Chiral Recognition of d-Tryptophan by Confining High-Energy Water

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Chiral Recognition of D-Tryptophan by Confining High-Energy Water Molecules Inside the Cavity of Copper-Modified beta-Cyclodextrin Yongxin Tao, Xiaogang Gu, Linhong Deng, Yong Qin, Huaiguo Xue, and Yong Kong J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 27 Mar 2015 Downloaded from http://pubs.acs.org on March 29, 2015

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Chiral Recognition of D-Tryptophan by Confining High-Energy Water

Molecules

Inside

the

Cavity

of

Copper-Modified

beta-Cyclodextrin Yongxin Tao,† Xiaogang Gu,† Linhong Deng,‡ Yong Qin,† Huaiguo Xue,§ and Yong Kong*, † †

Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, School of Petrochemical Engineering, Changzhou University, Changzhou 213164, P. R. China

‡

Institute of Biomedical Engineering & Health Sciences, Changzhou University, Changzhou 213164, P. R. China

§

School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002, P. R. China

1 ACS Paragon Plus Environment


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ABSTRACT: We report a Cu2+-modified β-cyclodextrin, Cu2-β-CD, that was self-assembled on poly(L-glutamic acid) (P-L-Glu) for chiral recognition of D-tryptophan (D-Trp). Cu2+ formed binuclear hydroxy-bridge (Cu(OH)2Cu) at the wider opening of the cavity of β-CD and acted as a cap to prevent high-energy water molecules from being released while forcing Trp isomers to enter through its narrower opening. Because H-bonds were favored to form between the high-energy water molecules and the amino groups of D-Trp inside the cavity of β-CD, D-Trp was thus recognized. The recognition capability of the Cu2-β-CD appeared to depend on the way the Cu2-β-CD self-assembled onto P-L-Glu, preferring via H-bonds rather than via electrostatic interactions, because electrostatic interactions between P-L-Glu and Cu(OH)2Cu could alter the configuration of the Cu2-β-CD cavity, resulting in weakened H-bonds between high-energy water molecules and D-Trp.

Keywords: Electrochemical recognition; Tryptophan isomers; Cu2+-modified β-cyclodextrin; High-energy water molecule; Self-assembly


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1. INTRODUCTION Enantioselective recognition of chiral compounds is one of the most challenging tasks in biochemical analysis because chiral compounds are optical isomers with similar physical and chemical properties that are difficult to discriminate.1 In the past, several techniques have been widely adopted to recognize chiral compounds including molecular imprinting based on the three-point interaction2,3 and ligand exchanging based on the diastereomeric ternary coordination formatted by chiral selector, metal ion and analyte.4-6 As an alternative approach, electrochemistry circumvents the problems such as high cost and time consumption associated with spectroscopic and chromatographic methods. Considering this preference and the abundance of L-amino acids in all life forms, most studies, therefore, have so far focused on development of chiral sensors for recognition of L-amino acids.

7-9

However, the importance of D-amino acids has been highlighted by recent

reports of their great influence on bacteria growth and biofilm formation.10-12 Accordingly, in addition to recognition of D-amino acids via spectroscopic and chromatographic approach has been proposed.13,14 The development of supramolecular chemistry has opened a new avenue for recognition of chiral compounds.15 In particular, the natural cyclodextrin (CD), due to its well-defined hydrophobic cavity, has emerged as an ideal host for forming supramolecular structures that are capable of selectively accommodating various guest molecules to be detected.16,17 For example, Song’s group found that β-CD exhibited a high recognition efficiency toward L18

D-Tryptophan (Trp) in the presence of 18-crown-6.

They also investigated the influence of

molecule-ion interactions on the precipitation-dissolution equilibrium of Li2CO3 in water and



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on the chiral recognition behaviors and binding abilities of CD to D- and L-Trp, which provided

a

new

insight

into

the

link

between

molecule-ion

interactions

and

precipitation-dissolution equilibriums of poorly dissolving inorganic salts.19 More recently, we have reported the fabrication of a supramolecular electrode with natural β-CD self-assembled onto poly(L-glutamic acid) (P-L-Glu) that can be used to electrochemically recognize chiral compounds such as L- and D-Trp.20 However, like other reports on the discrimination of amino acids enantiomers,21,22 the targets (L-/D-amino acids) to be recognized are entered into the cavity of natural β-CD from the wider opening. Herein, we report on the self-assembly of a Cu2+-modified β-CD (Cu2-β-CD),23-25 on P-L-Glu and its application in the chiral recognition preferable to D-amino acid. Here, Trp was selected as the target amino acid because it is an essential amino acid in most biological systems. And natural β-CD is a truncated cone-shaped molecule with a hollow, tapered cavity which has a wider opening of diameter 6.5 Å, and a narrower opening of diameter 6.0 Å, respectively.26 The inclusion complexation of guest molecules into natural β-CD is usually accompanied by the removal of high-energy water molecules from the cavity of β-CD.27-29 However, for the Cu2-β-CD, Cu2+ is coordinated at the wider opening of β-CD,24 and thus forming a cap-like binuclear hydroxy-bridge (Cu(OH)2Cu) (Figure 1, bottom). This prevents the high-energy water molecules from being removed from the cavity and forces the outer Trp isomers to enter the cavity from the narrower opening. In this way, the –NH2 at the chiral center of D-Trp and the high-energy water molecules inside the cavity form strong intermolecular H-bonds due to steric reasons, giving the Cu2-β-CD higher affinity to D-Trp. This work is also the fisrt example verifying the theory proposed by Altarsha et al., in which


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they stated that β-CD has a higher affinity to D-isomers entered into the cavity from the narrower opening based on the investigations of molecular dynamic (MD) simulations.30 Furthermore, the Cu2-β-CD can be self-assembled onto P-L-Glu by either H-bonds or electrostatic interactions (Figure 1). Interestingly, the Cu2-β-CD attached to P-L-Glu via H-bonds exhibited greater capability of recognition for Trp isomers compared to that attached via electrostatic interactions, which was mainly attributed to the different configurations of the Cu2-β-CD cavity by the two different self-assembly modes.

Figure 1. Electrodeposition of P-L-Glu on glassy carbon electrode and subsequent self-assembly of Cu2-β-CD onto P-L-Glu via H-bonds (pH 7.0) and electrostatic interactions (pH 9.0).

2. EXPERIMENTAL SECTION Reagents and Apparatus. D-Trp (99%) and L-Trp (99%) were purchased from Aladdin Chemistry Co., Ltd (China). L-Glutamic acid (L-Glu), potassium ferricyanide (K3Fe(CN)6) and potassium ferrocyanide (K4Fe(CN)6) were purchased from Shanghai Lingfeng Chemical Co., Ltd (China). β-CD was obtained from Zibo Qianhui Biological Technology Co., Ltd (China). Copper sulfate pentahydrate (CuSO4⋅5H2O), sodium hydroxide (NaOH), ethanol and



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potassium chloride (KCl) were purchased from sinopharm chemical reagent company (China). Electrochemical experiments were carried out in a standard three-electrode system connected to a CHI-660D electrochemical workstation (CH Instruments, Inc., China). Auxiliary electrode was a platinum foil (10 × 5 mm) and reference electrode was a saturated calomel electrode (SCE). Working electrode was a Cu2-β-CD or D- or L-Trp-Cu2-β-CD self-assembled P-L-Glu/glassy carbon electrode (GCE, φ = 3 mm). The FT-IR spectra of Cu2-β-CD and β-CD were recorded by a Nicolet FTIR-8400S spectrophotometer (Shimadzu, Japan) using KBr pellets. The UV spectra of β-CD, Cu2-β-CD and the inclusion complexes of Trp-Cu2-β-CD at different temperatures were measured using a variable-temperature UV-1700 UV/Vis spectrophotometer (Shimadzu, Japan). The content of copper in the Cu2-β-CD was determined by a Shimadzu AA-6300 atomic absorption spectrophotometer (Japan). The temperatures studied in the present work for self-assembly of Cu2-β-CD and inclusion complexation of the host-guest system were controlled by an intelligent thermostatic bath (Ningbo Scientz Biotechnology Co., Ltd, China). 1H NMR measurements were carried out in D2O solution on a Avance Ⅲ HD-400 NMR spectrometer (Switzerland). Synthesis of Cu2-β-CD. Cu2-β-CD was synthesized by the method described by Matsui et al.23,24 15 mL of 0.04 M CuSO4⋅5H2O solution was added to a 10 mL solution containing 0.5 M NaOH and 0.02 M β-CD, and blue precipitate (Cu(OH)2) was formed immediately. The solution was stirred at room temperature (25 °C) for 1 hour, and then it was filtered to remove the Cu(OH)2 precipitate. 200 mL of ethanol was then added to this filtrate until a light blue suspension was observed, and it was filtered after 24 hours’ standing. The obtained blue solid (Cu2-β-CD) was washed with ethanol and deionized water, respectively, and then it


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was air-dried at room temperature. Preparation of Cu2-β-CD Self-Assembled P-L-Glu/GCE. According to the previous report in our group,20 poly(L-glutamic acid) (P-L-Glu) was electrodeposited onto the surface of GCE by cyclic voltammetry in 0.1 M phosphate buffer solution (PBS, pH 7.0) containing 0.05 M L-Glu in the potential range of –0.6 ~ +2.0 V (vs. SCE) at a scan rate of 100 mV s-1. The electrolysis was finished after 25 cycles, and the P-L-Glu modified GCE (P-L-Glu/GCE) was then immersed into 25 mL of 0.1 M PBS containing 5 mM Cu2-β-CD at pH 7.0 or 9.0 at 8 °C overnight for the self-assembly of Cu2-β-CD onto the surface of P-L-Glu/GCE. Recognition of Trp Isomers at Cu2-β-CD Based Chiral Sensor. Electrochemical recognition of Trp isomers was achieved by differential pulse voltammetry. The as-prepared Cu2-β-CD self-assembled P-L-Glu/GCE was immersed into a 25 mL of 0.5 mM L-Trp or D-Trp solution of various pH values (pH 3.0 ~ 9.0) for 120 s with a constant potential of –0.1

V applied at the Cu2-β-CD self-assembled electrode. Besides pH, the host-guest inclusion complexation was also conducted at different temperatures (5 ~ 50 °C). After the inclusion complexation was finished, the host-guest system (L-Trp-Cu2-β-CD or D-Trp-Cu2-β-CD) was transferred into a 25 mL of 0.1 M PBS of pH 8.0 immediately, and the electrochemical responses of the two inclusion complexes were recorded by differential pulse voltammetry at 25 °C in the potential range of +0.4 ~ +1.0 V (vs. SCE) at a scan rate of 100 mV s-1. Each measurement in differential pulse voltammetry was repeated in triplicate and the mean and standard deviation was calculated. After each measurement, the host-guest system was treated in 0.1 M PBS (pH 8.0) by repeated potential scanning till a steady cyclic voltammogram was obtained.



3. RESULTS AND DISCUSSION Characterization of Cu2-β-CD. The obtained Cu2-β-CD was characterized by Fourier transfer infrared (FT-IR) spectroscopy (Figure 2), ultraviolet-visible (UV/Vis) spectroscopy (Figure 3) and atomic absorption spectroscopy (AAS). The FT-IR spectra shows the vibration of HOH activated by coordination to Cu2+ at 877 cm-1,31 accompanied by the deterioration of the vibration at 580 cm-1 due to the formation of Cu-O bond on β-CD. UV/Vis spectra also shows the characteristic peaks of Cu2+ at 247 and 674 nm,25 and AAS reveals that the content of Cu in the Cu2-β-CD was 7.78% (wt.%). 400 MHz 1H NMR measurement of Cu2-β-CD was also conducted. δ: 4.91 (s, 7H, C(1)H), 3.81 (s, 7H, C(3)H), 3.72 (s, 14H, C(6)H2), 3.50 (s, 7H, C(5)H), 3.48 (m, 14H, C(2)H and C(4)H), 1.02 (t, 2H, Cu(OH)2Cu). The peak at δ 1.02 is from Cu(OH)2Cu, and the cumulative peak area ratio of C(2-6)H to Cu(OH)2Cu is 21.5:1, agreeing well with the theoretical value (21:1). These results clearly suggest that Cu2+ were coordinately attached to β-CD.

Cu2-ß-CD

Transmittance (%)

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

877 580

ß-CD

1000

580

900

800

700

600

-1

500

Wavenumber (cm )

400

Figure 2. FT-IR spectra of β-CD and Cu2-β-CD.


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Cu 2- ß -CD

1.0

Absorbance

0.02

1.5

Absorbance

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


Cu 2 - ß -C D 0.01

0.00

-0.01

ß -CD

600

650

700

750

W avelength (nm )

0.5 ß -CD

0.0 200

300

400

500

600

700

Wavelength (nm)

Figure 3. UV/Vis spectra of 0.1 mM β-CD and 0.7 mM Cu2-β-CD dissolved in water.

Self-Assembly of Cu2-β β-CD onto P-L-Glu. We examined the self-assembly of the Cu2-β-CD on P-L-Glu electrodeposited on GCE. In neutral environment (pH 7.0), H-bonds formed between the primary hydroxyl groups (–(HO)P) of β-CD and the carboxyl groups of P-L-Glu, which facilitated self-assembly of β-CD onto P-L-Glu.20 In alkaline environment (pH 9.0), however, the carboxyl group of P-L-Glu turned to its anionic resonance form, which was beneficial to the occurrence of electrostatic interactions, i.e., the self-assembly of the Cu2-β-CD on P-L-Glu occurred more easily via electrostatic interactions between Cu2+ and the carboxyl group of P-L-Glu. For Cu2-β-CD self-assembled via H-bonds, the Cu(OH)2Cu located at the wider opening of β-CD existed in the manner of a plane; however, for Cu2-β-CD self-assembled via electrostatic interactions, Cu(OH)2Cu presented an appearance of an outward convex due to the strong interactions between P-L-Glu and Cu(OH)2Cu (Figure 1). Compared with bare GCE, the peak current at the P-L-Glu modified GCE (P-L-Glu/GCE) increased slightly (Figure 4), indicating that P-L-Glu facilitated the electron transfer at the electrode-solution interface. However, the peak current at the Cu2-β-CD self-assembled onto P-L-Glu/GCE via electrostatic interactions decreased significantly due to the poor conductivity of the Cu2-β-CD, and the self-assembly via



H-bonds further decreased the peak current, demonstrating that H-bonds mode resulted in greater amount of the Cu2-β-CD self-assembled to P-L-Glu/GCE. 10 5

I (mA)

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0 -5 GCE P-L-Glu/GCE via electrostatic interactions via H-bonds

-10 -15 -0.2

0.0

0.2

0.4

0.6

E (V vs. SCE)

Figure 4. Cyclic voltammograms of 5 mM Fe(CN)64-/3- in 0.1 M KCl solution at bare GCE, P-L-Glu/GCE, Cu2-β-CD self-assembled onto P-L-Glu/GCE via electrostatic interactions and via H-bonds.

Chiral Recognition of D-Trp by Confining High-Energy Water Molecules Inside Cu2-β β -CD. Next, we evaluated the capability of the Cu2-β-CD based chiral sensor for recognition of Trp isomers according to the differential pulse voltammograms (DPVs). For the Cu2-β-CD self-assembled on bare GCE, DPVs showed only a small peak current ratio (ID-Trp/IL-Trp) of 1.67 between L- and D-Trp (Figure 5). This might be due to the fact that in the absence of P-L-Glu, the amount of the assembled Cu2-β-CD with inherent chirality was too small to provide sufficient recognition of Trp isomers. However, when the Cu2-β-CD was self-assembled on P-L-Glu/GCE via either electrostatic interactions or H-bonds, the peak current ratio (ID-Trp/IL-Trp) of DPVs between L- and D-Trp (Figure 6a, b) was increased to 2.45 and 3.28, respectively, indicating that the recognition capability was largely improved when the Cu2-β-CD self-assembled onto P-L-Glu via electrostatic interactions, and further enhanced when self-assembled via H-bonds. Interestingly, this affinity to D-Trp of Cu2-β-CD on P-L-Glu was in contrast to that to L-Trp of natural β-CD on P-L-Glu (IL-Trp/ID-Trp=2.30).20


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When natural β-CD is adopted for recognition of Trp isomers, indole groups of both L- and D-Trp can enter the cavity of β-CD from the wider opening for inclusion complexation, but

the position of –NH2 at the chiral center of L-Trp makes an easier complexation as compared to D-Trp (Figure 7). The presence of Cu2+ on natural β-CD apparently changed the affinity of the cavity to D-Trp instead of L-Trp. Unlike its usual role as ligand exchange agent in chiral recognition of amino acid isomers,32-35 here Cu2+ was used to form binuclear hydroxy-bridge at the wider opening of β-CD, acting as a cap of the β-CD cavity for a dual role in the discrimination of Trp isomers. On the one hand, it prevented high-energy water molecules (usually six to seven water molecules36) inside the Cu2-β-CD cavity from being released during complexation. On the other hand, it also blocked the entrance of Trp isomers into the cavity through the wider opening, thus forcing them to enter the cavity through the narrower opening (Figure 6c). When the indole groups of L- or D-Trp entered the cavity from the narrower opening, the Cu2-β-CD preferably included D-Trp due to favorability of H-bonds formation between the high-energy water molecules inside the cavity and the –NH2 at the chiral center of D-Trp compared with that of L-Trp (Figure 6d, e). By forming H-bonds with D-Trp, the energy of high-energy water molecules was also lowered, thus stabilizing the

high-energy water molecules within the cavity and preventing them from escaping the cavity. 3.0 D-Trp

2.5 2.0

I (µA)

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1.5 L-Trp

1.0 0.5 0.4

0.5

0.6

0.7

0.8

0.9

E (V vs. SCE)


1.0


Figure 5. Differential pulse voltammograms of 0.5 mM L-Trp and D-Trp bound to GCE modified directly with Cu2-β-CD, i.e., L-Trp-Cu2-β-CD and D-Trp-Cu2-β-CD in 0.1 M PBS (pH 8.0) at 25 °C.

2.5

(a)

D -Trp

4.0

(b)

D -Trp

2.0

I (µA)

3.2

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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1.5

2.4 1.6

1.0

L -Trp

0.5 0.4

0.5

0.6

L -Trp

0.8

0.7

0.8

0.9

0.3

1.0

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

E (V vs. SCE)

E (V vs. SCE)

(c) L- or D-Trp

L- or D-Trp

(OH)P

(e)

(d)

Figure 6. Differential pulse voltammograms of L- and D-Trp at Cu2-β-CD self-assembled on P-L-Glu via electrostatic interactions (a) and H-bonds (b). (c) Illustration of incorporation of L- and D-Trp into Cu2-β-CD self-assembled via electrostatic interactions (left) and H-bonds (right) through the narrower opening of the cavity. Schematic diagram showing the optimal orientation of L-Trp (d) and D-Trp (e) on the basis of the highest degree of H-bonds and inclusion complexation.


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(a)

(b)

Figure 7. Schematic diagram showing the optimal orientation of L-Trp (a) and D-Trp (b) in the hydrophobic cavity of natural β-CD based on the highest degree of H-bonds and inclusion complexation.

It is noteworthy that the peak currents of both L- and D-Trp at the Cu2-β-CD self-assembled via H-bonds were higher than those via electrostatic interactions. This might be because that more Cu2-β-CDs were self-assembled to the electrode surface via H-bonds than that via electrostatic interactions so that more L- and D-Trp could be included into the cavity of the Cu2-β-CD. Moreover, the current ratio (ID-Trp/IL-Trp) for the Cu2-β-CD self-assembled via H-bonds was greater than that via electrostatic interactions (3.28 vs. 2.45), indicating a greater recognition ability of the Cu2-β-CD self-assembled via H-bonds compared to that via electrostatic interactions. This phenomenon could be explained by the configuration difference of the Cu(OH)2Cu cap at the Cu2-β-CD self-assembled via the two different assembly modes. For the Cu2-β-CD self-assembled via H-bonds, strong H-bonds were formed inside the cavity of Cu2-β-CD blocked by planar Cu(OH)2Cu bridge; however, for the Cu2-β-CD self-assembled via electrostatic interactions, the outward convex of Cu(OH)2Cu located at the wider opening would enlarge the size of Cu2-β-CD cavity to some extent (Figure 1), resulting in the increase in the average distance between high-energy water molecules and D-Trp, and this might weaken the intermolecular H-bonds between



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high-energy water molecules and D-Trp molecules. As a result, the enantioselectivity toward Trp isomers was higher by the Cu2-β-CD self-assembled via H-bonds. Temperature-pH Dual Sensitivity of the Chiral Recognition. Perhaps more important is that we further discovered a temperature-pH dual sensitivity associated with the Cu2-β-CD based chiral sensor. In other words, the chiral recognition of the Cu2-β-CD was both temperature and pH dependent. For both of the Cu2-β-CD self-assembled via either H-bonds or electrostatic interactions, the chiral recognition of Trp isomers, as quantified by the peak current ratio of ID-Trp/IL-Trp obtained from the DPVs, appeared to be highly dependent on the temperature when measured between 5 and 50 °C (Figure 8a). It seemed that the highest recognition efficiency was obtained when measured at 30°C, from which either increasing or decreasing the temperature led to rapidly reducing the recognition efficiency. And another obvious peak was observed at 8 °C, however, it was lower than that at 30 °C. In addition, although the Cu2-β-CD self-assembled via both H-bonds and electrostatic interactions exhibited similar pattern of temperature dependence, the former was generally more efficient (i.e. greater ID-Trp/IL-Trp) than the latter. Because the recognition was achieved based on the differences in the H-bonds between the high-energy water molecules inside the cavity and Trp isomers, it could be concluded that temperature significantly influenced the strength of these H-bonds. At low temperature such as 5 °C, real water would maintain local molecular correlation and strongly confine its motion,37 so that H-bonds between the internal water molecules would predominate in the system and the H-bonds between D-Trp and internal water molecules would be suppressed seriously, which would lead to very low recognition efficiency (near zero). As temperature increased, the water-molecule interactions attendant


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would be decreased to cause gradual breakdown of H-bonds between water molecules but to increase the H-bonds between D-Trp and water molecules, thus resulting in increasing efficiency of chiral recognition. It turned out that at 8 °C a relatively high current ratio of 2.28 (ID-Trp/IL-Trp) was achieved, and it was very close to that in our previous work (2.30)20 although the affinity was opposite (Figure 8b). In ref. 10, the chiral recognition was attributed to the H-bonds between L-Trp and the secondary hydroxyl groups (–(HO)S) on β-CD rims and each L-Trp molecule can form one H-bond with –(HO)S at 8 °C. So, it could be concluded that only one stable H-bond between D-Trp molecule and high-energy water molecule was formed at 8 °C (Figure 8c, left). Interestingly, the highest recognition capability of the host was achieved at 30 °C, probably because the D-Trp molecule was additionally stabilized by forming two strong H-bonds with the high-energy water molecule at 30 °C (Figure 8c, right). However, further increase of temperature would break the two stable H-bonds, resulting in deterioration of the host-guest interactions and thus weakening the recognition capability. Here, it should be pointed out that the role of temperature in chiral recognition was not limited to its influence on H-bonds, but it might as well also influence the conformation, position, and orientation of the guest molecules.38



4

ID- / IL-Trp

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

(a )

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(c)

v ia H -b o n d s

3 2 v ia e le c tr o sta tic in te r a c tio n s

1 0

10 20 30 40 T e m p e r a tu r e ( o C )

50 o

o

30 C

8C

(b)

D-Trp

L-Trp

P-L-Glu

Cu2-β-CD

P-L-Glu

ID/IL = 2.28

β-CD

IL/ID = 2.30

Figure 8. (a) Peak current ratio of D-Trp to L-Trp included with Cu2-β-CD at different temperatures. (b) Chiral recognition of Trp isomers by Cu2-β-CD self-assembled on P-L-Glu (left) and β-CD self-assembled on P-L-Glu (right) at 8 °C. (c) Predicted one H-bond and two H-bonds patterns between D-Trp molecule and high-energy water molecule at 8 °C and 30 °C, respectively. Error bars in (a) represent standard deviation for three independent measurements.

The influence of pH on the recognition capability of the Cu2-β-CD based chiral sensor was also investigated (Figure 9a). It is evident that the peak current of included L-Trp was hardly changed when pH was changed over the range of 3.0 to 9.0. This insensitivity to pH for L-Trp was probably due to the steric hindrance of H-bonds formation between L-Trp and the

high-energy water molecules. In contrast, the recognition capability, or ID-Trp/IL-Trp changed remarkably when pH was changed over this range, which clearly indicated that the amount of included D-Trp varied significantly as pH was changed. Since the isoelectric point of Trp is 5.89, D-Trp should be positively charged at pH < 5.0. On the other hand, the high-energy water molecules inside the cavity were also protonated to be H+(H2O)n (n = 6 or 736).


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Therefore, the electrostatic repulsion between the positively charged D-Trp and protonated water molecules was stronger than the force of H-bonds between them (Figure 9b, left). As a result, the inclusion of D-Trp into the host was weaker than that of L-Trp, and so the peak current ratio was low (ID-Trp/IL-Trp < 1). When pH was increased from 5.0 to 8.0, H-bonds gradually became predominant in the host-guest system when D-Trp entered the cavity from the narrower opening (Figure 9b, middle), which eventually led to the highest affinity to D-Trp at pH 8.0 (ID-Trp/IL-Trp = 3.28). When pH was further increased from 8.0 to 9.0, D-Trp

became negatively charged and predominantly interacted with the positively charged Cu2+ in this system, resulting in aggregation of D-Trp outside the cavity instead of inclusion of D-Trp inside the cavity (Figure 9b, right). And the recognition efficiency was decreased greatly accordingly. 4

(a)

3

3

2

2

1

1

0

3

4

5

6 pH

7

8

9

IL-Trp ( µA)

4 ID- / IL-Trp

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


0

(b)

pH < 5.0

pH 5.0 ~ 8.0

pH > 8.0

Figure 9. (a) Peak current ratio and peak current of L-Trp included with Cu2-β-CD at different pHs. (b) Diagram showing the inclusion of D-Trp with Cu2-β-CD at different pHs. Error bars in (a) represent standard deviation for three independent measurements.



Complexation Thermodynamics During Chiral Recognition. To understand the complexation thermodynamics of the Cu2-β-CD in recognition of Trp isomers, we measured the absorbance of UV spectra for 0.2 mM D- and L-Trp dissolved in solution of the Cu2-β-CD at temperature of 30°C and concentration ranging from 0.1 to 0.7 mM. The results showed that with increasing concentration of the host, the characteristic absorbance of Trp at around 278 nm20,39 increased consistently (Figure 10), suggesting increasing dissolution of the guest molecules.40 4

(a)

0.2 mM D-Trp 0.2 mM D-Trp + 0.1 mM Cu2-β-CD

4

(b)

0.2 mM L-Trp 0.2 mM L-Trp + 0.1 mM Cu2-β-CD

0.2 mM L-Trp + 0.4 mM Cu2-β-CD

0.2 mM D-Trp + 0.4 mM Cu2-β-CD

3

Absorbance


Absorbance

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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2

1

0


3


2 1 0

250

275

300

325

250

350

Wavelength(nm)

275

300

325

350

Wavelength(nm)

Figure 10. UV spectra of 0.2 mM D-Trp (a) and L-Trp (b) upon addition of Cu2-β-CD of various concentrations at 30 °C (pH 8.0).

Employing a classical method proposed by Benesi and Hildebrand,41 the stoichiometry ratio and binding constant for the inclusion complexation of Cu2-β-CD and Trp isomers can be determined by the following equation:

1 1 1 = + A - A0 ∆ε × [CTrp ] (∆ε × [CTrp ] × K × [Cu 2 - β - CD]0n ) where A and A0 are the absorbance of Trp at each Cu2-β-CD concentration and in the absence of Cu2-β-CD, respectively. K is the binding constant for the host-guest system. [Cu 2 - β - CD]0n and [CTrp] are the original concentrations of Cu2-β-CD and Trp isomers, respectively, and n represents the stoichiometry ratio of the inclusion complex. ∆ε is the differential molar 18 ACS Paragon Plus Environment

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extinction coefficient of Trp in the absence and presence of Cu2-β-CD. Different values of n were tested to give the plots of 1/∆A versus 1/ [Cu 2 - β - CD]n0 . A straight line is achieved only when the value of n equals to the stoichiometry ratio, and the value of the binding constant K can be calculated from the double reciprocal plot by dividing the intercept by the slope of the obtained straight line. When a 1:1 inclusion complex of Trp-Cu2-β-CD isomers was formed, the UV absorbance and the concentration of the Cu2-β-CD were linearly related (Figure 11), from which the binding constant K was calculated by dividing the intercept by the slope of the straight line plot. Thus, the values of K at 30 °C and various pH values were obtained and listed in Table 1. The results indicate that due to favorability of intermolecular H-bonds formation between high-energy water molecules inside the Cu2-β-CD cavity and the amino groups of D-Trp compared with those of L-isomer at pH 8.0, the Cu2-β-CD exhibited a higher affinity to D-Trp than L-Trp (KD-Trp/KL-Trp = 2.09). However, the recognition efficiency via UV is still inferior to electrochemisty (2.09 vs. 3.28), indicating the proposed electrochemical approach can provide an efficient platform for recognition of Trp isomers. 7

10

(b)

(a)

6

8 5

6

1 / (A-A0)

1 / ( A-A0)

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


4 2 0

4 3 2 R2 = 0.9892

1

R2 = 0.9889

0 0

2000

4000

6000

8000

1 / [Cu2-β-CD]

0

10000

2000

4000

6000

8000

1 / [Cu2-β-CD]

10000

Figure 11. Double reciprocal plots of D-Trp (a) and L-Trp (b) included with Cu2-β-CD at 30 °C (pH 8.0).



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Table 1. Binding constant (K) for 1:1 inclusion complexation of D- and L-Trp with Cu2-β-CD at 30°C with various pH values. Guest

pH

t (°C)

6.0 7.0 8.0 9.0 6.0 7.0 8.0 9.0

L-Trp

D-Trp

30

30

K 399.0 376.5 276.7 170.8 506.8 545.1 579.0 255.7

Other important thermodynamic factors governing the host-guest system, including standard Gibbs energy change (∆Go), enthalpy change (∆Ho) and entropy change (∆So), were further determined by analyzing the binding constants obtained at different temperatures according to the van’t Hoff equation (Table 2).42 Despite the uncertainty of van’t Hoff treatment for analysis of experimental data,27 these data are still informative revealing the exothermic, spontaneous and entropy increment nature of the inclusion complexation. That the complexation of D-Trp with Cu2-β-CD gave more negative ∆Go than that of L-Trp also indicated that the complexation favored D-Trp. Table 2. Binding constant (K), standard Gibbs energy (∆Go), enthalpy (∆Ho) and entropy changes (T∆So) for 1:1 inclusion complexation of D- and L-Trp with Cu2-β-CD at various temperatures with pH 8.0. Guest

pH

L-Trp

8.0

D-Trp

8.0

t(°C)

K

8 30 40 8

442.3 276.7 247.0 497.3

30 40

579.0 486.5

∆Go (kJ mol-1)

∆Ho (kJ mol-1)

T∆So (kJ mol-1)

-14.17

-8.962

5.208

-16.03

-13.74

2.290

Limitation of the Chiral Recognition. One limitation of this chiral sensor was that the self-assembled Cu2-β-CD tended to detach from the surface after repeated use, and thus 20 ACS Paragon Plus Environment

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reducing its reproducibility. For example, the recognition capability was found to decrease from 3.28 to 1.67 after three successive measurements. However, this problem could be overcome by simply immersing the electrode into 5 mM Cu2-β-CD solution overnight to allow reassembly of the Cu2-β-CD onto P-L-Glu surface, which was found to restore the sensor to 98.2 % (3.22) of its initial recognition capability.

4. CONCLUSIONS To summarize, we demonstrated that the self-assembly of the Cu2-β-CD onto P-L-Glu via either H-bonds or electrostatic interactions, forming a chiral sensor that was preferable to D-Trp as well as sensitive to both temperature and pH. The function of the Cu2-β-CD was to

prevent the high-energy water molecules inside the cavity from being released during complexation, as well as to force Trp isomers to enter the cavity from its narrower opening. By doing so, Trp isomers were enantioselectively recognized due to favorability of H-bonds formation between the high-energy water molecules inside the cavity and the amino groups of D-Trp at optimal temperature and pH. Moreover, the results provide a direct proof for the previous report, i.e., β-CD has a higher affinity to D-isomers entered into the cavity from the narrower opening.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Telephone: +86-519-86330256. Fax: +86-519-86330167.

ACKNOWLEDGEMENT This work was supported by National Natural Science Foundation of China (21275023, 21173183), Natural Science Foundation of Jiangsu Province (BK2012593), Jiangsu Key



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Laboratory of Advanced Catalytic Materials and Technology (BM2012110) and Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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Chiral Recognition of d-Tryptophan by Confining High-Energy Water

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