Novel enantioselective fluorescent sensors for malate anion based on acridine

Article abstract of DOI:10.1016/j.dyepig.2014.05.017

The compounds L-1 and L-2 were synthesized and the interactions of all of the compounds with malate anion were studied by fluorescent titration and ¹H NMR experiments. The Sensors L-1 and L-2 were found to present good enantioselective fluorescent sensing ability to malate anion. The results indicated that sensors L-1 and L-2 were very promising to be used as fluorescent sensors in determining malate anion in CH₃CN.

Full text of DOI:10.1016/j.dyepig.2014.05.017

Dyes and Pigments 109 (2014) 169e174
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Novel enantioselective fluorescent sensors for malate anion based on
acridine
,
, **
Qian Li a, Kuoxi Xu a *, Pan Song a, Yanpeng Dai a, Li Yang a, Xiaobin Pang b
a Institute of Fine Chemical and Engineering, College of Chemistry and Chemical Engineering, Henan University, Kaifeng, Henan 475004, PR China
b Institute of Pharmacy, Henan University, Kaifeng, Henan 475004, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
The compounds L-1 and L-2 were synthesized and the interactions of all of the compounds with malate
anion were studied by fluorescent titration and 1H NMR experiments. The Sensors L-1 and L-2 were
found to present good enantioselective fluorescent sensing ability to malate anion. The results indicated
that sensors L-1 and L-2 were very promising to be used as fluorescent sensors in determining malate
anion in CH3CN.
Received 25 February 2014
Received in revised form
3 May 2014
Accepted 12 May 2014
Available online 21 May 2014
© 2014 Elsevier Ltd. All rights reserved.
Keywords:
Enantioselective recognition
Fluorescent sensor
Malate anion
Fluorescence titration
Acridine
PET
1. Introduction
enantioselective recognition of fluorescent MA sensors developing
from acridine.
During the past two decades, considerable attention has been
focused on the design of chiral substrates that can recognize and
sense chiral anions selectively due to the important roles and po-
tential applications anions play in environmental, biological and
supramolecular sciences [1e5]. Among these chiral anions, malate
anion (MA) plays significant role in pharmaceutics. D-Malate can
only be found if the synthetic racemate is used as food additive. L-
Malate is used in the treatment of light-damaged or dry skin, acne,
and especially fibromyalgia when combined with magnesium. L-
Malate is also used to treat atherosclerosis [6,7]. Therefore, the
practical, rapid and accurate methods applied in enantioselective
recognition of D- and L-malate are an important analytical agenda.
Up to now, many analytical methods such as nuclear magnetic
resonance (NMR), high-performance liquid chromatography, cir-
cular dichroism have been developed for chiral anions determina-
tion, fluorescent sensors present many appealing advantages,
including high sensitivity and selectivity, low cost, easy detection,
and especially suitability as a diagnostic tool for biological concern
[8e14]. To the best of our knowledge, few chiral fluorescent sensors
have been reported up till now for the enantioselective recognition
of MA in CH3CN. In this paper, we reported the synthesis and
2. Experimental section
2.1. Materials
The reagents were used of commercial origin and were
employed without further purification. Purifications by column
chromatography were carried out over silica gel (230e400 mesh).
The IR spectra were performed on a Nicolet 670 FT-IR spectro-
photometer. 1H NMR and 13C NMR spectra were recorded on a
Bruker AV-400 spectrometer. Mass spectra were determined by ESI
recorded on an Esquire 3000 LCeMS mass instrument. Optical ro-
tations were taken on a PerkineElmer Model 341 polarimeter.
Elemental analyses were performed by the Vario Elemental CHSN-
O microanalyzer. Fluorescence spectra were obtained with an F-
7000 FL Spectrophotometer. The anions were used as their tetra-
butylammonium salts. 4,5-Bis(bromomethyl)acridine was pre-
pared according to the literature methods [15,16].
2.2. Syntheses
2.2.1. Syntheses of Boc-amino alcohol
(Boc)2O (2.10 g, 12 mmol) was added to a solution of amino
alcohol (10 mmol) and diisopropylethylamine (DIPEA) (1.44 g,
* Corresponding author. Tel./fax: þ86 371 23881589.
** Corresponding author.
E-mail addresses: xukx@henu.edu.cn, xkuox@163.com (K. Xu).
http://dx.doi.org/10.1016/j.dyepig.2014.05.017
0143-7208/© 2014 Elsevier Ltd. All rights reserved.

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Q. Li et al. / Dyes and Pigments 109 (2014) 169e174
Fig. 1. Fluorescent spectra changes of sensor L-1 (3.0 ꢃ 10ꢂ5 M) measured in CH3CN
upon the addition of 100 equivalent of various anions (lex ¼ 356 nm, lem ¼ 423 nm).
Fluorescent enhancement ((I ꢂ I0)/I0) of sensor L-1 (3.0 ꢃ 10ꢂ5 M) at 423 nm upon the
addition of different carboxylate anions (as tetrabutylammonium) in CH3CN at 25 ꢀC
(100 equiv, 3.0 ꢃ 10ꢂ3 M). A ¼ D-MA, B ¼ L-MA, C ¼ L-mandelic acid anion, D ¼ D-
mandelic acid anion, E ¼ L-phenyl-lactic acid anion, F ¼ D-phenyllactic acid anion,
Scheme 1. Synthesis of compounds L-1, D-1 and L-2.
12 mmol) in THF(40 mL) under N2 protection at 0 ꢀC. The mixture
was stirred for 6 h at room temperature, then the solvent was
removed under reduced pressure and the resulting residue was
dissolved in 100 mL ethyl acetate. This solution was washed twice
with 100 mL of water, once with 100 mL of saturated aqueous so-
dium chloride, dried over Na2SO4 and concentrated under reduced
pressure to give product L-3 or L-4 as yellow oil.
G
¼
L-methoxyphenylacetic acid anion,
H
¼
D-methoxyphenylacetic acid anion,
I ¼ Ltartaric acid anion, J ¼ D-tartaric acid anion, K ¼ L-dibenzoyltartaric acid anion,
L ¼ D-dibenzoyltartaric acid anion, M ¼ L-phenylglycine anion, N ¼ D-phenylglycine
anion, O ¼ L-phenylalanine anion, P ¼ D-phenylalanine anion.
3. Results and discussion
L-3: 1.70 g, yield 97%; ½aꢁ2D0 ¼ ꢂ8.4 (c 0.05, CHCl3); 1H
3.1. Synthesis
NMR(CDCl3):
d 4.71 (s, 1H), 3.69e3.60 (m, 1H), 3.57e3.42 (m, 2H),
2.21 (s, 1H), 1.36 (s, 9H), 1.12 (d, J ¼ 6.4 Hz, 3H).
L-4: 2.38 g, yield 95%; ½aꢁ2D0 ¼ ꢂ20.8 (c 0.05, CHCl3); 1H
The chiral fluorescence sensors L-1 and L-2 were efficiently
synthesized by the reaction of intermediate D-N-Boc-amino alcohol
L-3 or L-4 and 4,5-Bis(bromomethyl)acridine (Scheme 1). The
preparation procedure of compound D-1, the enantiomers of L-1,
was the same as that of L-1 by starting with D-N-Boc-amino alcohol
and 4,5-Bis(bromomethyl)acridine. The 1H NMR spectra exhibited
all the expected signals with the desired integral values and sup-
port the molecular structures. The structures of these compounds
were characterized by IR, MS, 1H NMR and 13C NMR spectra. We
chose these compounds to undertake the desired fluorescent
recognition of MA for the following two reasons: on one hand, the
oxygen atoms of the compounds could bind eOH of guest MA well
through multiple hydrogen bonds. On the other hand, when the
sensors interact with MA, their oxygen atoms were expected to
turn on the fluorescence of the sensors by inhibiting the
photoinduced-electron-transfer (PET) [17e19] of the oxygen atoms.
NMR(CDCl3):
d 7.36e7.21 (m, 5H), 4.71 (s, 1H), 3.87 (s, 1H),
3.66e3.55 (m, 2H), 2.84 (d, J ¼ 7.8 Hz, 2H), 2.23 (s, 1H), 1.41 (s, 9H).
2.2.2. General procedure for the preparation of compounds L-1, D-1
and L-2
To a solution of 4,5-bis(bromomethyl)acridine (0.37 g,1.0 mmol)
in dichloromethane (10 mL), 20% NaOH (10 mL), tetrabutylammo-
nium iodide (0.81 g, 2.2 mmol) and N-Boc-amino alcohol
(2.2 mmol) were added consecutively. The biphasic mixture was
then stirred at room temperature for 15 h and monitored via TLC,
after which it was poured into a separatory funnel over water and
extracted with dichloromethane. The combined organic extracts
were rinsed with brine, dried over anhydrous Na2SO4 and purified
by means of flash chromatography over silica gel using petroleum
ether: ethyl acetate (10:1) as eluent to obtain pure product L-1, D-1
and L-2 as yellow solid, respectively.
L-1: 0.23 g, m.p.: 132e133 ꢀC, yield, 42.2%. ½aꢁ2D0 ¼ ꢂ12.6
(c ¼ 0.20, CHCl3); D-1: The preparation procedure the same as that
of L-1 with the use of R-N-Boc-amine alcohol as the materials.
0.24 g, m.p.:135e136 ꢀC, yield, 42.4%, ½aꢁ2D0 ¼ þ12.2 (c ¼ 0.20,
CHCl3); 1H NMR (CDCl3) :
d
8.75 (s, 1H), 7.93(d, J ¼ 8.4 Hz, 2H),
7.88(d, J ¼ 7.6 Hz, 2H), 7.55(t, J ¼ 7.6 Hz, 2H), 5.38(s, 4H), 4.89(s, 2H),
3.98(s, 2H), 3.76e3.68(m, 4H), 1.43 (s, 18H), 1.29(d, J ¼ 6.8 Hz, 6H);
13C NMR (CDCl3): 155.49, 145.95, 136.84, 136.03, 127.40, 127.24,
125.56, 74.44, 69.49, 46.53, 28.45, 18.28, IR (KBr):3373, 1691, 1521,
1253, 1128, 758 cmꢂ1; HRMS m/z: calculated for C31H43N3O6,
[MþH]þ 554.3225, found 554.3229.
L-2: 0.37 g, m.p.: 141e142 ꢀC, yield, 52.8%. ½aꢁ2D0 ¼ ꢂ15.6
(c ¼ 0.20, CHCl3); 1H NMR (CDCl3):
8.78 (s, 1H), 7.95(d, J ¼ 8.4 Hz,
d
2H), 7.89(d, J ¼ 6.8 Hz, 2H), 7.57(t, J ¼ 7.6 Hz, 2H), 7.24e7.17(m,10H),
5.32(d, J ¼ 5.8 Hz, 4H), 5.03(s, 2H), 4.04(s, 2H), 3.66e3.40(m, 4H),
2.98(d, J ¼ 4.0 Hz, 4H), 1.41 (s, 18H); 13C NMR (CDCl3): 155.44,
146.00, 138.35 136.84, 136.09 129.52, 128.39, 127.62, 126.30, 126.23,
125.61, 71.04, 69.46, 52.09, 51.98, 38.12, 38.09, 28.44; IR (KBr):
3365, 1689, 1522, 1390, 1365, 1249, 1170, 757, 700 cmꢂ1; HRMS m/z:
calculated for C43H51N3O6, [MþH]þ 706.3851, found 706.3856.
Fig. 2. Fluorescent spectra of L-1 (3.0 ꢃ 10ꢂ5 M) with 100 equiv. of D- and L-MA in
CH3CN.

Q. Li et al. / Dyes and Pigments 109 (2014) 169e174
171
Fig. 3. (A) Fluorescent intensity changes of sensor L-1 (3.0 ꢃ 10ꢂ5 M) with D-MA in CH3CN. The D-MA equivalents are: 0 / 100. (B) The fluorescent intensity changes of sensor L-1
with L-MA .The L-MA equivalents are: 0 / 100.
3.2. Fluorescence spectroscopic studies
Because there was almost no change on the UVevis spectra of
sensor L-1 or L-2 upon the addition of carboxylate anions, such as
D- or L-phenylalanine, mandelate, methoxyphenylacetic acid,
phenyl-lactic acid, tartaric acid, dibenzoyltartaric acid, MA (as tet-
rabutylammonium salts) in CH3CN, even when the guests were
excessive ([guest]/[ L-1 or L-2] ¼ 100, see Supplementary Data
Fig. s1). The properties of enantioselective recognition of sensor
L-1 or L-2 were only investigated for D- or L-MA by the fluorescence
spectra.
When the solution of sensor L-1 was excited at 356 nm, it gave a
characteristic emission spectrum with monomeric acridine
maximum at ca 423 nm in CH3CN. Binding behaviors of sensor L-1
with different carboxylate anion in CH3CN were investigated. Fig. 1
showed the changes of emission spectra of L-1 (3.0 ꢃ 10ꢂ5 M) upon
the addition of various carboxylate (see Supplementary Data,
Fig. s2), only MA induced an increasing intensity. Other anions
did not show any apparent spectral changes. These results indicated
that MA could behave as a “turn-on” fluorescence sensor for
selectively sensing MA over other investigated guests. Fig. 2
showed different fluorescent intensity changes when the same
equivalent of D- or L-MA was added to sensor L-1 in CH3CN,
respectively. These experiments demonstrated that sensor L-1 had
a highly enantioselective recognition ability towards the MA.
The binding constant (Kass) of complexes of aforementioned
chiral fluorescent sensor with MA was determined by means of
titration fluorimetry. With the assumption of a 1:1 stoichiometry,
the complexation of MA (G) with sensor (H) was expressed by
Equation (1):
Fig. 4. Fluorescent intensity changes of sensor L-1 at 423 nm with the addition of
different amounts of D- or L-MA. The correlation coefficient (R) of the non-linear
curve-fitting is 0.9952 and 0.9985, respectively.
K
H þ G#HG
(1)
Under the conditions employed, the association constant (Kass
)
could be calculated by using Equation (2) from the Origin 7.5
software package [20,21]. I represented the fluorescence intensity,
Table 1
Association constants (Kass) of sensor L-1, D-1 and L-2 with D- or L-MA at 25 ꢀC.
a,b
Entry
Host
Guestc
Ks[Mꢂ1
]
K(D)/K(L)
R
1
2
3
4
5
6
L-1
L-1
D-1
D-1
L-2
L-2
D-MA
L-MA
D-MA
L-MA
D-MA
L-MA
290 2.1
74 1.6
95 3.2
198 4.1
147 2.8
47 3.4
3.92
0.9952
0.9985
0.9974
0.9959
0.9984
0.9993
0.45
3.13
a
The data were calculated from the results of the fluorescent titrations in CH3CN.
All error values were obtained from nonlinear curve-fitting.
MA were used as their tetrabutylammonium salts.
Fig. 5. Job plots of sensor L-1 with D- and L-MA. The total concentration of the sensor
L-1 ([H]) and D- or L-MA ([G]) is (1.0 ꢃ 10ꢂ4 M) in CH3CN. I0: fluorescent intensity of
sensor L-1 and I: fluorescent intensity of sensor L-1 in the presence of MA.
b
c

172
Q. Li et al. / Dyes and Pigments 109 (2014) 169e174
fraction of the sensor ([H]/([H] þ [G])). Fig. 5 showed the Job plots of
sensor L-1 with D- and L-MA. When the molar fraction of the sensor
was 0.50, the fluorescent intensity reached the maximum, which
demonstrated that sensor L-1 formed a 1:1 complex with D- and L-
MA, respectively [22,23].
In order to obtain better enantioselective fluorescent sensors,
we used the large unit phenylalaninol as the started material to
replace alaninol to get compound L-2. The enantioselectivities of L-
2 towards MA were compared with that of L-1 towards MA. When
compound L-2 was treated with D-MA under the same conditions,
the emission gradually enhanced; while treated with L-MA, the
emission of sensor L-2 lesser enhanced (Figs. s3 and s4), which gave
Ka(D)/Ka(L) 3.13 for MA (Fig. 6, Table 1). The fluorescent sensor L-2
was less enantioselective towards MA than sensor L-1. The
increasing of satiric bulk maybe caused the obstacle in recognizing
the fluorescent sensors towards MA.
Fig. 6. Fluorescence spectral changes of sensors L-1, D-1 and L-2 (3.0 ꢃ 10ꢂ5 M) were
measured upon the addition of 100 equiv. of D-or L-MA.
The enantioselective fluorescent recognition of MA by L-1 could
be confirmed by using sensor D-1, which gave the expected mirror
image responses for the enantiomers of MA (Figs. s5 and s6), that
was, the enantiomer of L-MA increased the fluorescence of L-1
more efficiently than the D-MA.
CH and CG were the sensor and anion concentrations respectively,
and C0 was the initial concentration of the sensor.
The chiral fluorescent sensors L-1 and L-2 containing nitrogen
were synthesized by simple steps, and their enantioselective
recognition abilities towards MA were evaluated by the fluores-
cence spectra. Probably NH group of the sensors interacted with
carboxylates of MA due to enantioselective recognition. To our
knowledge, this was the first reported fluorescent sensor for the
enantioselective recognition of MA in CH3CN. Sensor L-1 exhibited
the highest association constants and the best enantioselective
recognition towards MA. The sensors' structure-complementary,
steric effect with MA and multiple hydrogen bonding should be
responsible for the enantioselective recognition. Sensitive fluores-
cent response revealed that sensor L-1 could be used as a fluores-
cent sensor for MA in CH3CN.
.
n
I ¼ I0 þ ðIlim ꢂ I0Þ 2C0 CH þ CG þ 1=Kass
h
i
o
1=2
ꢂ ðCH þ CG þ 1=KassÞ2 ꢂ 4CHCG
(2)
In order to know more about the interactions between L-1 and
D- or L-MA, fluorescent titration was carried out. When L-1 was
treated with L-MA, the emission gradually enhanced; while treated
with D-MA under the same conditions, the emission of L-1 larger
enhanced (Fig. 3). The association constants (Kass) and correlation
coefficients (R) obtained by a nonlinear least squares analysis of I
versus CH and CG were listed in Table 1. The results (the correlation
coefficient R was over 0.99) of non-linear curve-fitting indicated
that a 1:1 complex was formed between sensor S-1 and D- or L-MA
(Fig. 4).
3.3. 1H NMR studies
The continuous variation methods were also employed to
determine the stoichiometric ratio of sensor L-1 with D- and L-MA.
The total concentration of sensor L-1 and D- or L-MA was constant
(3.0 ꢃ 10ꢂ4 M) in CH3CN, with a continuously variable molar
1H NMR experiments were undertaken to assess the enantio-
selective recognition properties between sensor and D- or L-MA
because it could directly provide structural and dynamic
Fig. 7. 1H NMR spectra (400 MHz, DMSO-d6 25 ꢀC) of L-1 and MA complex at ([L-1] ¼ [guest] ¼ 4.0 nM in DMSO-d6 at 400 MHz). (A) Racemic MA; (B) sensor L-1 þ D-MA; (C) sensor
L-1 þ L-MA; (D) sensor L-1.

Q. Li et al. / Dyes and Pigments 109 (2014) 169e174
173
Fig. 8. Modes of the proposed 1:1 complexation of sensor L-1 with the D- and L-MA.
information [24,25]. Studies on the enantioselective recognition
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