Article pubs.acs.org/ac
Engineering 1,3-Alternate Calixcarbazole for Recognition and Sensing of Bisphenol F in Water Gang Li, Liang Zhao, Peng Yang,* Zhaozheng Yang, Zhangmin Tian, Yan Chen, Hongyan Shen, and Chun Hu* Key Laboratory of Structure-Based Drug Design and Discovery (Shenyang Pharmaceutical University), Ministry of Education, Shenyang 110016, People’s Republic of China S Supporting Information *
ABSTRACT: Herein, we report a carbazolyl tubular macrocycle (3), which possesses a “π-tube” capable of encapsulating and sensing bisphenol F (BPF), which is a toxic industrial material. In this work, the synthesis of 3, its conformation in solution as well as its binding property to BPF have been investigated. Nuclear magnetic resonance (NMR), ultraviolet− visible light (UV-vis), fluorescence spectra, and molecular modeling studies reveal that the “π-tube” of 3 in 1,3-alternate conformation could encapsulate BPF with 1:1 binding stoichiometry in water, with its orthogonal tweezers sandwiching the two phenol units of BPF. Moreover, 3 could serve as a sensitive fluorescent probe for BPF (limitation of detection = 157 nM).
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structurally complementary bis-tweezer may be able to sandwich the double phenol units of BPF. The pity is that the bis-tweezers are rare.11−14 In a way, the calix[4]pyrrole in 1,3-alternate conformation could be regarded as a model of bistweezers, since Hudhomme and Sallé have noted that it contains two perpendicular tweezers.12 If one agrees with this viewpoint, the 1,3-alternate calix[4]arene is also a “bis-tweezer”, because it also contains two orthogonal tweezers. Aside from the “bis-tweezer” geometry, the “cylindrical inner tunnel” (or “π-basic tube”) of the 1,3-alternated calix[4]arene, constituted by its two pairs of the orthogonal tweezers, has ever been reported.22−26 Compared with geometries of other tubular hosts (e.g., cyclodextrin, cucurbit[n]urils and pillar[n]arenes),27−31 this calix[4]arene-based tube seems to be more analogous/complementary to that of BPF. This charming structural feature make this calix[4]arene-based “bis-tweezer” or the “tube” appears to be a suitable host of BPF. However, its small cavity could only encapsulate ionic guests (e.g., Ag+, NO+), rather than a molecular guest. As such, a novel tubular host, possessing the analogous but “expanded” geometry of 1,3alternate calix[4]arene, may be able to encapsulate a conformation mobile, molecular guest (e.g., BPF). We recently reported a novel series of non-phenol-based macrocycles: calixcarbazoles.32 The cavity of calix[3]carbazole, the smallest one among those carbazolyl macrocycles, is already larger than that of calix[4]arene. As such, the π-tube constituted
onstruction of synthetic receptors capable of molecular recognition, as well as optically sensing neutral guests in aqueous solution, remains challenging.1−3 Growing concerns have been given to recognizing and sensing those toxic industrial compounds, e.g. Bisphenol F (4,4′-dihydroxydiphenylmethane, denoted hereafter as BPF). It is known that exposure to BPF, which is a widely used material in the plastics industry, even at very low concentrations, would increase the incidences of metabolic syndrome, chromosomal defect, breast and prostate cancers, etc.4−6 Various analytic strategies, such as GC/MS, LC/MS, and electrochemical sensing, have been developed to detect BPF.7−10 However, these methods either rely on expensive instruments or require skilled operators. Fluorescence sensing is a highly valuable alternative technique, because it possesses the advantages of high sensitivity, high spatial and temporal resolutions, and convenient operability. As a fluorescent sensor is generally composed of a fluorophore and a receptor moiety, because of the lack of specific receptor of BPF, its selective fluorescent probe has never been reported yet. The molecular tweezer (or clip) is one of extensively investigated supramolecular receptors.11−14 It is generally constructed by a syn conformation/configuration of two more or less rigid aromatic rings. The open cavity of the tweezer constituted by its two aromatic arms can sandwich guests via various noncovalent interactions, including hydrogen bonding, van der Waals force, electrostatic, π−π stacking, or even hydrophobic forces.15−17 Several groups have reported that the aromatic rings of phenol derivatives could be sandwiched by the open cavity of glycoluril-based tweezers via π−π stacking interaction.18−21 As such, it is not illogical to envisage that a © XXXX American Chemical Society
Received: August 30, 2016 Accepted: September 30, 2016
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DOI: 10.1021/acs.analchem.6b03398 Anal. Chem. XXXX, XXX, XXX−XXX
Article
Analytical Chemistry
Syntheses. Compound 1. Compound 1 was synthesized based on refs32−34. Compound 2. A total of 43 mg of compound 1 and 1 mL of N,N′-dimethylethylenediamine were dissolved in 1.5 mL DMF and then heated at 70 °C for 25 h. Afterward, the cooled mixture was dropped in methanol to give white precipitate. The precipitate was collected and washed with diethyl ether, methanol, and DCM, respectively, to give 30.0 mg of compound 2. Thin-layer chromatography (TLC) results (dichloromethane:methanol:ammonium hydroxide, 25:2:1, v/ v/v), Rf = 0.3, white solid. Yield = 84.3%. 1H NMR (600 MHz, DMSO-d6) δ 7.84 (t, J = 5.3 Hz, 4H), 7.04 (s, 8H), 6.92 (s, 8H), 4.97 (s, 8H), 3.88 (s, 8H), 3.80 (s, 24H), 3.14 (q, J = 5.3 Hz, J = 6.5 Hz, 8H), 2.22 (t, J = 6.5 Hz, 8H), 2.04 (s, 24H).13C NMR (150 MHz, DMSO-d6) δ 167.9, 156.0, 140.5, 121.0, 119.6, 115.7, 92.2, 58.5, 55.9, 46.4, 45.4, 37.1, 30.0. HRMS (ESI/TOF-Q) Calcd for [M + 2H] 2+: 735.3865; found: 735.3867. Compound 3. Thirty milligrams (30 mg) of compound 2 and 1.5 mL of methyl iodide were dissolved in 2 mL of DMF and then heated at 60 °C for 20 h. Afterward, the cooled mixture was deposited dropwise in methanol to form a dark yellow precipitate. The precipitate was collected and washed with diethyl ether, methanol, and DCM, respectively, to give 28 mg of product (dark yellow solid, yield = 67.3%). 1H NMR (600 MHz, DMSO-d6) δ 8.19 (s, 4H), 7.07 (s, 8H), 7.00 (s, 8H), 5.10 (s, 8H), 3.89 (s, 8H), 3.81 (s, 24H), 3.52 (s, 8H), 3.38 (s, 8H), 3.06 (s, 36H).13C NMR (150 MHz, DMSO-d6) δ 169.0, 156.0, 140.5, 121.0, 119.7, 115.7, 92.3, 63.8, 56.1, 52.9, 46.3, 33.8, 30.2. HRMS (ESI/TOF-Q) Calcd for [M − 4I]4+: 382.2125; found: 382.2137.
by 1,3-alternate calix[4]carbazole would be certainly bigger than that of calix[4]arene-based tubes, as long as it could adopt a 1,3-alternate conformation. In that work, we found that H4 or 5 of calix[4]carbazole was situated at δ ≈ 7.70 ppm. Its chemical shift was higher than those of all the other cyclomers (δH4 or 5: 7.90 ppm for calix[3]carbazole, 7.84 ppm for calix[5]carbazole, and 7.88 ppm for calix[6]carbazole).32 The similar NMR spectral behaviors were also observed for our very recently prepared calix[n]dimethoxycarbazole.33 As the characteristic upfield shift of the interior shielded protons of the calix[4]arene is a powerful indicator for its 1,3-alternate conformation, our findings drop a hint that the carbazolyl tetramers might adopt the 1,3-alternate conformation. With this hypothesis in kind, we synthesized 3 (Scheme 1) and studied its conformation as well as its binding property to BPF in aqueous solution. We report our findings herein. Scheme 1. Synthetic Route of 3
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RESULTS AND DISCUSSION Our previous work32 showed that calix[4]carbazole could not be easily synthesized/purified, because of the poor yield of the cyclization step (0.8%). Since the 2,7-dimethoxy-substituted carbazole unit possesses richer electrons than carbazole itself, because of the electronic-donating properties of methoxy group, we envisage that the Friedel−Crafts cyclization reaction of this substituted carbazole may be favored. If so, the yield of calix[4]dimethoxycarbazole will be enhanced. On the other hand, many natural carbazole alkaloids contain the units of 2,7dioxygenated carbazoles,35−39 so that 2,7-dimethoxy-substituted carbazole cyclo-oligomers may possess the potential biological properties. Taken together, we then used 2,7-dimethoxy-9Hcarbazole to construct a water-soluble host and start our study. 2,7-Dimethoxy-9H-carbazole was synthesized based on ref 34. Its consequent N-alkalation by bromoacetic acid and esterification by triethylene glycol monomethyl ether, as well as Friedel−Crafts cyclization, proceeded following our previously developed conditions.32 As expected, the Friedel− Crafts cyclization yield for calix[4]dimethoxycarbazole (1) is much higher than that for our previously prepared nonsubstituted calix[4]carbazole.32,33 Since 1 was not watersoluble, even at micromolar concentrations, it was then reacted with N,N′-dimethylethylenediamine, followed by N-alkylation to get a water-soluble one (3) (Scheme 1). Both 2 and 3 were fully characterized (see Figure 1, as well as Figures S2−S9 and S27−S32 in the Supporting Information). It could be seen from Figure 1 that the bridging methylene of 3 showed a singlet resonance at ∼3.89 ppm at room temperature. Variable-temperature nuclear magnetic resonance (VT-NMR) spectra of 3 recorded in DMF-d7 (Figure S10 in
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EXPERIMENTAL SECTION Unless otherwise noted, materials were purchased from commercial suppliers and used without further purification. Thin layer chromatography (TLC) analysis of reaction mixtures was performed on Dynamic adsorbents for silica chromatography (F-254 TLC plates). Column chromatography was performed on a silica chromatography device with 200−300 mesh. Fluorescence emission spectra were obtained using Shimadzu Model RF-5301 PC Spectrofluorophotometer. UVvis absorption spectra were obtained on Beijing purkinje TU1810 at 298 K. 1H NMR (600 MHz) and 13C NMR (150 MHz) spectra were recorded with Bruker Avance-III 600 spectrometers at 298 K. Chemical shifts were reported in units (ppm) and all coupling constants (J values) were reported in Hertz (Hz), using TMS as the internal standard. Highresolution mass spectra (electrospray ionization: ESI) were carried out using a QTOF-MS instrument. For all the measurements, the solutions of compound 3 were freshly prepared before use. The stock solutions of 3 and various guests were prepared by dissolving them either in dimethyl sulfoxide (DMSO) (for UV-vis and fluorescence titrations) or in DMSO-d6 (for 1HNMR titrations), respectively. Before the spectra were recorded, the sample solutions were mixed for 2 min after the addition of guests. UV-vis and fluorescence measurements were performed in a 1 cm cuvette. The excitation wavelength was 317 nm, and the excitation and emission slit widths were 3 nm. All the experiments were repeated at least three times. B
DOI: 10.1021/acs.analchem.6b03398 Anal. Chem. XXXX, XXX, XXX−XXX
Article
Analytical Chemistry
Figure 1. Partial 1H NMR spectrum of 3 (DMSO-d6).
the Supporting Information) showed that this singlet peak was not split into AB quartets, even at lower temperatures. This behavior indicated that 3 possessed either a single conformation or a very rapid interconverted conformation on the NMR time scale. It could also be observed from VT-NMR that there existed only one set sharp resonance signals for this cyclic tetramer at all the tested temperatures. Moreover, as we mentioned in the beginning of this Article, H4 (or 5) of the cyclic tetramer is situated at the relatively higher field than that of other cyclomers, because of it being a shielded feature. According to the literature,40,41 the above NMR spectra suggested that 3 most possibly adopted the 1,3-alternate (or the rapidly inverting 1,3-alternate) conformation in solution. Two-dimensional nuclear Overhauser spectroscopy (2D NOESY) of 3 (Figure 2a and Figure S32 in the Supporting Information) further revealed its 1,3-alternate conformation: H4 (or 5) was correlated to both Hb and Hc. An energyminimized molecular model42 depicted its 1,3-alternate conformation. It could be seen in Figure 2b that Hb and Hc of one carbazole unit of 3 were closed to H4 (or 5) of its adjacent carbazole unit. This modeling result is consistent with findings deduced from NMR studies. Having studied the potential conformation of 3 in solution, we then explored its capability of serving as a receptor for BPF, using p-cresol (4-methylphenol) as a control guest (see their structures in Figure 3). It should be noted that, at millimolar concentrations, both 3 (host) and BPF (guest) are not absolutely soluble in water, so that 1H NMR spectra were recorded in the mixture of DMSO-d6/D2O (V/V = 3/7) (see Figure 3, as well as Figures S23−S25 in the Supporting Information). The following observations could be made from Figure 3. For the host: H4 (or 5) of 3 shifted upfield (Δδ ≈ 0.58 ppm) upon the addition of 5 equiv BPF. For the guest: when the [BPF]/[3] ratio is
