Acridine-based enantioselective fluorescent sensors for the malate anion in water

Article abstract of DOI:10.1039/c3nj01341c

Compounds S-1, S-2, S-3 and R-1 were synthesized and the interactions of all of the compounds with the malate anion were studied in H₂O (comprising 0.3% DMSO and 0.01 M HEPES at pH = 7.4) by fluorescence titration experiments. Sensor S-1 was found to present good enantioselective fluorescent sensing ability towards the malate anion. The results indicated that sensor S-1 was very promising as a fluorescent sensor for recognition of the malate anion in H₂O solution.

Full text of DOI:10.1039/c3nj01341c

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Acridine-based enantioselective fluorescent
sensors for the malate anion in water†
Cite this: New J. Chem., 2014,
38, 1004
Kuo-Xi Xu,*a Hua-Jie Kong,a Ping Li,a Li Yang,a Jing-Lai Zhanga and
Chao-Jie Wangb
Received (in Porto Alegre, Brazil)
31st October 2013,
Accepted 11th December 2013
Compounds S-1, S-2, S-3 and R-1 were synthesized and the interactions of all of the compounds with
the malate anion were studied in H2O (comprising 0.3% DMSO and 0.01 M HEPES at pH = 7.4) by
fluorescence titration experiments. Sensor S-1 was found to present good enantioselective fluorescent
sensing ability towards the malate anion. The results indicated that sensor S-1 was very promising as a
fluorescent sensor for recognition of the malate anion in H2O solution.
DOI: 10.1039/c3nj01341c
www.rsc.org/njc
physiological investigations, but the recognition !only func-
tions in nonprotic organic solvents.15 Usually, the structure of
Introduction
The recognition and sensing of chiral substrates by synthetic the molecular fluorescent sensors includes three parts—the
receptive molecular systems have continued to attract much binding unit, the fluorophore and the linker between the first
attention because of the fundamental roles of chiral anions in two parts.16 Our recent studies in this field were mainly
many chemical and biological processes.1–5 Dicarboxylates dedicated to synthesis and complex formation of chiral fluor-
are biologically important because of their considerable roles escent sensors with chiral a-carboxylates in aqueous solu-
in numerous metabolic processes such as the generation of tions.17,18 To the best of our knowledge, no fluorescent
high-energy phosphate bonds and the biosynthesis of impor- chiral carboxylate anion sensor has been reported up to now
tant intermediates.6 Malate, a chiral molecule, plays a signifi- for the enantioselective recognition of MA in protic solvents.
cant role in pharmaceutics. D-Malate can only be found if In this paper, we report the synthesis and enantioselective
the synthetic racemate is used as a food additive. L-Malate is fluorescent sensing ability of acridine-based fluorescent MA
used in the treatment of light-damaged or dry skin, acne, sensors in H2O.
and especially fibromyalgia when combined with magne-
sium.7,8 Therefore, the development of a practical, rapid and
accurate method for chiral recognition of D- and L-malate is
Results and discussion
Binding experiments
an important analytical agenda. Amongst many of the enantio-
selective methods under investigation,9–11 the use of fluores-
cence has attracted
a sizable interest because it has A general procedure for the fluorescent measurements: all
some advantages of real-time analysis, high sensitivity, multi- solutions were prepared in volumetric syringes, pipettes, and
ple sensing modes, widely available instrumentation, and volumetric flasks. A DMSO stock solution of compound S-1,
remote detection capabilities.12 Among the studies on the S-2, S-3 or R-1 (0.1 M) was freshly prepared before each
fluorescence-based chiral recognition, only limited reports measurement. The tetrabutylammonium salts were prepared
on the malate anion (MA) have appeared.13,14 We noticed by adding 1.0 equiv. of tetrabutylammonium hydroxide in
that the enantioselective recognition of chiral carboxylate methanol to the solution of the corresponding carboxylic acid
has been achieved using the sensors which are based on the (2.0 equiv. tetrabutylammonium hydroxide to dicarboxylic
hydrogen binding motif and cannot be used in biochemical or acid) in methanol. The resulting syrup was dried under high
vacuum for 24 h, analyzed by 1H NMR spectroscopy, and
a Institute of Fine Chemical and Engineering, College of Chemistry and Chemical
Engineering, Henan University, Kaifeng, Henan, China.
E-mail: xukuoxi@gmail.com; Tel: +86 378 3881589
b Key Lab of Natural Medicinal and Immunal Engineering, Henan University,
Kaifeng, Henan, China
stored in a desiccator, then the stock solutions of the salts
were prepared in HEPES-buffered H2O (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 diluted to 3.0 mL with HEPES-buffered H2O.
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c3nj01341c
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Scheme 1 Synthesis of compounds S-1, S-2, S-3 and R-1.
After being shaken for several minutes, the test solutions were
analyzed immediately (Scheme 1).
Design of new chiral sensors
We chose these compounds to undertake the desired fluores-
cent recognition of a-carboxylate anions in H2O for the follow-
ing two reasons: on one hand, the nitrogen atoms of the
compounds should bind –NH2 or –OH of a-carboxylate anions
well through multiple hydrogen bonds. When the polarity of
solvent is too strong, the binding process will be affected, and
even cannot transfer to the fluorophore. Therefore, a short
linker or a fluorescent group directly involves in the recognition
process, which helps identify the fluorescence sensing. Among
many possible fluorophores, acridine, which is attractive
because of its rigidity, has stronger hydrogen bond-accepting
ability. Rigid receptors often perform better in recognition. On
the other hand, when the sensor interacts with the chiral
carboxylate, their nitrogen atoms are expected to turn on the
fluorescence of the sensors by inhibiting the photoinduced-
electron-transfer (PET)19–21 of the nitrogen lone pair electrons.
Fig. 1 UV-vis spectra of sensor S-1 (2.0 ꢀ 10ꢁ4 M) upon the addition of
various amounts of the anion in H2O (comprising 0.3% DMSO and 0.01 M
HEPES buffer at pH = 7.4). The equivalents of the added anion are 0, 2, 6,
10, 20, 30, 40, 50, 60, 70, 80, 90 and 100.
excessive ([guest]/[S-1, S-2 or S-3] = 100, Fig. 1). The properties of
enantioselective recognition of sensor S-1, S-2 or S-3 were only
investigated for D- or L-a-carboxylate by the fluorescence
spectra.
UV-vis spectroscopic studies
Because there is almost no change in the UV-vis spectra of
sensor S-1, S-2 or S-3 upon the addition of a-carboxylate anions,
such as D- or L-phenylalanine, mandelate, methoxyphenylacetic
Fluorescence spectroscopic studies
acid, phenyl-lactic acid, tartaric acid, dibenzoyltartaric acid, MA When the solution of sensor S-1 was excited at 356 nm, sensor
(as tetrabutylammonium salts) in H2O (comprising 0.3% DMSO S-1 gave a characteristic emission spectrum with a monomeric
and 0.01 M HEPES at pH = 7.4), even when the guests are acridine maximum at ca. 453 nm in H2O (comprising 0.3%
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Fig. 2 Fluorescent spectra changes of sensor S-1 (3.0 ꢀ 10ꢁ5 M) measured in H2O upon the addition of 97 equivalent of various anions (lex = 356 nm,
lem = 453 nm). Fluorescent enhancement ((I ꢁ I0)/I0) of sensor S-1 (3.0 ꢀ 10ꢁ5 M) at 453 nm upon the addition of different a-carboxylic acid anions
(as tetrabutylammonium) in 10 mM HEPES buffer (pH = 7.4) at 25 1C (97 equiv., 3.0 ꢀ 10ꢁ3 M). A = L-MA, B = D-MA, C = L-mandelic acid anion,
D = D-mandelic acid anion, E = L-phenyl-lactic acid anion, F = D-phenyl-lactic acid anion, G = L-methoxyphenylacetic acid anion, H =
D-methoxyphenylacetic acid anion, I = L-tartaric acid anion, J = D-tartaric acid anion, K = L-dibenzoyltartaric acid anion, L = D-dibenzoyltartaric acid
anion, M = L-phenylglycine anion, N = D-phenylglycine anion, O = L-phenylalanine, P = D-phenylalanine.
DMSO and 0.01 M HEPES buffer at pH = 7.4). Binding behaviors added to the solution as shown in Fig. 3, which suggested that
of sensor S-1 with different a-carboxylate anions in H2O were this kind of enantioselective recognition process could be
investigated. Fig. 2 shows the changes in emission spectra of observed by a visual color change. These experiments demon-
sensor S-1 (3.0 ꢀ 10ꢁ5 M) upon addition of various carboxylates. strated that sensor S-1 has a highly enantioselective recognition
It can be seen that upon addition of carboxylate, only MA ability towards the MA.
induced an increasing intensity. Other carboxylates did not
The binding constant (Kass) of complexes of the aforemen-
show any apparent spectral changes. These results indicated tioned chiral fluorescent sensor with MA was determined by
that S-1 could behave as a ‘‘turn-on’’ fluorescence sensor for means of titration fluorimetry. With the assumption of a 1 : 1
selectively sensing MA over other investigated guests. Fig. 3 stoichiometry, the complexation of MA (G) with the sensor (H)
shows different fluorescent intensity changes when the same was expressed using eqn (1):
equivalent of D- or L-MA was added to sensor S-1 in H2O,
K
Ð
respectively. The enantiomer D-MA enhanced 192% and L-MA
H þ G
HG
(1)
enhanced 39%, that is, ef = 4.88 [ef: enantiomeric fluorescent
difference ratio = (ID ꢁ I0)/(IL ꢁ I0). I0 is the fluorescent intensity
of the sensor. ID and IL are the fluorescent intensity of the
Under the conditions employed, the association constant
sensor in the presence of D- and L-MA, respectively]. The colors (Kass) can be calculated by using eqn (2) from the Origin 7.5
of solution were also different when 97 equiv. of L- or D-MA was software package.22 I represents the fluorescence intensity,
Fig. 3 (a) Fluorescent spectra of sensor S-1 (3.0 ꢀ 10ꢁ5 M) with 97 equiv. of D- and L-MA in H2O. (b) Color changes of sensor S-1 (3.0 ꢀ 10ꢁ4 M) in H2O
upon addition of D-/L-MA at room temperature. D- or L-MA has been added with the amount of 97 equiv. of the sensor S-1.
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Fig. 4 (A) Fluorescent intensity changes of sensor S-1 (3.0 ꢀ 10ꢁ5 M) with D-MA in H2O. The D-MA equivalents are: 0 - 97. (B) The fluorescence intensity
changes of sensor S-1 with L-MA. The L-MA equivalents are: 0 - 97.
CH and CG are the sensor and anion concentrations, respec-
tively, and C0 is the initial concentration of the sensor.
I = I0 + (Ilim ꢁ I0)/2C0{CH + CG + 1/Kass
ꢁ [(CH + CG + 1/Kass)2 ꢁ 4CHCG]1/2
}
(2)
In order to know more about the interactions between
sensor S-1 and L- or D-MA, fluorescence titration was carried
out. When sensor S-1 was treated with D-MA, the emission
gradually enhanced; when treated with L-MA under the same
conditions, the emission of sensor S-1 slightly enhanced
(Fig. 4). The association constants (Kass) and correlation coeffi-
cients (R) obtained by a nonlinear least square analysis of
I versus CH and CG are 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. 5).
Fig. 5 Fluorescent intensity changes of sensor S-1 at 356 nm with
the addition of different amounts of D- or L-MA. The correlation coeffi-
cient (R) of the non-linear curve-fitting is 0.9952 and 0.9958,
respectively.
The continuous variation methods were also employed to
determine the stoichiometric ratio of sensor S-1 with D- and
L-MA. The total concentration of sensor S-1 and D- or L-MA was
constant (3.0 ꢀ 10ꢁ4 M) in H2O (comprising 0.3% DMSO and
0.01 M HEPES buffer at pH = 7.4), with a continuously variable
molar fraction of the sensor ([H]/([H] + [G])). Fig. 6 shows the
Job plots of sensor S-1 with D- and L-MA. When the molar
Table 1 Association constants (Kass) of sensors S-1, S-2, S-3 and R-1 with
D- or L-MAs at 25 1C
Entry
Host
Guestc
Ks [Mꢁ1
a,b
]
K(D /K(L)
R
)
1
2
3
4
5
6
7
8
S-1
S-1
S-2
S-2
S-3
S-3
R-1
R-1
D-MA
L-MA
D-MA
L-MA
D-MA
L-MA
D-MA
L-MA
195 ꢂ 4
29 ꢂ 3
85 ꢂ 3
22 ꢂ 2
224 ꢂ 6
144 ꢂ 3
15 ꢂ 2
111 ꢂ 6
0.9952
0.9958
0.9972
0.9983
0.9908
0.9978
0.9974
0.9989
6.72
3.86
1.56
1/7.50
Fig. 6 Job plots of sensor S-1 with the D- and L-MA. The total concen-
tration of the sensor S-1 ([H]) and D- or L-MA ([G]) is (3.0 ꢀ 10ꢁ4 M) in H2O.
I0: fluorescent intensity of sensor S-1 and I: fluorescent intensity of sensor
S-1 in the presence of MAs.
a
The data were calculated from the results of the fluorescence titra-
b
tions in H2O. All error values were obtained from nonlinear curve-
fitting. MAs were used as their tetrabutylammonium salts.
c
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Fig. 7 Fluorescence spectral changes of sensors S-1, S-2 and S-3 (3.0 ꢀ 10ꢁ5 M) were measured upon the addition of 97 equiv. of D- or L-MA.
fraction of the sensor was 0.50, the fluorescent intensity To our knowledge, this is the first reported fluorescent sensor
reached the maximum, which demonstrated that sensor S-1 for the enantioselective recognition of MA in H2O solutions.
formed a 1 : 1 complex with D- and L-MA, respectively.23,24
Sensor S-1 exhibited the highest association constants and the
In order to obtain better enantioselective fluorescent sen- best enantioselective recognition towards MA. The sensors’
sors, we used the large units phenylalanine and tryptophan as structure complementarity, steric effect with MA and multiple
the starting materials to replace alanine to get compounds S-2 hydrogen bonding may be responsible for the enantioselective
and S-3. The enantioselectivities of compounds S-2 and S-3 recognition. Sensitive fluorescent response revealed that sensor
towards MA were compared with that of sensor S-1 towards MA. S-1 can be used as a fluorescent sensor for MA in H2O.
When compound S-2 or S-3 was treated with D-MA under the
Quantum chemical calculations
same conditions, the emission gradually enhanced; when trea-
ted with L-MA, the emission of sensor S-2 or S-3 slightly A computational study of the 1 : 1 complex of sensor S-1 and MA
enhanced (Fig. S3–S6, ESI†), which gave Ka(D)/Ka(L) 3.86 and was performed using the Gaussian 03 program25 and the
1.56 for MA (Fig. 7 and Table 1). The fluorescent sensor S-2 or density functional theory method B3LYP.26,27 Geometries were
S-3 was less enantioselective towards MA than sensor S-1. The optimized using the 6-31G basis set and energies were esti-
increase of satiric bulk maybe caused the obstacle in recogniz- mated using the 6-31G* basis set. The spatial arrangements of
ing the fluorescent sensors towards MA.
D- or L-MA in the two diastereomeric complexes are different.
The enantioselective fluorescence recognition of MAs by S-1 The sensor S-1 exhibited weak fluorescence response towards
could be confirmed by using sensor R-1, which gave the L-MA because of their complex being less favorable. While D-MA
expected mirror image responses for the enantiomers of MA interacted with sensor S-1 forming a stable complex, and there
(Fig. S7 and S8, ESI†), that is, the enantiomer of L-MA increased were multiple functional group interactions between sensor S-1
the fluorescence of R-1 more efficiently than the D-MA.
and D-MA. The NH group and the nitrogen atom of sensor S-1
The chiral fluorescent sensors S-1, S-2 and S-3 containing had strong base and acid interactions with the –COOꢁ and the
nitrogen were synthesized by simple steps, and their enantio- –OH group in MA, respectively. Therefore, the two proposed
selective recognition abilities towards MA were evaluated by using modes are shown in Fig. 8, the a-hydroxy group of D-MA may
the fluorescence spectra. Probably NH groups of the sensors inter- form hydrogen bonds with the nitrogen atom of the sensor (A)
acted with carboxylates of MA due to enantioselective recognition. but not (B), according to the orientation of the acidic group in
Fig. 8 Modes of the proposed 1 : 1 complexation of sensor S-1 with the D- or L-MA.
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D-MA combined with the NH group. From calculations, we
S-2: 0.25 g, yield, 44.7%. [a]2D0 = +46.17 (c = 0.20, CHCl3),
found that the structure of mode A was calculated to be 1H NMR (CDCl3,400 MHz): d(ppm) = 8.33 (s, 2H), 8.27 (s, 1H),
ꢁ204.14 kJ molꢁ1, which was more stable than mode B 7.62 (d, J = 8.1 Hz, 2H), 7.42 (d, J = 6.8 Hz, 2H), 7.30 (t, J = 4.2 Hz,
(ꢁ198.35 kJ molꢁ1) (Fig. S9, ESI†).
2H), 6.98 (d, J = 4.0 Hz, 2H), 6.75 (t, J = 6.4 Hz, 4H), 6.60–6.56
(m, 2H), 4.36 (s, 1H), 4.32 (s, 1H), 3.69 (s, 6H), 3.66 (t, J = 7.0 Hz,
4H), 3.48 (s, 4H), 3.09 (s, 2H). 13C NMR (CDCl3): 175.09, 146.38,
135.78, 129.32, 128.74, 127.43, 126.45, 126.08, 124.78, 123.29,
123.08, 122.18, 121.57, 121.37, 119.54, 118.59, 53.49, 48.96,
29.75, 22.75. IR (KBr): 3394, 2957, 2921, 1732, 1619, 1523,
Conclusions
We have demonstrated that by introducing alanine methyl ester
into the acridine, the chiral fluorescent sensors S-1 and R-1 are
easily available. To our knowledge, it is the first excellent
enantioselective fluorescent sensor towards MA in H2O.
1460, HRMS m/z: calculated for
C35H35N3O4, [M +
H]+
562.2700, found 562.2702.
S-3: 0.31 g, yield, 48.1%. [a]2D0 = +22.82 (c = 0.20, CHCl3),
1H NMR (CDCl3, 400 MHz): d(ppm) = 9.41 (s, 2H), 8.16 (s, 1H),
7.56 (d, J = 8.0 Hz, 2H), 7.42 (d, J = 6.8 Hz, 2H), 7.22 (dd, J = 8.3,
6.9 Hz, 2H), 6.87 (s, 2H), 6.57 (d, J = 8.0 Hz, 2H), 6.48 (d, J =
8.0 Hz, 2H), 6.42 (t, J = 7.4 Hz, 2H), 6.20 (t, J = 7.4 Hz, 2H), 4.20
(d, J = 14.0 Hz, 1H), 3.83 (d, J = 14.2 Hz, 1H), 3.59 (s, 6H), 3.47–
3.41 (m, 8H), 2.97 (s, 2H). 13C NMR (CDCl3): 175.02, 146.35,
139.48, 137.35, 136.74, 135.76, 133.36, 129.48, 129.31, 127.98,
126.47, 123.82, 121.40, 118.60, 118.02, 115.15, 53.48, 52.06,
29.74, 22.74. IR (KBr): 3397, 2951, 1738, 1624, 1528, 1461,
HRMS m/z: calculated for C39H37N5O4, [M + H]+ 640.2918,
found 640.2922.
Experimental section
General methods
The reagents used were of commercial origin and employed
without further purification. Purifications by column chromato-
graphy were carried out over silica gel (230–400 mesh). The IR
spectra were recorded on a Nicolet 670 FT-IR spectrophoto-
meter. 1H NMR and 13C NMR spectra were recorded on a Bruker
AV-400 spectrometer. High resolution mass spectra (HRMS)
were recorded on a Agilent 1290LC-6540 Accurate Mass
Q-TOF by using electrospray ionization (ESI). Optical rotations
were taken on a Perkin–Elmer Model 341 polarimeter. Fluores-
cence spectra were obtained with an F-7000 FL spectrophoto- Acknowledgements
meter. The anions were used as their tetrabutylammonium
salts. 4,5-Bis(bromomethyl)acridine and amino acid methyl
ester were prepared according to the literature methods.28,29
We thank the National Natural Science Foundation of China
(Grant No. 21172053), Technological Project of Henan Province
(132300410055, 132102210388) and the Education Department
of Henan Province of China (14A150050) for financial support.
General procedure for the preparation of compounds S-1, R-1,
S-2 and S-3
In the ice-bath cooled solution, amino acid methyl ester
(2.2 mmol) in dry CH3CN (20 mL) was added to a mixture of
4,5-bis(bromomethyl)acridine (36 mg, 1 mmol) and anhydrous
K2CO3 (72 mg, 5 mmol) in dry CH3CN (20 mL). The reaction
mixture was stirred for another 24 h at room temperature and
monitored via TLC. After completion of the reaction, the
solvent was evaporated under reduced pressure. Then water
(10 mL) was added, and the crude product was extracted with
CH2Cl2 (3 ꢀ 20 mL). The combined extract was dried over
Na2SO4. After removal of the solvent, the residue was purified
by column chromatography on silica gel eluted with CHCl3–
C2H5OH (20 : 1) to obtain pure products S-1, R-1, S-2 and S-3 as
yellow viscous oils, respectively.
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