Samples sample
MTP spiked
MTP recovered
Recovery
(µM)
(µM)
(%)
0
not detected
–
3
2.91 ± 0.21
97
10
9.39 ± 0.13
94
20
19.03 ± 0.33
95
0
not detected
–
3
2.85 ± 0.07
95
10
10.34 ± 0.34
103
20
19.11 ± 0.25
96
rainwater
Linde Lake water
CONCLUSIONS In summary, we have designed, synthesized and characterized a new fluorescent probe for thiophenols detection. The sensing mechanism is based on the desulfonylation process of a sulfonyl-ester via an enhanced nucleophilic aromatic substitution. Probe 1 displayed a fast response time (less than 2 minutes) and proved to be highly selective and sensitive for the fluorogenic detection of thiophenols. Probe 1 showed an excellent membrane permeability and low toxicity, allowing the detection of MTP in HepG2 cells. Moreover, 1 was also applied to the detection of MTP in real water samples.
Corresponding Author * E-mail:
[email protected]. Tel/Fax: +86-351-7011022
* E-mail:
[email protected]. Tel/Fax: +34 963877343 Author Contributions The manuscript was written through contributions of all authors. / All authors have given approval to the final version of the manuscript.
ACKNOWLEDGMENT The work was supported by the National Natural Science Foundation of China (No. 21472118), the Program for the Top Young and Middle-aged Innovative Talents of Higher Learning Institutions of Shanxi (TYMIT, No. 2013802), talents Support Program of Shanxi Province (No. 2014401), Shanxi Province Outstanding Youth Fund (No. 2014021002), and Shanxi University funds for study aboard 2014. The Generalitat Valenciana (Project PROMETEOII/2014/047) and the Spanish Government (Projects MAT2012-38429-C04 and MAT201564139-C4-1 MINECO/FEDER)) are also gratefully acknowledged.
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