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Highly Sensitive Bifunctional Probe for Colorimetric Cyanide and Fluorometric H2S Detection and Bioimaging: Spontaneous Resolution, Aggregation, and Multicolor Fluorescence of Bisulfide Adduct Sumit Kumar Patra,† Sanjoy Kumar Sheet,† Bhaskar Sen,† Kripamoy Aguan,‡ Debesh Ranjan Roy,§ and Snehadrinarayan Khatua*,† †

Centre for Advanced Studies, Department of Chemistry, North Eastern Hill University, Shillong, Meghalaya 793022, India Department of Biotechnology and Bioinformatics, North Eastern Hill University, Shillong, Meghalaya 793022, India § Applied Physics Department, S.V. National Institute of Technology, Surat 395 007, India ‡

S Supporting Information *

ABSTRACT: A 4-methylbenzothiazole linked maleimide-based single molecular bifunctional probe 1 has been synthesized for the colorimetric and fluorometric detection of highly competitive H2S and cyanide ion in aqueous DMSO media. The probe 1 selectively detected CN− under the UV−vis spectroscopy through the rapid appearance of deep pink color. The bright pink color developed due to ICT in the moderately stable cyano substituted enolate intermediate. The absorbance titration of 1 with CN− revealed a new band at 540 nm and the nonlinear curve fitting analysis showed good fit with 1:1 model. In fluorescence channel, 1 was found to be highly selective to H2S in 50% aqueous buffer (pH 7). It exhibited ∼16-fold fluorescence intensity enhancement at 435 nm after reaction with 1 equiv of H2S due to the inhibition of PET. The 1-SH adduct showed TICT phenomenon and behaved like molecular rotor. It further displayed aggregation behavior at higher concentration and excitation wavelength dependent multicolor emission properties. Most interestingly, the spontaneous resolution of chiral S-isomer of the 1SH adduct occurred during crystallization. The cell imaging study revealed the staining of the cell and multicolor emission in the presence of H2S.

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INTRODUCTION

of a probe with single binding site and detection through different channel interrogation is more facile and advantageous as well.3a,c,d Up to now, several single molecular probes for multi-cation,1c,5 multi-anion,6 and both cation−anion7 based on above two approaches have been revealed in the literature. Whereas highly selective and sensitive detection of two competitive analytes, namely cyanide and thiol, by a single probe without any interference is barely reported.8 Cyanide (CN−) is known as one of the most toxic anions in nature because of its ability to interact with the active sites of cytochrome a3 and it inhibits the mitochondrial electron-

Development of multi-ion responsive single molecular probe is now drawing much curiosity of modern scientists as it is much more useful than a particular probe devoted for a particular analyte.1 There are several approaches for detection and quantification of multianalytes through a single molecular probe, for instance (a) incorporation of several binding site for each analytes in single molecular probe, and (b) utilization of different spectroscopic channels for detection of multiple analytes by a molecular probe with single binding site.2,3 It is tedious to design and synthesize a probe with multiple binding sites for multianalytes as it requires a proper scaffold which will differently interact with analytes and also prevent the analytes from interacting with each other.2,4 On the contrary, synthesis © 2017 American Chemical Society

Received: July 12, 2017 Published: August 24, 2017 10234

DOI: 10.1021/acs.joc.7b01743 J. Org. Chem. 2017, 82, 10234−10246

Article

The Journal of Organic Chemistry Scheme 1. Synthetic Route for MBA and Probe 1 and the ORTEP Plot of Probe 1 (30% Thermal Ellipsoids)a

a

The molecule with the disordered nitrogen center is not shown here. Dihedral angles: N1−C8−N2-C12 = 0.76°, S1−C8−N2-C9 = 3.24°.

intramolecular charge transfer from the donor to the acceptor upon excitation.21 The interaction of analytes (cation or anion) with electron donating or electron withdrawing group obstructed the electron transfer process which causes enhancement of intensity in fluorescence or change in absorption band.21 Twisted intramolecular charge transfer (TICT) is a common phenomenon in compounds containing electron donor and acceptor group linked by single bond.22 TICT process can be strongly affected by local environment and it has great application in chemosensing and photovoltaic devices.22 Maleimide is well-known for exclusive detection of thiols as the thiol-maleimide adduct formation by 1,4-addition reaction has been accepted for several decades.23 So far, maleimide based probes have not been used for detection of both cyanide and thiol. In continuation of our research on single molecular bifunctional probes,1c,7c herein we report the synthesis of a simple maleimide based bifunctional probe, 1, and the aptness for highly sensitive and selective detection of two competitive analytes, namely toxic cyanide and biologically important hydrogen sulfide. In two different channels, the colorimetric CN− detection and fluorescence detection of H2S based on the well-known Michael addition reaction at the α,β-unsaturated carbonyl are presented. The probe 1 rapidly detects cyanide and produces bright pink color even in the presence of other competitive anions. The weakly emissive probe 1 selectively and rapidly reacts with H2S and emits strongly due to the inhibition of photo induced electron transfer. The bisulfide adduct (1-SH) is thoroughly characterized by 1H NMR and ESI-MS. Moreover, for the first time the solid-state structure and spontaneous resolution of a bisulfide adduct of a H2S probe is reported herein. The 1-SH adduct shows TICT phenomenon and behaves as a molecular rotor. At high concentrations, the aggregate formation and excitation wavelength dependent multicolor fluorescence is exhibited by 1-SH adduct. Finally the cytotoxicity and the intracellular H2S imaging in live HeLa cell are also discussed herein.

transport chain that leads to vomiting, convulsion, loss of consciousness, and eventual death.9 Even a small amount (0.5− 3.5 mg) of cyanide in per kg of body weight is lethal for humans. According to the World Health Organization (WHO) the maximum limit of cyanide in drinking water is to be 1.9 μM.10 Nevertheless, cyanide is widespread in industrial processes for instance synthetic fertilizers, resins, pharmaceuticals, plastic manufacturing, electroplating, metallurgy, and gold mining. Therefore, it is highly necessary to design and synthesize new probes for rapid and selective detection of cyanide in environmental samples.11,12 Unlike CN−, hydrogen sulfide (H2S) has long been considered as environmentally toxic gas with characteristic odor of rotten eggs and no physiological significance. But recent investigations imply that H2S is the third most important endogenous gaseous signaling compound (gasotransmitter) after NO and CO. H2S plays an essential role in enormous number of physiological and pathological processes, such as vasodilatation, ischemia reperfusion injury, neuronal transmission, angiogenesis, antioxidation, antiapoptosis, anti-inflammation, insulin signaling, and oxygen sensing.13 Conversely, the abnormal level of H2S causes various diseases, such as stroke, Alzheimer’s disease, cardiovascular disease, Down’s syndrome, diabetes, and liver cirrhosis.14 In the mammalian system, H2S is produced endogenously in cytosols and mitochondria from three main enzymes, cystathionine β-synthase (CBS), cystathionine γ-lyase (CSE), and 3-mercaptopyruvate sulfur-transferase.15 Recently, various fluorescence-based methods have been developed for highly selective and sensitive in vitro detection and in vivo imaging of H2S in biological systems. These probes are commonly based on H2S-induced specific reactions, such as reduction of azide and nitroso to amine16 metal sulfide precipitation,17 thiolysis of dinitrophenyl ether or other leaving groups,18 and nucleophilic 1,4-addition.19 Many such probes suffer from low selectivity (SO32−, cysteine, and glutathiones are interfering), sensitivity, and slow response time. Thus, adequate studies on the development of advanced H2S probe for rapid, sensitive, and reliable detection techniques to know the physiological functions of H2S is highly demanded. Among the variety of signal transductions, many scientists are fascinated in taking the advantages of the well-known photoinduced electron transfer (PET)20 and intramolecular charge transfer (ICT) phenomenon,21 which are tremendously sensitive and constructive. The ICT based molecular probe is essentially a multicomponent system containing an electronrich group connected to an electron-deficient unit, undergoes

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RESULTS AND DISCUSSION

Synthesis and Characterization. The benzothiazole attached maleimide based probe 1 was synthesized by two steps. At first the (Z)-4-((4-methylbenzo[d]thiazol-2-yl)amino)-4-oxo-2-butenoic acid (MBA) was prepared by stirring of 4-methylbenzothiazol-2-amine and maleic anhydride under N2 and it was isolated in high yield (93%). The probe 1 was obtained by refluxing MBA with sodium acetate in acetyl chloride medium under N2 and isolated as a greenish white 10235


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The Journal of Organic Chemistry

Figure 1. (a) UV−vis spectra of 1 (100 μM) with different anions (100 μM). (Inset) Photograph showing the development of bright pink color upon addition of CN−. (b) UV−vis titration of 1(100 μM) in DMSO: H2O (95:5) with increasing amount of CN− solution (100 μM). (Inset) Plot of absorbance vs [CN−] shows a good linear relationship from 0 to 60 μM concentration range.

Scheme 2. Plausible Mechanism of Colorimetric Cyanide Detection

SO42−, S2O32−, N3−, NO3−, ClO−, H2PO4− HSO4−, histidine (His), serine (Ser), cysteine (Cys), glutathione (GSH), except H2S (NaHS as a source of H2S). This result points to a selective detection of CN− through 1,4-addition reaction with the maleimide moiety of 1 which was further monitored by UV−vis titration. The addition of increasing amount of CN− (0−1.0 equiv) to 5% aqueous DMSO solution of 1 reveals an appearance and gradual enhancement of the band at λmax = 540 nm. No further change was observed after the addition of more than 1.0 equiv of cyanide which confirms the completion of reaction and 1:1 stoichiometry of the probe and CN− (Figures 1b, S7 in SI) interaction. The nonlinear curve fitting from the absorbance titration data shows good fit with the 1:1 model. The absorbance vs concentrations of CN− in the range of 0−60 μM (Figure 1b inset) showed an excellent linear relationship, which indicates that the probe is suitable for quantitative determination of CN−. It is assumed that a moderately stable enolate intermediate, [1-CN]− is formed after the nucleophilic attack of CN− to the α, β−unsaturated carbon of maleimide moiety which absorbs the light at 540 nm (ε = 2724 L mol−1 cm−1) due to intramolecular charge transfer (ICT) transition from cyano substituted maleimide to benzothiazole ring and produces bright pink color (Scheme 2). This assumption is strongly supported by theoretical calculation (vide inf ra). To the best of our knowledge, maleimide based reactive probes have not been previously demonstrated for selective colorimetric detection of cyanide through intramolecular charge transfer (ICT). The bright pink color of the enolate intermediate, [1-CN]−, is developed immediately ( 8) pH affect the fluorescence detection of the H2S. However, the physiological pH (7.0) is most suitable for the detection of H2S (Figure S16 in SI). The time-dependent fluorescence responses of probe 1 with H2S were monitored upon addition of 1.0 equiv of H2S in 50% aqueous buffer solution at room temperature (Figure 5). About 9-fold fluorescence enhancement was observed immediately within ∼2 min after addition of H2S. The maximum fluorescence of 1 was reached within 14 min and no further enhancement was observed. This observation points to a relatively fast reaction of 1 with H2S. Characterization of 1-SH Adduct by Mass Spectrometry, NMR Spectroscopy, and X-ray Crystallography. To verify the mode of interaction between hydrogen sulfide and 1, we performed a reaction of 1 with NaHS in methanol/water. A yellow bisulfide adduct, 1-SH, was obtained by stirring the solution for 1 h at room temperature. The 1-SH adduct 10239


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Figure 6. (a) Partial 1H NMR spectrum of 1 and 1-SH in DMSO-d6. (b) Crystal structure of the compound 1-SH. (c) View of the 2D packing diagram with zigzag chain along crystallographic b-axis. Dihedral angles (φ): S1−C1−N2-C12 = 0.3(4)°, N1−C1−N2-C9 = 4.1(4)°.

Scheme 4. Schematic Representation of Fluorescence Enhancement Due To the Restriction of Intramolecular Rotation (RIR) and Inhibition of TICT and (Inset) Molecular Structure of the “Molecular Rotor” ThT

Aggregation and Multicolor Emission Properties. Molecular rotors are a group of molecules which exhibit very weak fluorescence emission upon photoexcitation due to nonradiative decay from the TICT excited state.26 Numerous studies suggest that Thioflavin-T(ThT), consisting of a pair of benzothiazole and benzamine rings freely rotating around a shared C−C bond, behaves as a molecular rotor (Scheme 4 inset). It is assumed that ThT fluorescence increased with solvent viscosity due to the restriction of intramolecular rotation (RIR) of ThT rings and as a result, the electron cannot cross into the nonradiative “dark” charge transfer (CT) state. The locally excited (LE) state with high oscillator strength facilitates radiative relaxation from the excited state to the ground state.27 Herein, the 1-SH adduct bears a structural resemblance to the ThT which encouraged us to study the fluorescent property in confined environment where the intramolecular rotation around C−N is hindered. In order to check the TICT mechanism and molecular rotor behavior of 1-

enantiomers (R and S) in equal proportion. As expected, the HPLC traces of 1:1 mixture of probe 1 and HS− clearly indicates the formation of racemic mixture (Figure S19 in SI). But only the S isomer is identified from the single crystal structure (Figure 6b) analysis and the enantio purity is confirmed by examining the absolute structure (Flack) parameter (−0.06). Here the S isomer of the 1-SH adduct is spontaneously resolved during the crystallization. Spontaneous resolution is a relatively rare phenomenon and the spontaneous resolution of racemic mixture without using any chiral auxiliary is a challenging task.24 Although the crystal structure of an unique host guest complex with HS− is reported recently by Pluth et al.,25 to the best of our knowledge, 1-SH is the first structurally characterized bisulfide adduct of a H2S probe. Analysis of the crystal packing in 1-SH reveals that the complex forms a zigzag chain of 1-SH molecules running along crystallographic b-axis and a 2D-network through the noncovalent interactions (Figure 6c). 10240


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and viscosity-dependent fluorescence enhancement is a typical characteristic of TICT phenomenon.22 The viscosity dependent studies indicated the possibility of conformational planarization may give rise to the aggregation induced emission (AIE) phenomenon. This hypothesis is strongly supported by the crystal structure of 1-SH adduct which reveals that the two rings are coplanar in the solid state. On the basis of the result discussed above, the 1-SH adduct can act as a molecular rotor owing to the TICT and like thioflavin-T it may be further applied in β-sheet (G-quadruplex DNA, amyloid-β fibril, αsynuclein fibril) binding study.28 Recently, multicolor emissive materials have attracted a considerable attention owing to their diverse applications, e.g., in molecular machines, switches, logic gates, and opticalsensors.29 Small organic luminophores having AIE property display excellent multicolor fluorescence in different aggregation states which are normally controlled by concentration, temperature, and solvent polarity, etc.30 The probe 1 shows ∼16-fold fluorescence enhancement at 435 nm (λex = 410 nm) after H2S binding at 100 μM concentration and light blue fluorescence under laboratory UV lamp. On the contrary, the isolated solid 1-SH adduct shows bright yellow fluorescence under laboratory UV lamp (Figure 8a). This observation encouraged us to study the fluorescence property of the 1-SH adduct at different conditions. Herein, the formed 1-SH adduct in 50% aqueous buffer shows multicolor emission from blue to green to yellow under laboratory UV lamp at three different concentrations, i.e., 100 μM, 1 mM, and 5 mM (Figure 8a). The fluorescence emission wavelength (λem) was red-shifted from 435 to 560 nm when the concentration of 1-SH was increased from 100 μM to 5 mM and various excitation wavelength was used (Figure 8b). The 1-SH adduct (c = 1 mM) emits at 510 nm and shows bright green fluorescence upon excitation at 450 nm. Whereas, at higher concentration (5 mM) when excited at 450 and 500 nm, the 1-SH adduct shows green (λem = 530 nm) and dark yellow fluorescence (λem = 560 nm), respectively (Figure 8b). So, it is quite an interesting phenomenon that the emission color switching can be achieved simply by changing the excitation wavelength without any external stimuli. Although many inorganic luminescent complexes and hybrid quantum dot materials shows multicolor

SH, we studied the effect of solvent polarity and viscosity on the fluorescence spectra of the in situ formed 1-SH adduct (1:1 mixture of 1 and H2S). The fluorescence spectra of the in situ formed 1-SH adduct in solvents with different polarity were measured. The fluorescence spectra were very sensitive to the polarity of the solvent and the emission band was red-shifted with increasing solvent polarity (Figure S20 in SI). Further we studied the solvent dependent fluorescence using an aqueous buffer/glycerol gradient to find out whether the restriction to intramolecular rotation (RIR) in the 1-SH adduct will enhance its fluorescence (Figure 7). As expected, fluorescence intensity

Figure 7. Fluorescence spectra of 1-SH (100 μM, λex = 410 nm) adduct in aqueous buffer/glycerol.

is concomitantly enhanced with increasing solvent viscosity due to the restriction of intramolecular rotation around the C−N in 1-SH adduct. The probe shows only ∼16-fold fluorescence enhancement at 435 nm with a shoulder at 455 nm after binding with H2S (1 equiv) in 50% aqueous buffer (pH 7.0). Whereas the intensity is enhanced further ∼17-fold in 80% glycerol and the emission at 455 nm is vanished completely by showing only the locally excited (LE) emission at 445 nm (Figure 7). Such solvent polarity dependent red shift emission

Figure 8. (a) Digital photographs of solid 1-SH and 50% aqueous buffer solution of 1-SH adduct (1:1 mixture of 1 and NaHS) at different concentration showing multicolor fluorescence under UV illumination. (b) Fluorescence spectra of 1-SH adduct using various excitation wavelengths at 100 μM, 1 mM, and 5 mM concentration. 10241


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Figure 9. DLS data and TEM image (inset) of 1-SH adduct (1:1 mixture of 1 and NaHS) at concentration (a) 100 μM and (b) 5 mM.

Figure 10. Brightfield, fluorescence, and overlay images of (a−c) HeLa cells treated with only 1 (100 μM), (d−f) HeLa cells treated with 1 (100 μM) and NaHS (100 μM). The display of the fluorescence image of HeLa cells treated with 1 (100 μM) and NaHS (100 μM) in (g) blue (h) green and (i) red channel.

fluorescence, the small organic luminophore exhibiting multicolour fluorescence are still rare.31 Here we expect that the planarity between two rings is achieved and aggregate is formed at high concentration. The excitation wavelength dependent

multicolor emission may be due to the presence of aggregate from the different ground state conformers of 1-SH adduct.32 To verify the nature of the aggregate, the absorption spectra of the 1-SH adduct at different concentration in 50% aqueous 10242


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H2S than to its likely competitors, CN−, SO32−, and other biothiols (Cys and GSH), allowing quantification of H2S in the presence of these anions. The probe 1 acts as excellent bifunctional probe since two different channels and conditions have been used to detect two competitive analytes, CN− and H2S, without feasible interference. After the reaction with H2S, the resultant 1-SH adduct showed TICT phenomenon and performed like molecular rotor which has been supported by polarity and viscosity dependent studies. The 1-SH adduct shows aggregation at higher concentration and excitation wavelength dependent on multicolor emission properties. The unique multicolor fluorescence, including blue, green, and yellow, is achieved using a single compound, and may be applied in the field of information processing and light emitting optical devices. The bisulfide adduct has been characterized thoroughly by ESI-MS, 1H NMR, and for the first time by solid state structure of bisulfide adduct of a H2S selective probe. Interestingly the spontaneous resolution of chiral S-isomer occurred during crystallization. The cytotoxicity study and live cell imaging experiments using HeLa cells with the probe suggest that the probe has low cytotoxicity, good cell membrane permeability, and is capable of detecting intracellular H2S in a wide range of emission.

buffer was recorded (Figure S21 in SI). The bathochromic shift of both the absorption and fluorescence band could be attributed to J-aggregate formation in the solution (Figure S21 in SI and Figure 8b).33 The zigzag 1D chain and 2D crystal packing also supports the formation of J-aggregate in the solid state (Figure 6c). To gain further understanding of the self-aggregation behavior of the 1-SH adduct the sizes and morphologies were determined by dynamic laser scattering (DLS) and transmission electron microscopy (TEM) analyses. The DLS result revealed that the average diameter of the 1-SH adduct was increased by ∼5-fold with the change of concentration from 100 μM (d = 92 nm) to 5 mM (d = 458 nm) (Figure 9). The TEM images of 1-SH at low (100 μM) and high concentration (5 mM) is consistent with the DLS results, indicating the formation of aggregates of 1-SH at high concentration (5 mM) (Figure 9). Live Cell Imaging and Cytotoxicity Study. To demonstrate the biological applicability of probe 1, we assessed its cytotoxicity in HeLa cell for 24 h. With increasing amount of 1 from a concentration range of 5−100 μM the cell viability gradually decreased after 5 μM concentration. More than 85% of the cells survived at 100 μM of 1 after 24 h of treatment, signifying that 1 has a low cytotoxicity at wide concentration range (see Figure S22 in SI). We investigated the ability of 1 to detect H2S in biological systems by staining of living cells. Incubation of HeLa cells with 1 (100 μM) for 30 min in PBS buffer did not afford much staining. However, incubation of 1 for 20 min and after addition of NaHS for further 10 min in a separate well in PBS buffer (pH 7.4) reveals a brilliant blue fluorescence (Figure 10). These results indicate that probe 1 has the potential to detect H2S levels in living cells by forming a stable 1-SH compound. Further, as the 1 stains the live cells (in the presence of H2S) the compound is cell membrane permeable. Multicolor emission is also reflected during the cell imaging experiment. The 1-SH adduct shows blue (at excitation of BP 340−380 nm), green (at excitation of BP 450− 490 nm), and red emission (at excitation of BP 515−560 nm) inside the cell. Recently, similar multicolor emission by a single compound inside the cell is reported.34

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EXPERIMENTAL SECTION

Materials and Physical Measurements. All chemicals were used as received from commercial suppliers (Aldrich, Alfa Aesar, and Spectrochem India). 1H and 13C NMR spectra were measured on a Bruker Avance II (400 MHz) spectrometer and chemical shifts were expressed in ppm using solvent residual as an internal standard. ESIMS was performed with a Waters ZQ-4000 and QToF−Micro YA 263 mass spectrometer. Infrared spectra were recorded using a PerkinElmer FT- IR spectrometer with KBr pellets in the range of 4000 to 400 cm−1. Elemental analysis measurements were done using the PerkinElmer 2500 series II elemental analyzer. UV−visible and fluorescence spectra were recorded on a PerkinElmer Lambda 25 UV− vis spectrophotometer and Hitachi F-4500 spectrophotometer with quartz cuvette (path length = 1 cm). Dynamic light scattering (DLS) data were collected in Malvern Zen 3690 and HRTEM images were taken in JEOL JSM 100CX transmittance electron microscope. Measurements of PL Spectra. A stock solution (100 μM) of the probe was prepared 10−15 min earlier to the experiment. Samples for emission measurement were contained in 1 cm × 1 cm quartz cuvettes (3.5 mL volume). All spectroscopic measurements of 1 were performed in 50% aqueous buffer solution (DMSO: Tris HCl buffer; pH 7.0 1:1 v/v). An excitation wavelength at 410 nm was used. All anions were used as their tetra-n-butylammonium salt except H2S, SO32−, SO42−, S2O32− N3−, and ClO− (sodium salt). Excitation and the emission slit were set to 5 mm/10 nm and the PMT volt at 700. The fluorescence quantum yield (Φ) has been calculated using fluorescein as standard. Synthesis of (Z)-4-((4-Methylbenzo[d]thiazol-2-yl)amino)-4oxo-2-butenoic Acid (MBA). 4-Methylbenzothiazol-2-amine (0.492 g, 3.00 mmol) and maleic anhydride (0.441 g, 4.50 mmol) were dissolved in 15 mL of dry chloroform. Then the mixture was stirred for 15 min at room temperature under N2. A greenish yellow precipitate was obtained after completion of reaction. The precipitate was collected by filtration and then washed with diethyl ether and dried under vacuum. Yield: 93% (0.732 g). M.P = 179−184 °C. Anal. Calcd for C12H10N2O3S (MW. = 262.28): C, 54.95; H, 3.84; N, 10.68; Found: C, 55.13; H, 3.87; N, 10.81; FTIR in KBr disk (ν/cm−1): 3188, 2989, 2495, 1706, 1595, 1558, 1454, 1300, 857. ESI-MS: Calcd: m/z = 263.04; Found: m/z = 263.05; [C12H10N2O3S + H]+; 1H NMR (400 MHz, DMSO-d6): δ (ppm) = 12.99 (s, 1H, Hh), 12.82 (s, 1H, Hg), 7.80 (d, J = 7.6 Hz, 1H, Hc) 7.22−7.28 (m, 2H, Hd, He), 6.53 (d, J = 12.0 Hz, 1H, Hb),6.50 (d, J = 12.0 Hz, 1H, Ha), 2.58 (s, 3H), 13C

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CONCLUSION In conclusion, a 4-methyl substituted benzothiazole connected maleimide based probe, 1, has been designed and synthesized. Although in the past, maleimide-based probe was exclusively used as thiol probe, here we have shown that 1 acts as an efficient bifunctional probe wherein it quantitatively detects cyanide by UV−vis and H2S by fluorescence spectroscopy in aqueous DMSO and mixed aqueous buffer media, respectively. The cyanide is detected by fast development of pink color (less than 4 s) through the formation of moderately stable enolate intermediate which is supported by theoretical calculation. TDDFT calculation detailed the intramolecular charge transfer transition from the cyanide bound maleimide moiety to the substituted benzothiazole ring and concomitant dark pink coloration. The probe 1 can quantify cyanide at the submicromolar level even in the presence of other anions which is demonstrated by interference study. In the fluorescence channel, the probe rapidly detects H2S by fluorescence light-up through the inhibition of PET. The time dependent fluorescence study reveals that the 1,4-addition reaction of H2S with the probe has been completed within ∼14 min. The probe exhibits stronger selectivity and affinity toward 10243


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The Journal of Organic Chemistry NMR (100 MHz, DMSO-d6): δ (ppm) = 167.6, 163.8, 157.2, 148.1, 133.6, 131.6, 130.4, 128.1, 127.1, 124.1, 119.6, 18.4. Synthesis of 1-(4-Methylbenzo[d]thiazol-2-yl)-1H-pyrrole2,5-dione (1). MBA (0.262 g; 1.00 mmol) was mixed with sodium acetate (0.328 g; 4.00 mmol) in a three neck round-bottom flask. Then the mixture was dissolved in 20 mL distilled acetyl chloride and refluxed for 48 h under N2. Thereafter, the acetyl chloride was distilled out and unreacted sodium acetate was washed with distilled water. The greenish yellow solid was extracted by dichloromethane and the solvent was removed under rotary evaporator. This crude product was then purified by silica gel column chromatography using chloroform. The greenish white crystalline solid 1 was dried in vacuum desiccators. Yield: 45% (0.110 g). M.P. = 172−176 °C Anal. Calcd for C12H8N2O2S (MW. = 244.03): C, 59.00; H, 3.30; N, 11.47; Found: C, 59.34; H, 3.27; N, 11.69; FTIR in KBr disk (ν/cm−1): 2921, 2849, 1710, 1591, 1553, 1436, 1223, 973, 777. ESI-MS: Calcd: m/z = 245.03; Found: m/z = 245.00; [C12H8N2O2S + H]+; 1H NMR (400 MHz, DMSO-d6): δ (ppm) = 7.93−7.91 (m, 1H, Hc), 7.35−7.34 (m, 4H, Ha, Hb, Hd, He), 2.63 (s, 3H,Hf). 13C NMR (100 MHz, DMSOd6): δ (ppm) = 167.4, 150.6, 148.3, 135.4, 132.5, 132.0, 126.8, 125.4, 119.3, 17.8. Cytotoxicity Study. The cytotoxicity of the probe 1 against HeLa cells was determined by colorimetric cell cytotoxicity assay kit, ab112118 (abcam), in a 96-well cell culture plate. HeLa cells were seeded in a 96-well plate at a density of 5 × 103 cells/well and incubated at 37 °C, 5% CO2 incubator. Cells were treated with different concentration of 1 (5, 10, 50, and 100 μM) at 70% confluency and incubated for 24 h. Assay solution was thawed and warmed at 37 °C and 20 μL (1/5 volume) was added into each well. The reagents were mixed by shaking the plate gently for 30 s and incubated at 37 °C, 5% CO2 incubator. The absorbance change was monitored at 570 and 605 nm using microplate reader. The cell viability in each well was determined from the ratio of OD570 to OD605. The cell viability is proportional to increased in OD570 and decreased in OD605. Readings were taken in quadruplet and the % cell viability was calculated for samples and controls based on the following formula:

obtain optical absorption spectra for all the blank, intermediate, and final structures, we employed time dependent density functional theory (TDDFT).38 Crystallographic Studies. The X-ray data of 1 and 1-SH were collected at 293 K with Agilent Xcalibur (Eos, Gemini) 39 diffractometer using graphite-monochromated Mo Kα radiation. The data were collected and reduced in CrysAlis PRO software.39 The absorption corrections were done by SCALE3 ABSPACK multiscan method in CrysAlisPro. The structures were solved by using the program SIR-9240 and refined by full matrix least-squares calculations (F2) by using the SHELXL-2016 software41 in WinGX.42 All non-H atoms were refined anisotropically against F2 for all reflections. All hydrogen atoms were placed at their calculated positions and refined isotropically. The details of crystal data collection and structure refinement and labeled ORTEP plots for 1 and 1-SH are given in Table S3, Figures S23 and S24, respectively. The cif file was deposited with the Cambridge Crystallographic Data Centre, and the following code was allocated: CCDC-1556515 and CCDC- 1556516. This data can be obtained free of charge via the Internet: www.ccdc.cam.ac.uk/ data_request/cif.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.joc.7b01743. All NMR, ESI-MS, UV−vis/fluorescence data for 1, theoretical calculation data, associated figures, tables, and crystallographic information file (PDF) X-ray crystallographic data for compound 1 (CIF) X-ray crystallographic data for compound 1-SH (CIF) Movie of development of [1-CN] after addition of CN− (AVI)

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AUTHOR INFORMATION

Corresponding Author

%cell viability = 100 × (R sample − R 0)/(R ctrl − R 0)

*E-mail: [email protected]; [email protected]

where Rsample is the absorbance ratio of OD570/OD605 in the presence of the probe 1. Rctrl is the absorbance ratio of OD570/OD605 in the absence of the probe 1 (vehicle control). R0 is the averaged background (noncell control) absorbance ratio of OD570/OD605. Cell Culture and Imaging. For the live cell imaging a fluorescence inverted microscope (Leica DMI4000B) was used to visualize the fluorescence of the cells following the addition of the 1 with a 20× objective lens. Fluorescence detection of H2S was carried out using blue (at excitation of BP 340−380 nm), green (at excitation of BP 450−490 nm), and red excitation filter (at excitation of BP 515−560 nm). HeLa cells were cultured in DMEM media containing low glucose (Invitrogen) with 10% FBS (Invitrogen) at 37 °C in 5% CO2 incubator chamber. The cells were seeded in 24-well flat-bottomed plates just before 1 day of the imaging experiment. After 24 h cell growth, cells were washed with PBS (phosphate buffer saline) and fresh PBS (500 μL) was added in two successive wells. One well was treated with 100 μM aqueous solution 1 (20% DMSO as co solvent) and incubated for 30 min. Afterward 100 μM of NaHS was added and incubated for 10 min. The other well was treated with only 100 μM of 1 and incubated for 30 min. Theory and Computation. All geometry optimizations are carried out with density functional theory (DFT).35 A molecular orbital approach using a linear combination of atomic orbitals is applied to probe electronic structure. The wave functions for the cluster were expressed as a linear combination of Gaussian-type orbitals (GTO) situated at the atomic positions in the cluster. The actual calculation is performed using the implementation in the Gaussian 09 program.36 We have used a very successful and widely used hybrid exchangecorrelation functional, B3LYP (Becke’s three parameter exchange with Lee−Yang−Parr correlation),37 and a large all-electron basis set 6311+G(d,p) to probe the electronic structure of the compounds. To

ORCID

Snehadrinarayan Khatua: 0000-0003-0992-4800 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by DST, India (No. SB/ FT/CS/115/2012). We thank DST Purse program for single crystal X-ray diffraction facility at NEHU and Sophisticated Analytical and Instrumentation Facility (SAIF), North Eastern Hill University for NMR data. We are especially thankful to Dr. R. Thounaojam of Dept. of Biotechnology and Bioinformatics, NEHU, India, for cell culture and cytotoxicity study and D. Bhattacharjee of Dept. of Chemistry, NEHU for HPLC analysis. Dr. P. N. Chatterjee of NIT, Meghalaya, India, Prof. M. Halder of IIT Kharagpur, and Prof. A. K. Chandra of NEHU, India, are gratefully acknowledged for useful discussion. S.K.S. and B.S. thank RGNF and North Eastern Hill University for their research fellowship.

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DEDICATION The present work is dedicated to Prof. Manish Bhattacharjee (IIT, Kharagpur) on the occasion of his 54th birthday. REFERENCES

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