An easy assembled fluorescent sensor for dicarboxylates and acidic amino acids

Article abstract of DOI:10.3762/bjoc.7.11

Two mesitylene based neutral receptors 1 and 2 bearing two thiourea binding sites were constructed as fluorescent probes for sensing dicarboxylates. Their binding affinities toward dicarboxylates, aspartate and glutamate have been investigated in acetoni-

Full text of DOI:10.3762/bjoc.7.11

An easy assembled fluorescent sensor for
dicarboxylates and acidic amino acids
Xiao-bo Zhou, Yuk-Wang Yip, Wing-Hong Chan* and Albert W. M. Lee
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2011, 7, 75–81.
Department of Chemistry, Hong Kong Baptist University, Kowloon
Tong, Hong Kong SAR, China
doi:10.3762/bjoc.7.11
Received: 08 October 2010
Accepted: 06 December 2010
Published: 17 January 2011
Email:
Wing-Hong Chan* - whchan@hkbu.edu.hk
* Corresponding author
Associate Editor: S. C. Zimmerman
Keywords:
© 2011 Zhou et al; licensee Beilstein-Institut.
License and terms: see end of document.
amino acids; dicarboxylate ion recognition; enantioselectivity;
mesitylene scaffold
Abstract
Two mesitylene based neutral receptors 1 and 2 bearing two thiourea binding sites were constructed as fluorescent probes for
sensing dicarboxylates. Their binding affinities toward dicarboxylates, aspartate and glutamate have been investigated in acetoni-
trile solution by fluorescence titration experiments. Both fluorescent sensors exhibited some ability to discriminate the antipodal
forms of aspartate and glutamate.
Introduction
The recognition and sensing of anionic substrates by positively subunits (i.e., chromophore or fluorophore) as sensing probes
charged or electrically neutral synthetic receptive molecular for dicarboxylates [8-13]. Additionally, chiral recognition of
systems has emerged as a key research area because of the carboxylates has been actively explored in the sensor field [14-
fundamental roles of anions in many chemical and biological 16]. By using cholic acid as the molecular scaffold for the
processes [1-5]. Being the key structural moieties of many construction of sensing probes, we have developed fluorescent
bioactive molecules such as amino acids and proteins, dicar- probes for detecting dicarboxylates and trifunctional aminoacids
boxylates are one of the most attractive targets for anion recog- [17,18]. To continue our interest in this research direction, we
nition and sensing. In addition, dicarboxylates are essential report here the facile synthesis and molecular recognition prop-
components of a variety of metabolic processes such as the erties of two new sensing probes 1 and 2.
generation of high-energy phosphate bonds and the biosyn-
thesis of important intermediates [6,7]. In recent years, Trimethyl- or triethylbenzene have been widely used building
numerous endeavors have been devoted to the design and syn- blocks to prepare both tripodal or ditopic supramolecular
thesis of ditopic anion receptors bearing optical signal display systems for molecular recognition [19-24]. An obvious incen-
75

Beilstein J. Org. Chem. 2011, 7, 75–81.
tive for the use of the mesitylyl moiety is that the required ceptor” sensing motif for carboxylate was incorporated into the
1,3,5-trimethyl-2,4-bis(bromomethyl)benzene and 1,3,5- molecular platform via thiourea formation to give the key inter-
trimethyl-2,4,6-tri(bromomethyl)benzene as starting materials mediate 4 in 70% yield. As a result of steric hindrance, an
are readily prepared [25]. Upon suitable functionalization, the attempt to make the corresponding bis-thiourea adduct from 3
three alkyl groups of the substituted mesitylene would preorga- and 9-aminomethylanthracene was unsuccessful. However, 4
nize towards the same face of the benzene core to favor the for- underwent a smooth addition reaction with the structurally less
mation of a semi-rigid conformation [26]. We thus chose to bulky (+)-α-ethylphenylamine to afford sensor 1 in 50% yield
employ this mesitylene derivative as our starting material for (Scheme 1). It is noteworthy that by appending a chiral element
the design and synthesis of dicarboxylate sensors.
onto the sensor scaffold, 1 may demonstrate enantioselectivity
towards optically active materials such as amino acids. In
continuation of the pioneering work of Gunnlaugsson, the
Results and Discussion
The 2-step protocol developed by Banert et al. [27] was adopted charge neutral thiourea-based fluorescent PET sensor motif,
for the synthesis of the sensors. Thus nucleophilic substitution which is immune from cross pH interference, was adopted in
of the bromine atoms of 1,3-bis[bromomethyl]-2,4,6-trimethyl- our approach [28]. The congested mesitylene derivative type of
benzene with sodium azide in DMSO afforded the corres- structure built in 1 may confer the molecule with sufficient
ponding bis-azide. Transformation of the crude bis-azide into rigidity favoring its selective binding with guest molecules. The
bis-isothiocyanate 3 was achieved by treatment with triphenyl- receptor 1 was fully characterized by 1H, 13C NMR, and high
phosphine in the presence of CS2. The “fluorophore–spacer–re- resolution mass spectral analysis.
Scheme 1: Synthesis of sensor 1 and 2.
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Beilstein J. Org. Chem. 2011, 7, 75–81.
To evaluate the binding affinity of the synthetic host to dicar- acetate being a monoanion did not cause any fluorescence
boxylate guest molecules, fluorometric titration experiments quenching to sensor 1 (Figure S2, Supporting Information
were carried out with the concentration of 1 fixed at File 1).
5.0 × 10−6 M in acetonitrile and the guest concentration (as
tetrabutylammonium salts) was varied from 5.0 × 10−6 M to
Table 1: Association constants Kass (M−1) of sensor 1 with various
1.0 × 10−4 M. The typical change of emission spectra of 1
caused by isophthalate is shown in Figure 1. Operating on the
PET mechanism, the fluorescent emission band of the host at
413 nm was quenched gradually upon the addition of the guest.
dicarboxylates (as their tetrabutylammonium salts) in CH3CN.
Anion
Kassa
R2
Isophthalate
Phthalate
Terephthalate
Oxalate
(6.25 ± 0.61) × 104
(1.34 ± 0.11) × 104
(1.23 ± 0.58) × 104
(2.16 ± 0.12) × 104
(2.49 ± 0.24) × 104
(1.95 ± 0.21) × 104
(1.60 ± 0.24) × 104
0.9949
0.9955
0.9835
0.9974
0.9927
0.9909
0.9857
Malonate
Succinate
Glutarate
aKass is the apparent constant for the equilibrium of the 1:1 stoichio-
metric ratio between 1 and dicarboxylate in CH3CN.
On the basis of the fairly similar association constants between
1 and different dicarboxylates, the two tweezer-like thiourea
side-arms of 1 seem to be quite flexible and are able to accom-
modate dicarboxylates of different chain lengths. The 3–4 fold
stronger binding interaction between sensor 1 and isophthalate
in comparison with those of other dicarboxylates may be the
result of the good host–guest shape-complementary relation-
ship and the π–π interactions between the aromatic moieties of
the host and the guest. To examine the chiral discrimination
ability of sensor 1 toward chiral acidic aminoacids, fluores-
Figure 1: The changes in the fluorescence emission spectra of sensor
1 (5.0 × 10−6 M) upon addition of isophthalate in acetonitrile. λex =
366 nm. (Inset) Quenching ratio of sensor 1 as a function of
(guest)/(host).
On the basis of a Job plot, a 1:1 complex between 1 and isoph- cence titrations of sensor 1 with antipodal aspartates and gluta-
thalate was confirmed (Figure S1, Supporting Information mates were carried out separately. Using succinate as the refer-
File 1). To give a full picture of the binding characteristics of ence guest molecule, the additional amino group present in
the synthetic host, other aromatic and aliphatic dicarboxylates aspartates weakened their interaction with sensor 1, presum-
were included in the study. By analyzing the change of fluores- ably due to non-bonding repulsions.
cent intensity associated with the stepwise addition of guest
dicarboxylates, the “host–guest” complex association constants In contrast, in comparison with the association constant
(Ka) were calculated using non-linear least-squares curve fitting between sensor 1 and glutarate, sensor 1 exhibited a stronger
and the results are compiled in Table 1. It is noteworthy that binding affinity to D- and L-glutamate (Table 2). Interestingly,
Table 2: Association constants Kass (M−1) of sensor 1 and 2 with antipodal aspartate and glutamate (as their tetrabutylammonium salts) in CH3CN.
Sensor 1
Kass
Sensor 2
Kass
Anion
Enantioselectivity
—
Enantioselectivity
—
Succinate
(1.95 ± 0.21) × 104
(0.57 ± 0.05) × 104
(1.33 ± 0.15) × 104
(1.60 ± 0.24) × 104
(2.05 ± 0.05) × 104
(3.73 ± 0.08) × 104
(0.34 ± 0.03) × 104
(2.13 ± 0.31) × 104
(1.19 ± 0.25) × 104
(0.52 ± 0.05) × 104
(5.17 ± 0.31) × 104
(3.35 ± 0.36) × 104
D-Aspartate
L-Aspartate
Glutarate
KL/KD = 2.3
KL/KD = 0.56
—
—
D-Glutamate
L-Glutamate
KL/KD = 1.8
KL/KD = 0.65
77

Beilstein J. Org. Chem. 2011, 7, 75–81.
a marginal enantioselectivity toward the antipodal forms of and the two benzylic methylene protons, respectively, are split
aminoacids demonstrated by sensor 1 is evident. We were into three sets of doublet of doublets. Apparently, after coordi-
aware that sensor 1 can only exhibit two-point interaction with nating with the guest, bond rotation of host 2 was restricted,
aspartate and glutamate via the thiourea–carboxylate type of with the result that the two methylene protons adjacent to the
binding motif. For the host to exert a greater influence on the aromatic ring (i.e., He and Hf) became magnetically non-
preorganization of the guests, an additional binding site must be equivalent. At the same time, the Ha, Hb and Hd protons of the
introduced. Thus, (S)-phenylalaninol (obtainable from host experienced an upfield shift of ~0.07 ppm, whereas the Hc
L-phenylalanine) was reacted with isothiocyanate 4 to give protons were downfield shifted by 0.19 ppm. Conceivably, as a
sensor 2 in 40% yield. We envisaged that the additional alcohol result of complexation, the spatial relationship between the
functionality present in 2 could provide a binding site for the mesitylene and the anthracene moiety of the host may slightly
α-amino group of aspartate and glutamate. Experimental find- change, inducing a change in the chemical shifts for some
ings indeed corroborated well with such a supposition. protons. On the other hand, the guest induced shift of selected
Compared with the interaction of the control compounds (i.e., protons of sensor 2 caused by D- and L-glutamate was almost
succinate and glutarate), sensor 2 demonstrated a 3–10 fold identical, which is consistent with the low enantioselectivity
stronger binding affinity with aspartate and glutamate, respect- exhibited by the host. Interestingly, when 0.5 equiv of D-gluta-
ively. For instance, the association constant of sensor 2 and mate was added to the host, one of the benzylic methylene
D-glutamate is one order of magnitude greater than that of protons of the host became a broad singlet. This could be ratio-
sensor 2 and glutarate. In addition to the carboxylate–thiourea nalized as follows: The thiourea group adjacent to the phenyl-
interactions, additional H-bond interactions could arise from the alaninol moiety possessing two binding sites first complexes
alcohol group of the host and the amino group of the guest. A with the α-aminocarboxylate group of glutamate. On binding,
three site binding model is proposed (Figure 2) to rationalize the the mobility of the corresponding pendant side-arm is reduced
enhanced binding interaction between sensor 2 and glutamate.
thus leading to the broad singlet centered at δ 4.69 splitting into
a multiplet (Figure 3b). The observation corroborates well with
our binding model shown in Figure 2.
To address the issue of selectivity of sensor 1 towards other
anions, fluorescence titrations of 1 were conducted with di-
hydrogen phosphate, nitrate and bromide. Upon addition of up
to 20 equiv of these anions separately to the sensor solutions, no
fluorescence change of sensor 1 was observed (Figure S3,
Supporting Information Information File 1). On the other hand,
among common transition metal ions (i.e., Ag+, Zn2+, Cd2+,
Fe2+, Co2+, Ni2+, Hg2+, Pb2+, Cu2+), only Cu(II) led to suppres-
sion of the fluorescence of sensor 1 (Figure S4, Supporting
Information File 1). Apparently, sensor 1 is a fairly selective
probe for detecting dicarboxylates.
Figure 2: Proposed three sites binding model for sensor 2 and gluta-
mate complex.
In comparison with sensor 1, sensor 2 exhibited reversed enan-
tiomeric bias towards aspartate and glutamate. On the other
hand, it was quite surprising to see that the enantioselectivity
demonstrated by sensor 2 toward the antipodal forms of the
aminoacids was very small. Apparently, the rigidity of the host
molecule is not sufficient to generate a unique binding cavity so
as to effectively differentiate between the two enantiomeric
acidic aminoacids.
Conclusion
This study examined the binding properties of two ditopic
receptors 1 and 2 for various dicarboxylates using fluorescence
and 1H NMR spectroscopic methods. As revealed by the respec-
tive association constants, among all studied dicarboxylates,
sensor 1 showed the highest affinity to isophthalate. The preor-
ganization of sensor 2 permits three site binding with the guests
leading to enhanced complexation with aspartate and glutamate
To shed some light on the binding interaction between sensor 2 in contrast to their corresponding dicarboxylate counterparts.
and glutamate, an 1H NMR spectroscopic method was The chiral recognition ability of the sensors is however, only
employed. By adding one equivalent of either D- or L-gluta- moderate. In order to be a better enantioselective dicarboxylate
mate to a DMSO solution of sensor 2, as shown in Figure 3, the sensor, the flexibility of the two thiourea receptive sites of the
resultant 1H NMR spectra displayed two distinct changes. The host must be constrained. Design of new enantioselective
broad singlets at δ 5.62, 4.69 and 4.49 ascribed to anthracenyl sensors along this direction is underway.
78

Beilstein J. Org. Chem. 2011, 7, 75–81.
Figure 3: Partial 1H NMR spectra for (a) sensor 2 (free, 3 mM), (b) sensor 2 + 0.5 equiv D-glutamate, (c) sensor 2 + 1.0 equiv D-glutamate, (d) sensor
2 + 1.0 equiv L-glutamate in DMSO-d6.
Experimental
2.2 mmol) in dry DMSO (30 mL) was stirred for 3 h at 50 °C.
General. Melting points were determined with a MEL-TEMPII After cooling to room temperature, PPh3 (0.58 g, 2.2 mmol)
melting point apparatus and are uncorrected. 1H and 13C NMR was added with cooling. The mixture was stirred for 16 h at
spectra were recorded on a Bruker Avance-III spectrometer (at room temperature and CS2 (0.18 mL, 3.0 mmol) added drop-
400 and 100 MHz, respectively) in DMSO-d6 or CDCl3. High wise to the ice-cooled reaction mixture. When the vigorous
resolution mass spectra were recorded on a Bruker Autoflex reaction had subsided, the mixture was stirred for an additional
mass spectrometer (MALDI TOF). Fluorescent emission 4 h at room temperature, then poured into water, and extracted
spectra were taken on a Perkin Elmer LS 50B luminescence repeatedly with DCM (3 × 30 mL). The combined organic
spectrofluorimeter. Unless otherwise specified, all fine chemi- layers were washed several times with water, dried with MgSO4
cals were used as received.
and filtered. After removal of the solvent in vacuo, the residue
was purified by column chromatography on SiO2 with chloro-
Synthesis of 2,4-bis(isothiocyanatomethyl)-1,3,5-trimethyl- form:PE (1:10) as eluent to give 3 as a white solid (200 mg,
benzene (3): A mixture of 3,4-bis(bromomethyl)-1,3,5- 76% yield). Its characterization data is in agreement with that
trimethylbenzene (0.306 g, 1 mmol) and NaN3 (0.14 g, reported [29].
79

Beilstein J. Org. Chem. 2011, 7, 75–81.
Synthesis of 2-anthrancenylthioureamethyl-4-isothio- 36.36, 42.73, 56.37, 60.91, 124.27, 125.30, 125.95, 126.52,
cyanatomethyl-1,3,5-trimethylbenzene (4): Compound 3 (130 127.52, 128.10, 128.93, 129.09, 129.44, 129.68, 129.86, 130.98,
mg, 0.5 mmol) and anthrancen-9-yl-methanamine (110 mg, 0.5 132.55, 132.60, 136.24, 136.38, 136.77, 138.86, 181.82.
mmol) were dissolved in freshly distilled dry dichloromethane MALDI TOF HRMS: calcd for C37H40N4OS2 [M+H]+
(10 mL) and the mixture was stirred for 14 h. The crude pro- 621.2727; found 621.2732.
duct collected after work-up was purified by column chroma-
tography on SiO2 with chloroform:PE (2:1) as eluent to give 4 Preparation of fluorometric anion titration solutions: Stock
as a white solid (60 mg, 70% yield), mp 129–133 °C.
solutions (5 mM) of the tetrabutylammonium salts of phthalatic
acid, isophthalic acid, terephthalic acid, oxalic acid, malonic
1H NMR (400 MHz, CDCl3): δ 2.26 (s, 3H), 2.27 (s, 3H), 2.30 acid, succinic acid, glutaric acid, D- and L-aspartic acid, and D-
(s, 3H), 4.64 (s, 2H), 4.82 (s, 2H), 5.64 (s, 2H), 6.93 (s, 1H), and L-glutamic acid in CH3CN were prepared. Stock solutions
7.16 (br, 1H), 7.55-7.63 (m, 4H), 8.12 (d, 2H, J = 8.4 Hz), of hosts (1 mM) were prepared in DMSO. Test solutions were
8.41(d, 2H, J = 8.8Hz), 8.63 (s, 1H). 13C NMR (100 MHz, prepared by adding 25 µL of the stock host solution and
CDCl3): δ 15.19, 19.11, 19.56, 40.63, 43.24, 124.29, 125.29, different volumes (5–100 µL) of the anion solution to a series of
126.50, 127.51, 128.27, 128.92, 129.39, 129.45, 129.87, 130.03, test tubes followed by dilution to 5 mL with acetonitrile. After
130.97, 133.00, 136.19, 136.61, 137.68, 181.84. MALDI TOF being shaken for several minutes, the test solutions were
HRMS: calcd for C28H27N3S2 [M+H]+ 470.1719; found measured immediately. For all measurements, the solutions
470.1700.
were excited at 366 nm and emission was measured from
380–480 nm.
Synthesis of Sensor 1: Compound 4 (70 mg, 0.15 mmol) and
(+)-α-ethylphenylamine (24 mg, 0.2 mmol) were dissolved in Association constants (1:1) of 1 and 2 with anions were calcu-
dry DMSO (10 mL) and the mixture was stirred for 14 h at lated by non-linear least square curve fitting using the following
100 °C. The crude product collected after work-up was purified equation in Origin 7.5:
by column chromatography on SiO2 with chloroform as eluent
to give 1 as a yellow solid (45 mg, 50% yield), mp 244–246 °C.
[α]25D = −30.1 (c = 0.65, DMSO).
1H NMR (400 MHz, CDCl3): δ 1.35 (d, 2H, J = 6.8 Hz), δ
2.21(s, 6H), 2.26 (s, 3H), 4.64 (s, 2H), 4.82 (s, 2H), 5.64 (s, where I0 is fluorescent intensity of host without any anion, Ilim
2H), 6.93 (s, 1H), 7.16 (br, 1H), 7.55-7.63 (m, 4H), 8.12 (d, 2H, is fluorescent intensity limit on adding excess anion, CA is the
J = 8.4 Hz), 8.41(d, 2H, J = 8.4 Hz), 8.63 (s, 1H). 13C NMR concentration of the anion added, and CH is the concentration of
(100 MHz, CDCl3): δ 15.19, 19.11, 19.56, 40.63, 43.24, 124.29, the host molecule.
125.29, 126.50, 127.51, 128.27, 128.92, 129.39, 129.45, 129.87,
130.03, 130.97, 133.00, 136.19, 136.61, 137.68, 181.84.
Supporting Information
MALDI TOF HRMS: calcd for C36H38N4S2 [M+H]+ 591.2583;
found 591.2596.
A Job plot of sensor 1 with isophthalate, interference
studies and the 1H, 13C NMR and HRMS spectra of
Synthesis of sensor 2: Compound 4 (70 mg, 0.15mmol ) and
compound 1, 2 and 4 are available as Supporting
(S)-phenylalaninol (30 mg, 0.2 mmol) were dissolved in dry
Information.
DMSO (10 mL) and the mixture was stirred for 14 h at 100 °C.
Supporting Information File 1
Spectral data of compounds 1, 2 and 4 and Job plot of
sensor 1.
The crude product collected after work-up was purified by
column chromatography on SiO2 with chloroform as eluent to
give 2 as a yellow solid (36 mg, 40% yield): mp 224–225 °C.
[α]21D = −16.3 (c = 0.50, DMSO).
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-7-11-S1.pdf]
1H NMR (400 MHz, CDCl3): δ 2.21(s, 6H), 2.24 (s, 3H),
2.76–2.78(m, 2H), 3.30–3.33(m, 2H), 4.36(s, 1H), 4.48(s, 2H),
4.62 (s,2H), 4.87(t, 1H, J = 4.8 Hz), 5.62 (s, 2H), 6.86 (s, 1H), Acknowledgements
7.05 (br, 1H), 7.06–7.27(m, 7H), 7.51–7.63 (m, 4H), 8.11 (d, The financial support of this work by the University Grants
2H, J = 8.4 Hz), 8.39 (d, 2H, J = 8.4 Hz), 8.63 (s, 1H). Committee of the Hong Kong SAR, China (HKBU 200208) is
13C NMR (100 MHz,CDCl3): δ 15.39, 19.45, 19.51, 28.94, acknowledged.
80

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