A pair of chiral fluorescent sensors for enantioselective recognition of mandelate in water

Article abstract of DOI:10.1016/j.saa.2013.09.083

A pair of chiral compounds S-1 and R-1 derived from (1S, 2S) or (1R, 2R)-1, 2-diphenylethane-1, 2-diamine were designed and synthesized, the interactions of S-1 and R-1 with mandelate were studied in H₂O (0.01 M HEPES buffer, pH = 7.4) by fluorescence titration experiments. The sensors S-1 and R-1 were found to present enantioselective fluorescent sensing ability to mandelate. The results indicated that the sensors S-1 and R-1 were very promising to be used as fluorescent sensors in determining the enantiomeric composition of mandelate in H₂O.

Full text of DOI:10.1016/j.saa.2013.09.083

Accepted Manuscript
A pair of chiral fluorescent sensors for enantioselective recognition of mandelate
in water
Kuo-xi Xu, Hua-jie Kong, Fu-li Zu, Li Yang, Chen-juan Wang
PII:
S1386-1425(13)01090-1
http://dx.doi.org/10.1016/j.saa.2013.09.083
SAA 11062
DOI:
Reference:
To appear in:
Spectrochimica Acta Part A: Molecular and Biomo-
lecular Spectroscopy
Received Date:
Revised Date:
Accepted Date:
2 July 2013
12 September 2013
25 September 2013
Please cite this article as: K-x. Xu, H-j. Kong, F-l. Zu, L. Yang, C-j. Wang, A pair of chiral fluorescent sensors for
enantioselective recognition of mandelate in water, Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy (2013), doi: http://dx.doi.org/10.1016/j.saa.2013.09.083
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A pair of chiral fluorescent sensors for enantioselective
recognition of mandelate in water
*
Kuo-xi Xu , Hua-jie Kong, Fu-li Zu, Li Yang, Chen-juan Wang
Institute of Fine Chemical and Engineering, College of Chemistry and Chemical Engineering,
Henan University, Kaifeng, Henan, China
ABSTRACT: A pair of chiral compounds S-1 and R-1 derived from (1S, 2S) or (1R,
2
R)-1, 2-diphenylethane-1, 2-diamine were designed and synthesized, the interactions
of S-1 and R-1 with mandelate were studied in H O (0.01 M HEPES buffer, pH=7.4)
2
by fluorescence titration experiments. The sensors S-1 and R-1 were found to present
enantioselective fluorescent sensing ability to mandelate. The results indicated that
the sensors S-1 and R-1 were very promising to be used as fluorescent sensors in
determining the enantiomeric composition of mandelate in H
2
O.
Keywords: Enantioselective recognition, Fluorescent sensor, Mandelate.
1
. Introduction
Molecular recognition is a fundamental property of various natural systems, based
on the ability of a molecular receptor to form a complex preferentially with one of the
enantiomers of a chiral molecule by noncovalent interaction such as hydrogen
bonding, electrostatic interaction, and hydrophobic interaction [1-4]. Molecular
recognition in protic solvents, especially in aqueous media, is another attractive topic
since noncovalent forces derived from electrostatic interaction are weakened
according to the dielectric constant of the solvent. In addition, the designed formation
of hydrogen bonds between two molecules is especially difficult in protic solvent
since a large number of solvent molecules can act as a hydrogen donor and/or
acceptor [5-7]. Hydrogen bonding, capable of showing a high degree of
complementarity and directionality, is frequently used for chiral recognition. During
the past several years, there has been a growing interest in the enantioselective
recognition of α-hydroxycarboxylic acid [8-13], especially using fluorescence
spectroscopy, owing to the basic building blocks of many natural products, biological
*
Corresponding author. Tel.: + 86 378 3881589; fax: + 86 378 3881589. E-mail address: E-mail:
xukx@henu.edu.cn
1

molecules and drugs. Therefore, the development of enantioselective fluorescent
sensors for structurally diverse α-hydroxycarboxylic acids has become a major
challenge in this area. Several fluorescent sensors, including those developed within
our group, have been reported to show high enantioselectivity in the recognition of
α-hydroxycarboxylic acids [14-15]. However, the enantioselective recognition of the
chiral α-hydroxycarboxylic acid in aqueous media is poor. Here we report
enantioselective recognition mandelate based on fluorescent quenching mechanism in
water. The molecular structures of two synthesized fluorescent sensors were shown in
Scheme 1.
2
. Experimental
2.1 Apparatus
The reagents used were of commercial origin and were employed without further
purification. Purifications by column chromatography were carried out over silica gel
230–400 mesh). Melting point was determined with a MEL-TEMP melting-piont
apparatus (uncorrected). The IR spectra were performed on a Nicolet 670 FT-IR
(
1
13
spectrophotometer. H NMR and C NMR spectra were recorded on a Bruker
AV-400 spectrometer. High resolution mass spectra (HRMS) were measured on a
Agilent 1290LC-6540 Accurate Mass Q-TOF using electrospray ionization
(
ESI).Optical rotations were taken on a Perkin–Elmer Model 341 polarimeter.
Fluorescence spectra were obtained with F-7000 FL Spectrophotometer. Elemental
analyses were performed with the Vario Elemental CHSN-O microanalyzer. The
anions were used as their tetrabutylammonium salts. All other commercially available
reagents were used without further purification.
2.2 Synthesis
Synthesis of S-2: (1S, 2S)-1, 2-diphenylethane-1, 2-diamine (1.0 g, 4.7 mmol) was
dissolved in 30 mL of dried chloroform and cooled in the ice bath. 1.4 g (10 mmol)
anhydrous K CO was placed in the solution as the base. 2-Chloroacetyl chloride (1.6
2
3
g, 14.1 mmol) was dropwisely added to the mixture within 30 min. The mixture was
2

then stirred at room temperature for 3 h. After filtration, the filtrate was collected and
washed with 20 mL 10% HCl, saturated NaHCO and water respectively. The organic
layer was dried over anhydrous Na SO and the solvent was evaporated under reduced
3
2
4
pressure. The residue was then purified by column chromatography on silica gel using
1
CHCl
NMR (CDCl
br.s, 4H, Ph-H), 5.31 (d, 2H, J = 3.6 Hz, chiral-H), 4.07 (m, 4H, CH
analysis calcd (%) for C18 : C 59.19, H 4.97, N 7.67, Found: C 58.96, H
.02, N 7.63. The preparation procedure of compound R-2 was the same as that of S-2
3
/MeOH (50:1) as eluent to obtain pure products a as a solid (1.20 g, 70%). H
3
, 400 MHz) δ (ppm): 7.48 (s, 2H, CONH), 7.21 (br.s, 6H, Ph-H), 7.11
Cl). Elemental
(
2
2 2 2
H18Cl N O
5
by starting with (1R, 2R)-1, 2-diphenylethane-1, 2-diamine.
Synthesis of S-1. The synthesis procedure of compound S-1: 0.18 g（0.5 mmol）
compound S-2 and 0.28 g（2.0 mmol）anhydrous K
2
CO were dissolved in 6 mL dry
3
DMF firstly, then a solution of 0.15 g（1.1 mmol）α-naphthol dissolved in 4 mL dry
DMF was added dropwisely to the mixture. After that, the solution was stirred at
6
0 for 8 h, monitoring with TLC. When completed, the mixture was poured into
ice-water, adjusted to weak acid by using 10％ dilute HCl, extracted by using CH Cl
respectively. The organic phase was washed with saturated NaHCO (3×30 mL),
water (2×50 mL) before drying it with anhydrous Na SO . After removing the solvent
by rotary evaporation, the crude product was purified by silica column
2
2
3
2
4
chromatography using CH
2
Cl
2
/MeOH (50:1) as eluent to get 0.22 g product as a white
1
powder. Yield: 76.4%, m.p. 217-219 . H NMR (CDCl
3
, 400 MHz) δ(ppm): 8.25 (s,
1
H), 8.23 (s, 1H), 7.83 (d, J = 5.6 Hz, 2H), 7.64 (d, J = 4.8 Hz, 2H), 7.56 (d, J = 5.6
Hz, 2H), 7.53 (d, J =6.0Hz, 2H), 7.43 (d, J = 8.4 Hz, 2H), 7.27 (d, J = 8.0 Hz, 2H),
7
1
1
1
.19-7.13(m, 6H), 7.04-7.01(m, 4H), 6.67 (d, J = 7.6 Hz, 2H), 5.39 (s, 1H), 5.37 (s,
13
H), 4.54 (s, 2H), 4.44 (s, 2H); C NMR (CDCl
28.67, 128.13, 127.73, 127.53, 126.81, 125.87, 125.64, 125.18, 121.86, 121.65,
05.51, 67.51, 58.37. HRMS m/z: 580.2362, found (calculated for C38
3
) δ: 168.74, 152.87, 137.80, 134.57,
H
32
N
2
O
4
,
+
+
-1
[
M+H] requires 581.2439，[M+Na] requires 603.2259). IR: NH(3313.89 cm ), C=O
3

-
1
-1
-1
-1
-1
(
(
1668.22 cm ), Ar (1649.87 cm , 1597.66 cm , 1580.01 cm , 1528.03 cm ), -O-
-1
-1
20
D
1231.13 cm , 1106.06 cm ). Specific rotation, [α] = -101.09 (c=0.13, CHCl
3
).
Synthesis of R-1. The compound was prepared using a procedure analogous to that
described previously for S-1. Yield: 74.7%, m.p. 218-220 ℃. HRMS m/z: 580.2362,
+
+
found (calculated for C38
H
32
N
2
O , [M+H] requires 581.2437, [M+Na] requires
4
2
D
0
6
03.2260). Specific rotation [α] = +99.31(c=0.13，CHCl
3
).
Scheme 1. Synthetic Route for the compound S-1
2.3 Binding Experiments
General procedure for the fluorescent measurement: All solutions were prepared by
volumetric syringes, pipettes, and volumetric flasks. A DMSO stock solution of the
sensor (0.1 M) was freshly prepared before each measurement. The
tetrabutylammonium salts were prepared by adding 1 equiv. of tetrabutylammonium
hydroxide in methanol to a solution of the corresponding carboxylic acid in methanol.
The resulting syrup was dried under high vacuum for 24 h, analyzed by NMR
spectroscopy, and stored in a desiccator, then the stock solutions of the salts were
2
prepared in HEPES-buffered H O (0.01 M, pH=7.4). The test solutions were prepared
by adding different volumes of anion solution to a series of test tubes and then the
same amount of stock solution of the sensor was added to each of the test tubes and
2
diluted to 3.0 mL with HEPES-buffered H O. After being shaken for several minutes,
the test solutions were analyzed immediately.
3
. Results and discussion
3.1 Design and synthesis of the new chiral sensors
The chiral fluorescence sensor S-1 was efficiently synthesized using (1S, 2S)-1,
2
-diphenyl-ethane-1, 2-diamine. The preparation procedure of compound R-1 was the
4

same as that of S-1 by starting with (1R, 2R)-1, 2-diphenyl-ethane-1, 2-diamine
1
(
Scheme 1). The structures of these compounds were characterized by IR, HRMS, H
1
3
NMR and C NMR spectra. Compounds S-1 and R-1 containing oxygen atoms
adjacent to the naphthyl rings, as we have shown earlier, interaction of these oxygen
atoms with hydroxyl of α-hydroxycarboxylic acids could turn off the fluorescence of
the naphthalenes by enhancing the fluorescence quenching of the
photoinduced-electron-transfer (PET)[16-17] of the oxygen lone pair electrons. The
multiple chiral functional groups of the sensor also provided efficient binding sites for
functional organic molecules.
3.2 Fluorescence spectra study
Because there was almost no change observed on the UV–vis spectra of sensors
upon addition of the anions (see Fig.s1), the interaction between sensor and anion was
only evaluated by fluorescent spectra. Fluorescent spectra were recorded from a
solution of S-1 in the presence of various enantiomers α-hydroxylcarboxylic acid
anions, such as D- or L-mandelate (Man), phenylactic acid, methoxymandelic acid
2
and alanine anion (as tetrabutylammonium) in H O (including 0.3％DMSO, 0.01 M
HEPES-HCl buffer, pH=7.4) (by means of titration fluorimetry).
Fig. 1 (A): Fluorescent spectra of sensor S-1 (4.0×10-5 M) with D-alanine anion in buffered H2O
(
including 0.3％DMSO, 0.01 M HEPES buffer, pH=7.4). Equivalents of anion: 0→267. λex=245
nm (EX: 5; EM: 5). (B): changes in the fluorescence intensity of S-1 at 388 nm upon addition of
D-alanine anion. The line shown is a line-fitted curve. The correlation coefficient (R) of the
non-linear curve fitting is 0.9996.
First we studied on using the sensors for the fluorescent recognition of alanine
5

anion. The primary amine group in amino acid could quench the fluorescence of the
fluorophores. The fluorescence measurements were conducted in buffered water
solutions with the addition of a small amount of DMSO in order to increase the
solubility of S-1. The fluorescence emission of S-1 (λex = 245 nm; λem = 388 nm)
decreased gradually with the addition of D-alanine anion (Fig.1). Similar phenomena
were found when L-alanine anion was added into the solution of S-1 (see Fig.s2),
which indicated complex between the sensor S-1 and L- or D-alanine.
TABLE 1. Association constants (Kass) of sensor S-1 and R-1 with D- or L-α-carboxylic acid
anions in water.
–
1 [a,b]
Entry
Host
S-1
S-1
S-1
S-1
S-1
S-1
S-1
S-1
R-1
R-1
R-1
R-1
R-1
R-1
R-1
R-1
Guest
D-alanine
L-alanine
D-Man
Kass[M ]
K
(d)/K(l)
1.27
3.7
R
3
3
3
3
3
3
2
3
3
3
3
3
3
3
2
3
1
2
3
4
5
6
7
8
9
(3.31±0.11)×10
(2.61±0.18)×10
(4.85±0.23)×10
(1.31±0.09)×10
(3.00±0.24)×10
(1.49±0.16)×10
0.9996
0.998
0.9991
0.9989
0.9978
0.9974
0.999
L-Man
D-phenylactic
L-phenylactic
2.01
0.65
1.14
0.26
0.53
0.7
D-methoxyphenylacetic (7.71±0.60)×10
L-methoxyphenylacetic (1.19±0.10)×10
0.9991
0.9976
0.998
D-alanine
L-alanine
(3.92±0.29)×10
(3.43±0.23)×10
(1.30±0.15)×10
(4.96±0.36)×10
(1.58±0.12)×10
(2.96±0.16)×10
10
11
12
13
14
15
16
D-Man
0.9976
0.9979
0.9984
0.9989
0.9996
0.9973
L-Man
D-phenylactic
L-phenylactic
D-Methoxyphenylacetic (7.87±0.46)×10
L-methoxyphenylacetic (1.13±0.12)×10
[
a]The data were calculated from results of fluorescence titrations in a aqueous solution
including 0.3％DMSO, 0.01 M HEPES buffer, pH 7.4).
b]All error values were obtained from nonlinear curve fitting.
(
[
On the basis of the change of fluorescent intensity associated with the stepwise
addition of guest anions, the complex association constants (K ) compiled in Table 1,
a
were calculated by nonlinear least-squares curve fitting using the following equation
from the Origin 7.5 software package [18]. Where I represents the fluorescence
6

H G 0
intensity, C and C were the host and guest concentrations, and C was the initial
concentration of the sensor.
2
1/2
I = I
0
+ (Ilim - I
0
) / 2C
0
{C
H
+ C
G
+ 1 / Kass - [(C
H
+ C
G
+ 1/ Kass ) - 4 C
H
C
G
] }
The fitting (R>0.99) of the titration data points shown in Figures (Fig.1, Fig.s2)
demonstrated that S-1 formed 1:1 complexes with alanine anion. The K of S-1 with
a
3
-1
3
-1
D- and L-alanine anion were (3.92±0.29)×10 M and (3.43±0.23)×10 M ,
respectively. Similar phenomena were observed when D- or L-alanine anion was
added into a solution of R-1(see Fig.s3, Fig.s4). Their fluorescent responses were
indicative of poor enantioselectivity (Table 1). We believed that the hydrogen in
primary amine of alanine anion participated in the formation of hydrogen bond with
oxygen of the sensor which resulted the recognition.
Then we studied the enantioselectivity of S-1 with D- or L-Man. Upon addition of
D- or L-Man anions, different fluorescent-quenching degrees of S-1 were observed.
Fig. 2 showed the different fluorescence intensity changes when the same equivalent
of D- or L-Man anion was added to the sensor S-1. The quenching efficiency was
4
3% when 19.8 equiv. of D-Man anion was added to the solution of S-1, while the
quenching efficiency was only 30% when 19.8 equiv. of L-Man anion was added. The
quenching efficiencies (ΔI /ΔI = 3.12) indicated that the sensor S-1 had a good
D
L
enantioselective recognition ability between the D- and L-Man anions. The quenching
efficiency of D-Man anion was much higher than the L-Man anion.
Fig.2 Fluorescent spectra of sensor S-1with 19.8 equiv of D- and L-Man anions in H
2
O (including
0.3％DMSO, 0.01 M HEPES buffer, pH 7.4)
7

Fig. 3(A) and 4(A) showed the fluorescence emission spectra of a mixture of S-1
and different concentrations of the D- or L-Man anion in H O, respectively. The Fig.
(B) and 4(B) illustrated the fluorescence intensity changes of receptor S-1 upon
2
3
addition of D- and L-Man anions, which gave Ka(D)/Ka(L) 3.70 for Man. We also
prepared R-1, the enantiomer of S-1, and studied its interaction with Man anion,
which showed the opposite enantioselectivity, that was, the enantiomer of L-Man
quenched the fluorescence of R-1 more efficiently than the D-Man (Fig.s5, Fig.s6).
The results of fluorescence titrations experiments indicated that the expected mirror
image responses for the enantiomers of Man anions. We believed the sensors both S-1
and R-1 had excellent enantioselective recognition ability due to their good
preorganized structures, which can be easily combined the sensors with Man anions
through hydrogen bonds. Moreover, we demonstrated the formation of a 1:1 inclusion
complex between sensor S-1 or R-1 and Man by ESI-MS studies. When Man were
added to the sensor S-1 or R-1, the ESI-MS results provided strong evidence for the
formation of the 1:1 complex of the sensor S-1 or R-1 and Man acid anion (Fig.s7,
Fig.s8). The results suggested that the S-1 and R-1 formed a 1:1 complex with D- or
L-Man.
–5
–1
Fig. 3 (A) Fluorescence spectra of sensor S-1 (4.0×10 mol L ) with D-Man in H
.3％DMSO, 0.01 M HEPES buffer, pH 7.4). The anion equivalents are: 0→115. λex= 245nm.
B)The fluorescence intensity changes in the S-1 at 388 nm upon addition of D-Man. The line
2
O (including
0
(
shown is a line-fitted curve. The correlation coefficient (R) of the nonlinear curve fitting is 0.9991.
8

–5
–1
Fig. 4 (A) Fluorescence spectra of sensor S-1 (4.0×10 mol L ) with L-Man in H
.3％DMSO, 0.01 M HEPES buffer, pH 7.4). The Man equivalents are: 0→82. λex = 245nm.
B)The fluorescence intensity changes in the S-1 at 388 nm upon addition of D-Man. The line
2
O (including
0
(
shown is a line-fitted curve. The correlation coefficient (R) of the nonlinear curve fitting is 0.9989.
To study the effect on the structure of the guests, the binding properties of
phenylactic acid anion and methoxyphenylacetic acid anion towards S-1 and R-1
were also studied using the same fluorescent titration method (Fig.s9-Fig.s16). As
shown in Table 1, the sensors S-1 and R-1 showed more enantiomeric recognition
abilities towards the enantiomers of Man than phenylactic acid anions. The possible
reason was that the guest phenylactic acid anions had a more methene group on their
stereogenic centre moiety. This demonstrated that the steric hindrance around the
stereogenic centers in the ligand played an important role on the degree of
enantiomeric recognition. It was also observed that the stability constants of the
complexes of S-1 and R-1 with methoxyphenylacetic acid anions were smaller than
what with Man, which could be attributed to the complex of the chiral sensors S-1 and
R-1 with Man possibly occurs through interaction of the oxygen atoms in the sensors
S-1 and R-1 with the hydroxyl of Man. So the steric hindrance around the stereogenic
centers and the hydroxyl of the guests may also play important roles in
enantioselectivity recognition in the tested cases.
The large fluorescence decreasing could be attributed to the photoinduced electron
transfer (PET) mechanism. In the absence of anions, PET between the naphthyl unit
and the oxygen substituents might result in quenched fluorescence. In the presence of
guest anions, the fluorescence quenching of sensor S-1 most likely arose from the
9

change of the free energy (ΔGPET) of the electron transfer between the excited
fluorophore and the sensor [19-23]. Therefore, fluorescence quenching was observed.
The phenomenon of fluorescence intensity decreasing upon the addition of guest
anion was similar to the anion-induced fluorescence quenched reported previously.
4
. Conclusion
We have demonstrated that the easily accessible chiral fluorescence sensor S-1 and
R-1 exhibited excellent enantioselective fluorescent recognition abilities toward Man
anions and formed 1:1 complexes. It was clear that the nature of the receptor, good
structural preorganization, multiple hydrogen-bonding interactions and the
complementary stereogenic center interactions induced might be responsible for the
enantiomeric recognition of Man anion. To our knowledge, it is the first good
enantioselective fluorescent sensor for mandelic acid anion in water. Sensors S-1 and
R-1 are promising in their use as fluorescence sensors for Man anions in water.
Acknowledgments
We thank China Post doctoral Science Foundation (2011M501177) and Henan
provincial department of technology (132300410055) for financial support.
Supplementary data
1
13
The H and C NMR spectra of R-1 and S-1, the fluorometric titration experiment,
ESI-MS studies with this article can be found, in the online version, at http://
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1
1

Highlights
The developed a pair of chiral fluorescent sensors selectively enantioselective detects mandelate in
water.
In the binding with mandelate, the large fluorescence decrease can be attributed to the PET mechanism.
The chemosensors exhibited selectively enantioselective for mandelate in water (0.01 M HEPES,
buffer, pH=7.4).
1
2

Graphical abstract
The sensors S-1 and R-1 were found to present enantioselective fluorescent sensing
ability to mandelate in H O.
2
1
3