Smart Chiral Sensing Platform with Alterable Enantioselectivity

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A Smart Chiral Sensing Platform with Alterable Enantioselectivity Yin Yu, Yongxin Tao, Baozhu Yang, Datong Wu, Yong Qin, and Yong Kong Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b03783 • Publication Date (Web): 10 Nov 2017 Downloaded from http://pubs.acs.org on November 13, 2017

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Analytical Chemistry

A

Smart

Chiral

Sensing

Platform

with

Alterable

Enantioselectivity

Yin Yu, Yongxin Tao, Baozhu Yang, Datong Wu, Yong Qin, and Yong Kong* Advanced Catalysis and Green Manufacturing Collaborative Innovation Center, Changzhou University, Changzhou 213164, China

Email: [email protected] Tel.: 86-519-86330256; fax: 86-519-86330167. 1 ACS Paragon Plus Environment


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ABSTRACT: A quinine (QN)-based chiral sensing platform with alterable enantioselectivity is constructed for electrochemical chiral recognition of tryptophan (Trp) isomers. The electrochemical signals of L- and D-Trp on the QN modified electrode depend closely on temperature, and more particularly are reversed at certain temperatures, which could be attributed to the temperature-sensitive H-bonds and π-π interactions between QN and the Trp isomers. The mechanisms of the reverse chiral recognition are investigated by density functional theory (DFT), variable-temperature UV spectra and variable-temperature 1H NMR spectra. In addition, the chiral recognition is highly specific to the isomers of Trp compared with other chiral amino acids. This study is the first example showing how temperature influences the reverse recognition of electrochemical chiral interfaces.

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Enantioselective recognition of chiral amino acids is of great importance for understanding of the living world,1–3 since amino acids are the molecular building blocks of life and different configurations of amino acids play different even opposite roles in life science.4,5 Among the reported methods for chiral recognition, electrochemistry has drawn considerable attention since it can easily convert a chiral recognition event to identifiable changes in electrochemical signals.6 Recently, electrochemical chiral recognition of tryptophan (Trp) isomers with polysaccharides-based chiral interfaces has been reported by our group.7–9 Although

temperature-responsive

recognition

can

be

achieved

with

these

polysaccharides-based chiral sensing platforms due to the existence of temperature-sensitive H-bonds within the host-guest systems, the electrochemical signals generated from L- and D-Trp show the same tendency over a wide temperature range. That is to say, the

electrochemical signal of L-Trp is always larger than that of D-Trp (or vice versa). In the past decades, L-Trp, which is called the second amino acid, is determined to be an essential constituent used in the synthesis of protein, while D-Trp is not participated in protein synthesis.10 However, the importance of D-Trp has been highlighted recently due to its significant influence on biofilm formation and bacteria growth.11,12 Therefore, design of smart and convenient chiral sensors showing high sensitivity to both L-Trp and D-Trp is a challenging task for biochemical analysis. Herein, we report on a quinine (QN)-based chiral sensing platform with alterable enantioselectivity for Trp isomers. Due to the intrinsic chirality of QN,13–15 recognition of Trp isomers can be achieved with the proposed electrochemical chiral sensing platform. Of particular interest, the relative intensity in the electrochemical signals generated from L- and 3 ACS Paragon Plus Environment


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D-Trp varies greatly at different temperatures. Especially, the peak current ratio changes

from 2.40 (ID-Trp/IL-Trp) at 5 °C to 3.06 (IL-Trp/ID-Trp) at 25 °C, and the fantastic reverse recognition could be attributed to the temperature-sensitive H-bonds and π-π interactions between QN and the Trp isomers. To further understand the mechanisms of the reverse chiral recognition, density functional theory (DFT), variable-temperature UV and 1H NMR spectra are studied. Finally, the specificity of the QN-based chiral interfaces is investigated at 25 °C, and the results show that the developed chiral interfaces are highly specific to the isomers of Trp compared with other chiral amino acids (cysteine, phenylalanine and tyrosine). DFT study reveals that double H-bonds can be formed between QN and L-Trp, while only one H-bond between QN and D-Trp and other amino acids isomers. EXPERIMENTAL SECTION Reagents and Apparatus. L-Tryptophan (L-Trp), D-tryptophan (D-Trp), L-tyrosine (L-Tyr), D-tyrosine (D-Tyr), L-cysteine (L-Cys), D-cysteine (D-Cys) and quinine (QN) were purchased from Aladdin Chemistry Co., Ltd. (Shanghai, China). L-phenylalanine (L-Phe), D-phenylalanine (D-Phe), potassium ferricyanide (K3[Fe(CN)6]), potassium ferrocyanide

(K4[Fe(CN)6]) and other chemicals not mentioned were analytical grade and purchased from Sinopharm Chemical Reagent Co., Ltd. (SCRC, China). All solutions were prepared with ultrapure water (Milli-Q, Millipore). Electrochemical

experiments

including

potentiostatic

electrodeposition,

cyclic

voltammetry (CV), electrochemical impedance spectroscopy (EIS) and differential pulse voltammetry (DPV) were carried out on a CHI-660E electrochemical workstation in a conventional three-electrode cell with a glassy carbon electrode (GCE, 3 mm in diameter) or 4 ACS Paragon Plus Environment

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QN modified GCE (QN/GCE) as the working electrode, a platinum foil (10 × 5 mm) as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode, respectively. The temperatures for the inclusion of Trp isomers to QN and the consequent DPV experiments were precisely controlled with a model DC-0510 intelligent thermostatic bath (Ningbo Scientz Biotechnology Co., Ltd. China). The surface topography of the QN films was characterized with a JPK NanoWizard3 atomic force microscope (AFM, Germany). The molecular dynamics (MD) simulation was accomplished by Gaussian 09 software package. The variable-temperature UV spectra of the complexes of QN and the Trp isomers were recorded on a UV-1700 UV-vis spectrophotometer (Shimadzu, Japan), and the variable-temperature 1H NMR spectra were recorded on an Avance III HD-500 NMR spectrometer (Switzerland) using deuterium chloride (ClD) as the solvent. Electrodeposition of QN on the Surface of GCE. 40 mg of QN powders was dissolved in 25 mL of 0.1 M HCl, and then a three-electrode system using a bare GCE as the working electrode was immersed into the QN solution (1.6 mg mL-1) for 600 s with a constant potential of –1.0 V (vs. SCE) exerted on the GCE for the electrodeposition of QN. Chiral Recognition of Trp Isomers with QN/GCE. Electrochemical chiral recognition of Trp isomers was carried out by DPV. The as-prepared QN/GCE was placed into 25 mL of 0.1 M phosphate buffer solution (PBS) containing 0.5 mM L- or D-Trp (pH 7.0) for 90 s, and then the differential pulse voltammograms of L- and D-Trp included with QN were recorded, respectively, in the potential range from 0.4 to 1.2 V at a scan rate of 8 mV s-1. Next, the peak current ratios of L-Trp to D-Trp (IL-Trp/ID-Trp) or D-Trp to L-Trp (ID-Trp/IL-Trp) were calculated as the indicator of recognition efficiency. To further understand the QN-based chiral sensing 5 ACS Paragon Plus Environment


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platform with alterable enantioselectivity, temperature was precisely controlled at 0, 4, 5, 8, 10, 15, 20, 25, 30, 35, 37 and 40 °C, respectively, for the incorporation of the Trp isomers to the QN films electrodeposited on GCE. Each DPV experiment was repeated in triplicate and the standard deviation was obtained for labeling the error bars. Finally, the electrochemical chiral recognition of the isomers of Tyr, Phe and Cys was investigated at 25 °C according to the same procedures to understand the chiral specificity of the proposed chiral interfaces. RESULTS AND DISCUSSION Construction of QN-Based Electrochemical Chiral Interfaces. As previously reported, protonation of both N atoms on QN occured at pH < 3.5,16 and therefore QN was positively charged in 0.1 M HCl. When a negative potential of –1.0 V (vs. SCE) was exerted on the GCE, QN was subject to be electrodeposited onto the surface of GCE owing to the strong electrostatic attractions, as shown in Figure 1. Due to the intrinsic chirality of QN, the obtained QN/GCE was endowed with chiral microenvironment and could be used as the chiral interfaces for enantioselective recognition.

Figure 1. Electrodeposition of QN films on the surface of GCE.

Characterization of Electrodeposited QN Films and QN/GCE. Figure 2 showed the two- and three-dimensional AFM images of the electrodeposited QN films, in which the darker and the brighter contrast correspond to the lower and the higher area of the surface, 6 ACS Paragon Plus Environment

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respectively. The hillocks on the surface were surrounded by lower areas with a maximum height variation of 41.7 nm, and the root mean square (RMS) surface roughness was measured to be 9.486 nm. These results implied that the protonated QN films were successfully electrodeposited at a negative potential.

Figure 2. Two- (left) and three-dimensional (right) AFM images of the electrodeposited QN films.

As a redox couple, Fe(CN)64-/3- was commonly used as a probe to understand the electrochemical properties of modified electrodes. Here, the cyclic voltammograms of bare GCE and QN/GCE in 0.1 M KCl aqueous solution containing 5 mM Fe(CN)64-/3- were recorded and compared. As shown in Figure 3, the couple of Fe(CN)64-/3- exhibited a pair of well-defined redox peaks at bare GCE (curve a). After the modification of QN, the redox peak currents were greatly increased (curve b), and the improved electrochemical activity of the QN/GCE could be attributed to the modification of protonated QN. Since the electrodeposited QN was positively charged, the electrostatic attractions between both protonated N atoms on QN and the negatively charged Fe(CN)64-/3- were favorable for the electron transfer at the electrode-solution interfaces, resulting in improved electrochemical behaviors of the QN/GCE. 7 ACS Paragon Plus Environment


b a

0.15 0.10 0.05

I / mA

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0.00 -0.05 -0.10 -0.15 -0.2

0.0

0.2

0.4

0.6

E (vs. SCE) / V Figure 3. Cyclic voltammograms of bare GCE (a) and QN/GCE (b) in 0.1 M KCl aqueous solution containing 5 mM Fe(CN)64-/3-. Scan rate, 100 mV s-1.

The successful modification of QN on the GCE was further proven by the EIS analysis. Figure 4 was the Nyquist plots of bare GCE and QN/GCE in 0.1 M KCl containing 5 mM Fe(CN)64-/3-, and the impedance semicircle in the high frequency region was an indicator reflecting the charge transfer resistance (Rct) at the electrode-solution interfaces.17 The couple of Fe(CN)64-/3- exhibited a larger value of Rct (83 Ω) at the bare GCE than that at the QN/GCE (44 Ω), and the significantly decreased Rct at the QN/GCE-solution interfaces could be ascribed to the strong electrostatic attractions between the positively charged QN and the negatively charged redox couple. The results of EIS agreed well with the results of CV (Figure 3).


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350

Z-imaginary / ohm

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


300 250

b

200

a

150 100 50 0

50

100

150

200

250

300

350

Z-real / ohm Figure 4. Nyquist plots of bare GCE (a) and QN/GCE (b) in 0.1 M KCl aqueous solution containing 5 mM Fe(CN)64-/3-. Inset is the corresponding equivalent circuit, where Rs is the solution resistance, Rct is the interfacial charge transfer resistance, Wd is the Warburg resistance, and Q represents constant phase elements (CPE).

Temperature Induced Reverse Chiral Recognition for Trp Isomers. First, the differential pulse voltammograms (DPVs) of 0.5 mM L- and D-Trp at bare GCE were recorded at 25 °C (Figure 5A). As previously reported, the electrochemical oxidation of Land D-Trp is taken place via a two-electron and two-proton transfer process.18,19 As can be seen, the DPVs of L- and D-Trp at bare GCE were seriously overlapped, implying that the Trp isomers could not be resolved with bare GCE. Obviously, the poor enantioselective ability of bare GCE was attributed to the lack of chiral sites with bare GCE. Next, we evaluated the capability of QN/GCE for recognition of Trp isomers at 25 °C, and it showed a significantly higher selectivity for L-Trp (IL-Trp/ID-Trp, 3.06) (Figure 5B), which was most likely due to the intrinsic chirality of QN. It was amazing to find that although a discernible peak current ratio (ID-Trp/IL-Trp, 2.40) was still observed at 5 °C, the QN/GCE exhibited an opposite selectivity for the Trp isomers (Figure 5C), i.e., QN exhibited a higher selectivity for 9 ACS Paragon Plus Environment


D-Trp at 5 °C. To further understand the interesting temperature induced chiral recognition

phenomenon of the QN/GCE, the QN-based chiral recognition was studied at varying temperatures from 0 to 40 °C (Figure 5D). It seemed that the chiral selectivity for the Trp isomers depended on the testing temperature. At low temperatures (below 10 °C), the chiral sensor exhibited a higher selectivity for D-Trp especially at 5 °C. However, the chiral selectivity happened in reverse with increasing temperature, and the highest selectivity for L-Trp occurred at 25 °C. 2.0

7

A

D-Trp

5

I / µA

D-Trp

I / µA

L-Trp

B

6

L-Trp

1.5

1.0

IL / ID = 3.06

4 3 2

0.5

1 0 0.4

0.6

0.8

1.0

0.4

1.2

0.6

E (vs. SCE) / V

0.8

1.0

1.2

E (vs. SCE) / V 3.5

3.0 L-Trp

C 2.5

3.0

D-Trp

ID / IL = 2.40

1.5 1.0 0.5

ID / IL

2.0

3.5

D

3.0

2.5

2.5

2.0

2.0

1.5

1.5

1.0

1.0

0.5

0.5

IL / ID

0.0

I / µA

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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0.0 0.4

0.6

0.8

1.0

1.2

0.0

0

10

20

30

40

0.0

o

E (vs. SCE) / V

T/ C

Figure 5. Differential pulse voltammograms of L-Trp and D-Trp at bare GCE (A) and QN/GCE (B, C) at 25 °C (B) and 5 °C (C). (D) Influence of temperature on the recognition efficiency of QN/GCE toward L- and D-Trp. Errors bars represent the standard deviation for three independent measurements.

Obviously, the temperature induced reverse recognition was essentially decided by the 10 ACS Paragon Plus Environment

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interactions between QN and the Trp isomers. Here, the density functional theory (DFT) was employed to optimize the ground state structure without symmetry constrains,20–22 and all the calculations were accomplished by Gaussian 09 software package.23 The 6−311G (d, p) basis sets were adopted for C, N, O and H atoms to give the optimized structures of the complexes of QN-D-Trp and QN-L-Trp, and the H atoms attached to the C atoms were omitted (except for H atoms that formed H-bonds). As shown in Figure 6A (left), D-Trp was included with QN via a single H-bond and π-π interactions, and the interatomic distance between H and N participated in the H-bond formation was 1.2 Å. For the complex of QN-L-Trp, double H-bonds could be formed. One was formed between the H atom of −OH on QN and the N atom of −NH2 on L-Trp with a H---N distance of 1.46 Å; the other was between the O atom of −OH on QN and the H atom of the indole ring on L-Trp with an O---H distance of 1.23 Å (Figure 6B, left). The interactions between QN and the Trp isomers were also schematically shown in Figure 6 (right). Based on the classification of Jeffrey and Steiner,24,25 strong H-bond could be formed when the length of the H-bond was in the range of 1.2−1.5 Å. Therefore, both L- and D-Trp could be steadily included with QN via strong H-bond(s) or π-π interactions, and the temperature induced reverse recognition for the Trp isomers could be attributed to the temperature-sensitive characteristics of H-bond and π-π interactions.



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Figure 6. Optimized structures of the complexes of QN-D-Trp (A, left) and QN-L-Trp (B, left) by DFT, and the schematic diagrams showing the interactions between QN and D-Trp (A, right) and L-Trp (B, right).

As previously reported, low temperature was favorable for π-π interactions,26 however, H-bonds were seriously suppressed owing to the confined molecular motion at low temperatures.9,27 The QN-based chiral interfaces exhibited a higher selectivity for D-Trp at low temperatures (especially at 5 °C, Figures 5C and 5D), because stable π-π interactions could occur between QN and D-Trp at low temperatures. The molecular motion was accelerated with increasing temperature owing to the weakened intramolecular interactions, which was beneficial to the formation of H-bond(s) between QN and Trp isomers. That is to say, H-bond(s) gradually became the main force between QN and Trp isomers instead of π-π interactions with increasing temperature. As a result, a higher selectivity for L-Trp was observed at high temperatures (especially at 25 °C, Figures 5B and 5D), and a temperature induced reverse recognition for the Trp isomers was achieved. In fact, the high selectivity for 12 ACS Paragon Plus Environment

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L-Trp at 25 °C could be further verified by the theoretical calculations based on DFT. To

explore the interactions between L-Trp, D-Trp and QN, we optimized the molecular structures and did frequence calculations for these complexes. According to the single point energies and Gibbs free energies (Table 1), the energy differences of Gibbs free energy (ΔG) and single point energy (ΔE) could be calculated based on the following formulae: ΔG=G(QN-D(L)-Trp)–G(D(L)-Trp)–G(QN) and ΔE=E(QN-D(L)-Trp)–E(D(L)-Trp)–E(QN), and the calculated ΔG and ΔE for QN-D-Trp and QN-L-Trp were listed in Table 2, in which the unit is converted from Hartree to kcal mol-1 (1 Hartree = 627.51 kcal mol-1). As shown in Table 2, for the complex of QN-L-Trp, both ΔG and ΔE were decreased more than those of QN-D-Trp, suggesting that the interactions between QN and L-Trp were stronger than those between QN and D-Trp at 25 °C. In addition, an excessively high temperature (30−40 °C) would lead to the breakage of stable H-bond(s) between QN and Trp isomers and the subsequent decrease in recognition efficiency (Figure 5D). Table 1. The calculated values of Gibbs free energies and single point energies. Molecule

Energy (G)/Hartree

Energy (E)/Hartree

D-Trp

−685.981542

−686.1615563

L-Trp

−685.984382

−686.1630412

QN

−1035.832461

−1036.1957406

QN-D-Trp

−1722.049107

−1722.6141231

QN-L-Trp

−1722.411375

−1722.6609582

Table 2. ΔG and ΔE values for the QN-D-Trp and QN-L-Trp complexes at 25 °C. Complex

ΔG/Hartree (kcal mol-1)

ΔE/Hartree (kcal mol-1)

QN-D-Trp

−0.235104 (−147.5)

−0.2568267 (−161.2)

QN-L-Trp

−0.594532 (−373.1)

−0.3021768 (−189.6)



Variable-Temperature UV Spectra of the Complexes of QN-L-Trp and QN-D-Trp. To further understand the temperature induced reverse recognition, variable-temperature UV spectra of QN-L-Trp and QN-D-Trp were studied (Figure 7). For QN-L-Trp formed via double H-bonds, the absorbances varied little at low temperatures (0 and 5 °C), and then it increased obviously from 5 to 25 °C. For QN-D-Trp formed via a single H-bond and π-π interactions, the absorbances were a little higher than those of QN-L-Trp at 0 and 5 °C, and the changes in the absorbance were greatly smaller than those of QN-L-Trp from 5 to 25 °C. By comparing the changes in the absorbances on the two spectra, two important conclusions could be drawn: (1) H-bond(s) between QN and Trp isomers was more sensitive to temperature than π-π interactions; (2) π-π interactions played a dominant role at low temperatures (0 and 5 °C) while the influence of H-bonds became more significant with gradually increasing temperature. In addition, the maximum absorbance appeared at 25 °C on both the spectra, and then it greatly declined at 35 °C. This phenomenon was most likely due to the fact that an excessively high temperature would lead to the breakage of H-bond(s) between QN and the Trp isomers, which agreed well with the results shown in Figure 5D. 3.0

3.0

A

B 2.5

2.0

2.0

A

2.5

A

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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1.5 1.0 260

0 °C 5 °C 15 °C 25 °C 35 °C 270

1.5

1.0 280

260

290

0 °C 5 °C 15 °C 25 °C 35 °C 270

280

290

 / nm  / nm Figure 7. Variable-temperature UV spectra of the complexes of QN-L-Trp (A) and QN-D-Trp (B)

prepared from 0.5 mM L-Trp, 0.5 mM D-Trp and 0.1 mM QN.


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Variable-Temperature 1H NMR Spectra of QN-L-Trp and QN-D-Trp. Next, the variable-temperature 1H NMR spectra of QN-L-Trp and QN-D-Trp complexes in deuterium chloride (ClD) were studied. As shown in Figures 8B and 8D, an upfield chemical shift of the QN−2H (Figure 8A, left) was observed as the temperature was raised from 0 to 8 °C, indicating favorable π-π interactions at low temperatures especially at 8 °C.26 When the temperature was further increased, the π-π interactions between QN and D-Trp were destroyed and a downfield shift of the 1H NMR aromatic resonances occurred.28 As shown in Figures 8C and 8E, the L-Trp−8H (Figure 8A, middle) which participated in the formation of H-bond with QN was downfield shifted with increasing temperature from 0 to 25 °C. In most H-bonds several nuclei may be observed from NMR,24 and particularly the proton is increasingly deshielded with increasing strength of H-bond, which leads to the downfield shift of 1H.24,25,29,30 When the temperature was further raised from 25 to 40 °C, an upfield chemical shift of the L-Trp−8H was observed since the H-bond between L-Trp and QN was weakened at high temperatures.



QN-D-Trp

Bo

QN-L-Trp o

C

40 C o 37 C o 35 C o 30 C o 25 C o 20 C o 15 C o 10 C o 8C o 5C o 4C

40 C o 37 C o 35 C o 30 C o 25 C o 20 C o 15 C o 10 C o

8C o 5C o 4C QN–2H

o

0C

7.8

7.6

7.4

7.2

7.0

6.8

6.6

6.4

L-Trp– H 8

6.2

/ ppm

6.0 7.6

7.4

7.2

7.0

6.8

6.6

6.4

o

0C

6.2

/ ppm

6.80

6.80

(QN—2H)

D

(L-Trp—8H)

E

6.75

6.75

 / ppm

 / ppm

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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6.70

6.70

6.65

6.65 6.60

0

10

20

30

40

0

10

o

20

30

40

o

T/ C

T/ C

Figure 8. (A) Chemical structures of QN, L-Trp and D-Trp. (B, C) Variable-temperature 1H NMR spectra of QN-D-Trp (B) and QN-L-Trp (C) in deuterium chloride (ClD). Each concentration: 10 mg mL-1. Plots of chemical shifts of QN−2H (D) and L-Trp−8H (E) against temperature.

Chiral Specificity of QN-Based Chiral Interfaces. Finally, the chiral specificity of the QN/GCE was studied using the isomers of tyrosine (Tyr), phenylalanine (Phe), cysteine (Cys) and Trp as the targets. The chiral recognition for the different isomers was carried out under 16 ACS Paragon Plus Environment

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the same conditions (25 °C, pH 7.0), and the results were shown in Figure 9. It showed that the recognition efficiency of Trp was greatly higher than those of Tyr, Phe and Cys, implying that the QN-based chiral interfaces had high chiral specificity for Trp isomers. To explain the high specificity for Trp isomers, the optimized structures of the complexes of QN and L/D-Trp, L/D-Tyr, L/D-Phe and L/D-Cys obtained by DFT as well as the chemical structures

of the four chiral amino acids were provided (Figure 10). The indole ring of Trp was consisted of a benzene ring and a five-membered pyrrole ring, and the H atom on the pyrrole ring and the N atom of −NH2 on L-Trp had a sufficient distance in spatial configuration to simultaneously form double H-bonds with the O and H atoms, respectively, of the −OH group on QN. However, only one H-bond could be formed between QN and D-Trp due to the steric hindrance from D-Trp. The different number of H-bonds between QN and L/D-Trp resulted in the high specificity for Trp isomers. The benzene ring on Tyr and Phe is a six-membered ring, and the H atom on the benzene ring and the N atom of −NH2 could not form double H-bonds with QN considering the spatial configuration of Tyr and Phe. Cys is a kind of aliphatic-α-amino acid, and only one H-bond could be formed between Cys and QN owing to the short chain of Cys.



6

6 L-Tyr

A 5

D-Phe

4

IL / ID = 1.023

3

I / µA

I / µA

L-Phe

B 5

D-Tyr

4

2

2 1

0

0 0.6

0.8

1.0

IL / ID = 1.012

3

1

0.4

1.2

0.4

0.6

E (vs. SCE) / V

0.8

1.0

1.2

E (vs. SCE) / V

3.0

3.5 L-Cys

C 2.5

3.0

D-Cys

D

2.5

IL / ID = 1.016

1.5 1.0

IL / ID

2.0

I / µA

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2.0 3.065

1.5 1.0

0.5

0.5

1.023

1.012

1.016

Tyr

Phe

Cys

0.0 0.4

0.6

0.8

1.0

0.0

1.2

E (vs. SCE) / V

Trp

Different amino acids

Figure 9. Differential pulse voltammograms of L-/D-Tyr (A), L-/D-Phe (B) and L-/D-Cys (C) at QN/GCE. Each concentration, 0.5 mM; temperature, 25 °C; pH, 7.0. (D) Comparison of recognition efficiency of QN/GCE for the isomers of four chiral amino acids. Error bars represent standard deviation for three independent measurements.


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Figure 10. (A) Chemical structures of Trp, Tyr, Phe and Cys. (B, C) Optimized structures of the complexes of QN and L-isomers (B) and D-isomers (C) of the four chiral amino acids.

CONCLUSIONS To summarize, we designed a novel QN-based smart chiral sensing platform with alterable enantioselectivity for Trp isomers. Of particular interest, the selectivity for the Trp isomers could be reversed at certain temperature owing to the temperature-sensitive interactions between QN and L-Trp (double H-bonds) and D-Trp (single H-bond and π-π interactions), which was further verified by the variable-temperature UV and 1H NMR spectra. The mechanisms of the temperature induced reverse recognition were discussed with the aid of density functional theory. Also in this work, the chial specificity of the developed chiral interfaces was studied, and the results indicated that the QN-based chiral interfaces exhibited significantly higher specificity for Trp isomers. With more attention paid to the 19 ACS Paragon Plus Environment


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importance of D-isomers of chiral amino acids, this work might open up a new avenue for the construction of smart and convenient chiral sensing platform with alterable enantioselectivity for both L- and D-isomer of amino acids. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This research was supported financially by National Natural Science Foundation of China (21775013), Natural Science Foundation of Jiangsu Province (BK20171194) and Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYLX16_0627).


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For TOC only


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Smart Chiral Sensing Platform with Alterable Enantioselectivity

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