Dynamic Interaction between Host and Guest for Enantioselective

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Dynamic Interaction between Host and Guest for Enantioselective Recognition: Application of #-CyclodextrinBased Charged Catenane as Electrochemical Probe Datong Wu, and Yong Kong Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.9b00378 • Publication Date (Web): 09 Apr 2019 Downloaded from http://pubs.acs.org on April 10, 2019

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Analytical Chemistry

Dynamic Interaction between Host and Guest for Enantioselective Recognition: Application of β-Cyclodextrin-Based Charged Catenane as Electrochemical Probe Datong Wu, Yong Kong*

Jiangsu Key Laboratory of Advanced Materials and Technology, School of Petrochemical Engineering, Changzhou University, Changzhou 213164, China *Email:

[email protected]

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ACS Paragon Plus Environment

Analytical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Abstract Charged catenane has attracted an ever increasing attention as prototypical molecular switches in their synthesis and function. In this study, β-cyclodextrin and ionic liquid interlocking mechanically with each other are carried out to form charged catenane through ionic hydrogen bonding. When the charged catenane is treated with Cu(II) ion, it will generate blue precipitate quickly with a rigid and robust structure. Given the synthesized molecule bears good electroactivity and stability, it is used as modifies on the surface of the bare glassy carbon electrode and further performed as an electrochemical probe. The probe shows a clear chiral discrepancy in the response of oxidation peak current (IP) toward four isomers, including tryptophan, tyrosine, cysteine, and malic acid. That is, L form has a much higher IP and meantime it is hard to observe the electrochemical signal for D form. More interestingly, the recognition ability between L and D forms of tryptophan can be reversed in the buffer solution with different pH values. These results show that the dynamic switch process of steric hindrance based on the catenane can enlarge the chiral discrepancy. In a word, we believe that this study would enrich the synthetic pathways of electroactive molecules and lead to a deeper fundamental understanding of their function.

Keywords: Charged catenane; Ionic liquid; Electrochemical analysis; Chiral recognition

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INTRODUCTION Much different with spectral or chromatographic methods,1–5 electrochemical chiral analysis has its advantages of rapidness and high sensitivity.6–17 It is no doubt that construction of functional surface is of great importance for efficient recognition. Up to date, diverse materials have been developed such as chiral polymers,6–9 nano/micro-composites,10–14 and metal complexes.7,16,17 In most cases, the synthesis of the target material with optical activity and electroactive units often requires genious design and complex reaction process. Developing a new strategy to prepare the material with mild conditions and simple reaction steps is desirable. Furthermore, to possess good discriminative ability, the rigid spatial structure between the probe and tested isomers should be formed through multiple interactions.18,19 For example, hydrophobic effect plays a key role when β-cyclodextrin (β-CD) is chosen as the chiral selector.20–22 But it occasionally performs unsatisfactory recognition efficiency by using β-CD based on the hydrophobic effect. Given that, more intermolecular interaction should be taken into consideration. We thus investigate the possibility of the synthesis of the artificial molecular machines via self-assembly, like catenanes, which can change the steric hindrance actuated by external stimuli modes. Catenanes have stimulated an ever increasing interest to the researchers on the potential application for the rich physicochemical properties.23–30 In general, they are synthesized through self-assembly of two molecules (host and guest). To make sure the successful interlocking, intermolecular effects (hydrogen bonding,25,26 π-π donor-acceptor interaction,27,28 and hydrophobic effect29,30) between the host and guest are necessary. Besides that, some groups reported the synthesis of charged catenanes derived from ion-pair effect recently.31–37 A dynamic [3]catenane is prepared with the units containing anion-binding motifs.36 Inspired by this finding, we start to consider the possibility that ionic hydrogen bonding can replace hydrophobic effect as the driving force and contribute to the interlocking for β-CD. Ionic liquids (ILs), composed of entire cation and anion pairs, are chosen as the feasible medium. They have the

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promising properties with wide electrochemical stability windows, high thermal stability and tunable combination.38–42 We wonder if the hydrophobic chain of ILs can thread inner cavity of β-CD during the synthetic process of the anion-cation pair in noncompetitive solvent media. Moreover, in its real application, two problems should be resolved: (a) the effect of the addition of β-CD to the electroactivity and (b) the effect of dynamic switch process of steric hindrance between β-CD and IL to chiral recognition efficiency.

Figure 1. (A) Schematic illustration of the synthesis of IL-CD-1 and IL-CD-Cu-2. (B) Chemical structures of (rac)-Trp, (rac)-Tyr, (rac)-Cys, and (rac)-Mal.

Thus, in the present study, we demonstrate a direct approach to synthesize a charged catenane (IL-CD-1) (Figure 1A). The obtained molecule can coordinate with Cu(II) ion to form a stable structure (IL-CD-Cu-2). NMR, MS, FTIR, XRD, and SEM are applied to confirm the successful synthesis and probe the possible structure. Much different from previous study in our lab7,10, the current work is unnecessary to synthesize chiral ILs derived from

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chiral molecules directly, which can simplify the synthetic steps. Further, the charged molecules are used as modification on the surface of bare glassy carbon electrode (GCE) to construct a novel electrochemical probe with enhanced performance of electrochemical assays. This probe attempts to perform a clear chiral distinction in the response of electrochemical signals for tryptophan, tyrosine, cysteine, and malic acid (Figure 1B) with rapid identification.

EXPERIMENTAL SECTION Chemicals. All reagents used in this study are of analytical reagent grade at least. 4,4’-Bipyridine, 1,6-dibromohexane, copper(II) chloride dehydrate, and β-CD were purchased from Aladdin Bio-Chem Technology Co., Ltd. (Shanghai, China) for the synthesis of IL-CD-Cu-2. The chiral objects including D-Tryptophan, L-tryptophan, D-tyrosine, L-tyrosine, D-cysteine, L-cysteine, D-malic acid, and L-malic acid were obtained from Sinopharm Group Co. Ltd. (Shanghai, China). A Milli-Q system (Millipore, Bedford, USA, 18.2 MΩ) was carried out to provide ultrapure water. Apparatus. 1H NMR and 13C NMR spectra were recorded at 400 MHz NMR spectrometer (Bruker, Switzerland), with TMS as internal standard. Fourier transform infrared (FTIR) spectrum (KBr) was obtained on a Nicolet NEXUS-470 FTIR spectrometer (Thermo Nicolet, USA) in the range of 400–4000 cm–1. X-ray diffraction (XRD) analysis was performed on a D/max 2500PC X-ray diffractometer (Rigaku, Japan). An S-4800 scanning electron microscope (SEM) (Hitachi, Japan) was used to examine the surface microstructures of IL-CD-Cu-2. Molecular weight was obtained by using a Bruker Esquire 3000plus ion trap mass spectrometer (MS) (Brucker, Germany) equipped with an electrospray ionization (ESI) interface in the negative-ion mode. Electrochemical experiments with a three-electrode system, including differential pulse voltammetry (DPV), cyclic voltammogram (CV), and Nyquist plot were performed using a CHI 660D electrochemical workstation (CH Instruments, Inc., China).

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Synthesis of IL-1. To compare the difference between the presence and absence of β-CD, the catenane IL-1 was synthesized without the addition of β-CD. Its chemical structure is shown in Figure 2A. 1,6-Dibromohexane (244 mg, 1.0 mmol) was added to a well stirred solution of 4,4’-bipyridine (156 mg, 1.0 mmol) in acetonitrile (30 mL) at room temperature. Then the reaction mixture was heated and stirred at 75 oC for 10 h. Next, the product was concentrated under reduced pressure and washed with ethyl acetate (3 × 20 mL) to remove the unreacted substrates, and dried in vacuo to give IL-1. 1H NMR (400 MHz, D2O): δ ppm 9.00−8.89 (m, 4H), 8.42−8.36 (m, 4H), 4.62−4.58 (t, 2H), 3.47−3.43 (t, 2H), 1.98−1.97 (m, 2H), 1.43-1.25 (m, 6H). 13C NMR (100 MHz, D2O): δ 147.8, 145.5, 145.4, 145.3, 129.5, 129.4, 128.5, 64.0, 33.4, 33.0, 27.5, 27.3, 26.9. HRMS (ESI, m/z): [M−2H++Na+]− for C16H20BrN2 found: 421.0332 (Figure S1). Synthesis of IL-CD-1. As shown in Figure 1, to a well stirred solution of β-CD (227 mg, 0.2 mmol) in water/ethanol (v/v=1/1, 40 mL) were added 4,4’-bipyridine (156 mg, 1.0 mmol) and 1,6-dibromohexane (244 mg, 1.0 mmol) at room temperature. Then the reaction mixture was heated and stirred at 75 oC for 10 h. Next, the product was concentrated under reduced pressure and washed with ethyl acetate (3 × 20 mL), ethanol (3 × 20 mL) to remove the unreacted substrates. The redundant β-cyclodextrin was removed by recrystallization with ice water and then dried in vacuo to give IL-CD-1. 1H NMR (400 MHz, D2O): δ ppm 9.01−8.88 (m, 4H), 8.44−8.38 (m, 4H), 4.94 (d, 1H), 4.58−4.52 (t, 2H), 3.78−3.66 (m, 4H), 3.56−3.44 (m, 4H), 1.98−1.92 (m, 2H), 1.44-1.26 (m, 6H). 13C NMR (100 MHz, D2O): δ 154.2, 147.8, 145.7, 129.4, 128.4, 104.3, 83.5, 75.6, 74.4, 64.1, 63.9, 33.7, 33.4, 27.5, 26.9. Synthesis of IL-CD-Cu-2. IL-CD-1 (0.20 g) dissolved in water (30 mL) was stirring at room temperature. Then the CuCl2 solution (10 mg mL–1) was added dropwise to it, which would generate precipitate followed by centrifuge. The crude product was washed with water (3 × 30 mL) and dried at room temperature to give IL-CD-Cu-2 as a blue solid (0.21 g).

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Preparation of IL-CD-1-GCE and IL-CD-Cu-2-GCE. No matter for IL-CD-1-GCE and IL-CD-Cu-2-GCE, it has the same preparation procedures. For example, the glassy electrode surface was polished carefully with the chamois leather and alumina slurries before use. Next, IL-CD-Cu-2 was added into ultrapure water to obtain suspension at the concentration of 5.0 mg mL−1. A 5 μL aliquot of the suspension was added on the GCE surface and then dried at room temperature. Electrochemical enantioselective sensing. DPV measurements were performed for enantioselective sensing at room temperature with the parameters containing scan rate (8 mV s−1), standing time (20 s), pulse width (0.2 s), and pulse period (0.5s). Taken L-Trp for example, 0.5 mM L-Trp was dissolved in PBS buffer solution (0.1 M, 30 mL) with different pH values. Then the three-electrode system was immersed into the prepared solution and the results were recorded using a CHI 660D electrochemical workstation.

RESULTS AND DISCUSSION

Figure 2. (A) 1H NMR of β-CD (a), IL-1 (b), and IL-CD-1 (c). (B) Possible chemical structure of IL-CD-Cu-2.

Characterization of IL-CD-1 and IL-CD-Cu-2. To prove the successful synthesis of IL-CD-1 and IL-CD-Cu-2, it should confirm three points: (i) interlocking with each other, (ii) the molar ratio between β-CD and IL-1, and (iii) the site of inter-molecular interaction. Thus NMR, MS, FTIR, SEM, and XRD were carried out in this study. 1H NMR spectroscopic investigation of β-CD (curve a), IL-1 (curve b), and IL-CD-1 (curve c) were performed in 7



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D2O (Figure 2A). In the absence of β-CD, the ring is formed with a molecule of 4,4’-bipyridine and 1,6-dibromohexane, proved by the molecular weight of IL-1 with the value of 421.0332 ([M−2H++Na+]−, Figure S1). Unlike IL-1, in IL-CD-1, the molar ratio of β-CD and IL-1 is 1:7 after calculation according to the characteristic peaks (Ha and H1) (curve b). It means that one cyclic molecule (IL-CD-1) is composed of seven IL-1 and one β-CD. Moreover, in curve b, it shows two peaks (doublet of doublets, Ha and Hb) at 8.97 and 8.38 ppm originating from the resonances associated with the protons of pyridinium cation and the −CH2− protons next to an N atom at 4.60 ppm in the aliphatic region. It is worth noting that obvious changes are observed in the 1H NMR spectrum of IL-CD-1. That is, the half peak intensity of Ha and Hb (doublets, curve c) decreases and downshifts to overlay with the other half. Meantime, H1 in curve c is deconvoluted into two peaks and one smaller peak upshifts to the position at 5.24 ppm. It indicates that bromide ion can form hydrogen bonds with the hydroxyl group at the wider opening of β-CD (Figure 2B), which would further weaken the electrostatic interaction between the ion pair. Similarly, Cu(II) ion can also coordinate with hydroxyl group of β-CD to form a stable hydroxyl bridge.43 Thus, the multiple interaction can produce the rigid and stable structure and generate precipitate.

Figure 3. (A) FTIR and (B) powder XRD patterns for β-CD (a), IL-1 (b), and IL-CD-Cu-2 (c).

FTIR spectra of individual β-CD (line a), IL-1 (line b), and IL-CD-Cu-2 (line c) are shown in Figure 3A. In curve a, the band between 3100 and 3700 cm−1 is assigned to νO−H. The stretching mode of C−O is recorded at the

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broad absorption band ranging from 997 to 1028 cm−1. In curve b, the peaks around 2858 and 2923 cm−1 are attributed to the asymmetric and symmetric stretching of C−H, respectively, whereas the peak centered at 3040 cm−1 is ascribed to the stretching vibration of −C=C−H. The signal at 1637 cm−1 is the characteristic stretching vibration of C=C, which is belonged to pyridium moiety. In addition, the FTIR spectrum of IL-CD-Cu-2 reveals the typical absorption features of curve a and b. Especially, small peaks located at 1012 and 1045 cm−1 are the characteristic absorption bands for the C−O bond of β-CD. Most importantly, it is worth noting that the C=C bond in line c has the redshift of 27 cm−1 compared to line b. This redshift shows that the intermolecular interaction between the host and guest increases the rigidity of the pyridium moiety.

Figure 4. SEM image of IL-CD-Cu-2.

The powder XRD patterns of all the samples are given in Figure 3B for the determination of the crystallization degree. XRD pattern of β-CD (curve a) shows distinct crystalline peaks, while IL-1 (curve b) has broaden peaks ranging from 20° to 40° reflecting the presence of armorphous nature. Furthermore, the crystalline peaks of β-CD (10.6°, 14.3°, 15.7°, 22.9°, 25.6°, 27.2°, and 31.7°) and broad peaks of IL-1 from 20° to 40° are both shown in curve c. These data confirm that the cores of IL-CD-Cu-2 have inhomogeneous and armorphous structures, which is consistent with the microstructure diagram in SEM. It has an interesting anthemia shape (Figure 4). Their diameters are ranging from 10–15 μm.

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Figure 5. (A) Cyclic voltammograms and (B) Nyquist plots obtained at bare GCE (a), IL-CD-1-GCE (b), and IL-CD-Cu-2-GCE (c) in a 0.1 M KCl solution containing 5.0 mM K3[Fe(CN)6]/K4[Fe(CN)6].

Electrochemical properties of IL-CD-1-GCE and IL-CD-Cu-2-GCE. Cyclic voltammograms (CVs) were carried out to evaluate the electrochemical sensing performance of IL-CD-1 and IL-CD-Cu-2. As shown in Figure 5A, oxidation and reduction waves are recorded for three electrodes when 5 mM [Fe(CN)6]3−/4− was used as the redox probe. It is noted that IL-CD-Cu-2-GCE (curve c) shows smaller peak current compared with IL-CD-1-GCE (curve b), which both of them are lower than that of naked GCE (curve a). It may be attributed to an effect of uncompensated ohmic drop.44,45 The diameter of semicircle in the impedance spectrum is performed as electron transfer resistance (Rct) with the ZSIMPWIN300 soft-ware (Inset of Figure 5B). The fitted values of Rct at GCE, IL-CD-1-GCE, and IL-CD-Cu-2-GCE are 67 ohm, 86 ohm, and 138 ohm, respectively (Figure 5B). Compared with naked GCE (curve a), the Rct value of IL-CD-1-GCE (curve b) and IL-CD-Cu-2-GCE (curve c) increases. Furthermore, IL-CD-Cu-2-GCE shows a bigger Rct than IL-CD-1-GCE, implying that the formation of coordination bonds decelerates the electron transfer. These differences prove the successful attachment of IL-CD-1 and IL-CD-Cu-2, which can be used as the electrochemical probe for enantiorecognition in the following study.

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Figure 6. DPV curves obtained for 0.5 mM L/D-Trp at (A) IL-CD-1-GCE with pH = 7.0, (B) IL-CD-Cu-2-GCE with pH = 7.0, (C) IL-CD-Cu-2-GCE with pH = 5.5, and (D) bare GCE with pH = 5.5.

Electrochemical enantiorecognition of Trp. DPV analysis was carried out to verify the chiral selectivity of IL-CD-1-GCE and IL-CD-Cu-2-GCE. Taken L/D-Trp for example, optimization of the experimental variables (pH and temperature) based on the reported methods is discussed in detail and the obtained results are presented in Figure S2 and S3. It is observed that IL-CD-1-GCE and IL-CD-Cu-2-GCE present much different recognition ability. As shown in Figure 6A, IL-CD-1-GCE has a higher oxidation peak current (IP) response toward L/D-Trp in the PBS solution (pH = 7.0) than IL-CD-Cu-2-GCE. But the peak current ratio (IL/ID) between L- and D- forms is only 2.2. Furthermore, pH shows a small difference to IL/ID and the DPV curves are similar for L/D-Trp at IL-CD-1-GCE. In contrast, IL-CD-Cu-2-GCE is sensitive to the buffer solution with different pH. With the increasing of pH from 5.5 to 7.0, IP for L-Trp decreases obviously and meantime IP for D-Trp increases gradually. Specifically, when the probe is treated with D-Trp at pH = 7.0, it exhibits higher IP than L-Trp (Figure 6B). On the contrary, at pH = 5.5, IL-CD-Cu-2-GCE shows a higher IP response toward L-Trp (4.3 μA), while a much lower IP 11



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response toward D-Trp (0.1 μA) (Figure 6C). It is interesting to note that the identification signal is reverse in the buffer solution with different pH. This is reasonable, since the coordination complex is sensitive to the acid solution. With the enhancing of the acidity, it will weaken strength of the coordination bond and L-Trp can enter into the hydrophobic cavity of β-CD with much lower intermolecular resistance. Furthermore, compared with the electrochemical signal at unmodified GCE (Figure 6D), the oxidation peak potential shifts positively with difference of 72 mV, which indicates that higher applied potential is necessary at IL-CD-Cu-2-GCE. Despite the IP of L/D-Trp obtained at IL-CD-Cu-2-GCE is lower than that at IL-CD-1-GCE, it presents a obvious chiral discrepancy between enantiomers, which can be applied as the probe for enantioselective recognition. The mechanism would be discussed in the following part.

Figure 7. DPV curves obtained at IL-CD-Cu-2-GCE in 0.1 M PBS solution (pH = 5.5) for 0.5 mM (A) tyrosine, (B) cysteine, and (C) malic acid.

Electrochemical enantiorecognition of other enantiomers. We further investigate the chiral selectivity for tyrosine, cysteine, and malic acid by using the same method. As shown in Figure 7, DPV analysis has a sufficient difference for each pair of the detected enantiomers. Better performance is obtained compared with other reported electrodes.46–49 That is, IL-CD-Cu-2-GCE presents a much higher IP response toward L isomer than that toward D isomer. In addition, the sensor shows two oxidation peaks when treated with malic acid (Figure 7B), which is much different to the reported work.7,11,46,47 A little difference occurs at the first oxidation peak (IL/ID = 1.5), while the second oxidation peak shows a clear chiral discrepancy (IL/ID = 12.1). It can be attributed to the reason that the 12


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molecule has a two-stage oxidation process. In a word, these differences demonstrate the potential for the novel material to be applied in enantioselective recognition.

Figure 8. Possible bonding modes of (A) IL-CD-Cu-2-L-Trp and (B) IL-CD-Cu-2-D-Trp.

Mechanism of chiral recognition. The mechanism for chiral recognition of amino acids using β-CD has been discussed in detail before.50,51 In this study, given the special structure of IL-CD-Cu-2, we propose a possible binding mode for Trp based on our electrochemical results. As presented in Figure 8, Cu(II) ion can form a stable hydroxyl bridge with β-CD on the edge of wider opening.43 Once the tested L-isomers enter into the cavity of β-CD, it will coordinate with Cu(II) ion with much lower steric hindrance. At the same time, the pyridinium cation of IL-1 will be far away from β-CD (Figure 8A). For D-Trp, the aliphatic chain is too short to provide enough space, which results in that it is hard to form stable coordinated complex in the cavity (Figure 8B) in the presence of strong repulsive effect at short ranges, leading to a very low electric signal. Unlike the above recognition mechanism in the absence of Cu(II) ion, both L- or D-isomers can enter into the cavity of β-CD, which would decrease the recognition efficiency.

CONCLUSIONS In summary, this work is the first report of the preparation of a coordinated complex derived from the charged 13



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catenane via host−guest recognition. Most importantly, the obtained molecule maintains its structure-switching characteristics, which is used as surface modification for the naked GCE. The functional GCE presents good capability for resolution of enantiomers (tryptophan, tyrosine, cysteine, and malic acid). That is, L-form shows much higher peak current response than that of D-form. Results indicate that the smart chemical structure of charged catenane contributes to the steric hindrance actuated by external stimuli modes, which can improve recognition efficiency. In a word, we believe that this study confirms the possibility of functional catenanes as electrode modification materials for chiral resolution.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. MS image of IL-CD-1 and optimization of the experimental variables (pH, concentration, and temperature) for chiral recognition (PDF).

AUTHOR INFORMATION Corresponding Author *Email:

[email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS We are grateful to National Science Foundation of China (21804013), Natural Science Foundation of Jiangsu Province (BK20171194), Natural Science Foundation of the Higher Education Institutions of Jiangsu Province 14


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(18KJB150003),


Advanced

Catalysis

and

Green

Manufacturing

Collaborative

Innovation

Center

(ACGM2016-06-27), and Qing Lan Project of Jiangsu Higher Education Institutions for the financial support.

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Dynamic Interaction between Host and Guest for Enantioselective

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