Fluorescence 'on-off-on' chemosensor for sequential recognition of Fe 3+ and Hg2+ in water based on tetraphenylethylene motif

Article abstract of DOI:10.1016/j.bmc.2012.11.005

A novel selective and sensitive fluorescence 'on-off-on' probe based on tetraphenylethylene (TPE) motif for sequential recognition of Fe³⁺ and Hg²⁺ in water has been developed. Especially the complex 6-Fe³⁺ could behave as a 'turn on' fluorescent sensor over a wide-range pH value for detection of Hg²⁺. The selectivity of this complex for Hg²⁺ over other heavy and transition metal ions is excellent, and its sensitivity for Hg²⁺ is at 2 ppb in water.

Full text of DOI:10.1016/j.bmc.2012.11.005

Bioorganic & Medicinal Chemistry 21 (2013) 508–513
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Bioorganic & Medicinal Chemistry
journal homepage: www.elsevier.com/locate/bmc
Fluorescence ‘on-off-on’ chemosensor for sequential recognition of Fe3+
and Hg2+ in water based on tetraphenylethylene motif
⇑
Yuanyuan Yan, Zhiping Che, Xiang Yu, Xiaoyan Zhi, Juanjuan Wang, Hui Xu
Laboratory of Pharmaceutical Design & Synthesis, College of Sciences, Northwest A&F University, Yangling 712100, Shannxi Province, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel selective and sensitive fluorescence ‘on-off-on’ probe based on tetraphenylethylene (TPE) motif
for sequential recognition of Fe3+ and Hg2+ in water has been developed. Especially the complex 6-Fe3+
could behave as a ‘turn on’ fluorescent sensor over a wide-range pH value for detection of Hg2+. The selec-
tivity of this complex for Hg2+ over other heavy and transition metal ions is excellent, and its sensitivity
for Hg2+ is at 2 ppb in water.
Received 25 September 2012
Revised 4 November 2012
Accepted 5 November 2012
Available online 15 November 2012
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Fluorescence probe
Tetraphenylethylene
Mercury ion
Water-soluble
Dithioacetal
1. Introduction
6 (Fig. 1) by incorporating TPE fragment with dithioacetals, which
contained four carboxylic groups. Four carboxylic groups would
Iron(III) ions play an important role in the biological systems,
such as cellular metabolisms, and a catalytic site for proteins and
enzymes.1 Meanwhile, mercury ions can be extremely dangerous
environmental pollutants because they can bioaccumulate
throughout the food chain and are chemically toxic to liver, kidney,
and brain function when they are ingested or inhaled by human
beings.2 Therefore, a lot of excellent sensors have been developed
make 6 selectively bind with a metal ion and soluble in aqueous
solution, and dithioacetals would let 6 selectively and sequentially
recognize Hg2+ over other HTM ions. To our delight, the probe 6
exhibited selective and sensitive fluorescence ‘on-off-on’ for
sequential recognition of Fe3+ and Hg2+ ions in water.
2. Results and discussion
3
for recognition of Fe3+ and Hg2+ ions,4 respectively. However, a
chemosensor for sequential recognition of Fe3+ and Hg2+ ions in
water, to the best of our knowledge, has not been reported.
Recently, many attractive probes for the detection of mercury
ions based on the specific mercury-promoted desulfurization reac-
tion have been reported.5 More recently, although another series of
new sensors based on the mercury-promoted deprotection of the
dithioacetal reaction has been explored, those probes for recogni-
tion of Hg2+ was conducted in organic solutions.6 Based upon the
Hg2+-promoted deprotection reaction of the dithioacetal, we have
recently reported a 1,8-naphthalimide-based water-soluble ‘turn
on’ chemosensor for recognition of Hg2+ ion over other heavy and
transition metal (HTM) ions with high selectivity and sensitivity.7
On the other hand, tetraphenylethylene (TPE) motif is usually used
as the fluorophore to prepare chemosensors for recognition of dif-
ferent metal cations.8 Encouraged by the above-mentioned inter-
esting results, we herein designed a water-soluble chemosensor
Sensor 6 was efficiently synthesized as shown in Scheme 1. The
chemical structures of 2–6 were well characterized by IR spec-
trometry, 1H and 13C NMR spectroscopy, and high-resolution mass
spectrometry (HRMS).
The fluorescent intensity of 6 was almost a constant value when
pH value was between 3.65 and 5.71 in water. The fluorscence
quantum yield of 6 was 0.06993. First, Figure 2 showed the fluores-
cent intensity ratio (I0/I) of 6 in water after adding Ag+, Zn2+, Fe3+
,
Cd2+, Pb2+, Ca2+, Cu2+, Mg2+, and Ba2+ ions, respectively. The fluores-
cent behavior of 6 was strong. Interestingly, once Fe3+ was added to
the water solution of 6, an decreased 8.71-fold of the fluorescent
intensity was observed at 465 nm. Because four carboxylic groups
of 6 was able to bind with Fe3+, accompanied with transforming a
high fluorescence (cyan) to a weak fluorescence (colorless, inset of
Fig. 2). The fluorscence quantum yield of complex 6-Fe3+ was
0.003046. The time of 6 (1 ꢀ 10ꢁ5 M) towards Fe3+ (1 equiv) in
water for the reaction time curve reaching a stable plain was
needed for 3 min after Fe3+ addition (Fig. S5, see Supplementary
data).
⇑
Corresponding author. Tel./fax: +86 29 87091952.
E-mail address: orgxuhui@nwsuaf.edu.cn (H. Xu).
0968-0896/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmc.2012.11.005

Y. Yan et al. / Bioorg. Med. Chem. 21 (2013) 508–513
509
Hg2+-promoted deprotection reaction
fluorophore
O
S
S
OH
OH
O
O
O
O
O
binding with a metal ion
S
S
OH
OH
O
6
O
improving water solubility
Figure 1. Chemical structure of 6.
O
Br
O
O
Br
Br
OH
OH
diphenyl ketone
BrCH2CH2CH2Br
O
O
McMurry reaction
4h, 30%
K2CO3/DMF
rt/24 h, 44%
O
Br
3
1
2
O
O
O
O
CHO
CHO
p-hydroxybenzaldehyde
K2CO3/KI/CH3CN
reflux/8h, 67%
4
O
S
OCH3
OCH3
O
O
O
S
O
O
HSCH2COOCH3
.
BF3 Et2O/CH2Cl2
0oC/5h, 94%
S
S
OCH3
OCH3
O
5
O
(1) 0.1M NaOH/THF/CH3OH
rt./24h
(2) 0.1M HCl
6
44%
Scheme 1. Synthesis of 6.
To obtain more insight into the fluorescent properties, the titra-
tion reaction curve of 6 toward Fe3+ was investigated (Fig. 3). The
fluorescent intensity of 6 decreased in response to the increases
in the concentration of the added Fe3+ in water, and the titration
reaction curve showed a steady increase until a plain was reached
when 1 equiv of Fe3+ ion was added. Job0s plots of fluorescence ob-
tained for 6-Fe3+ system in water obviously demonstrated a forma-
tion of 1:1 stoichiometry (Fig. 4). Meanwhile, the dissociation
constant (Kd) was calculated to be 1.41 ꢀ 10ꢁ4 M according to
the fluorescence titration profile (Fig. S12).
Then fluorescent intensity ratio (I/I0) of complex 6-Fe3+ in water
after adding different metal ions such as Ag+, Zn2+, Fe3+, Cd2+, Pb2+
,
Ca2+, Cu2+, Mg2+, Hg2+, and Ba2+ was examined in Figure 5. The fluo-
rescent intensity of complex 6-Fe3+ was almost a constant minimal
value when pH >4.0 in water (Fig. S13). After adding Hg2+ to the
water solution of complex 6-Fe3+, an increased 5.86-fold of the
fluorescent intensity was observed at 474 nm. Because complex
6-Fe3+ was deprotected the dithioacetal by Hg2+ to give aldehyde
4, accompanied with transforming a weak fluorescence (colorless,
inset of Fig. 5) to a high fluorescence (cyan). The fluorscence quan-

510
Y. Yan et al. / Bioorg. Med. Chem. 21 (2013) 508–513
Figure 5. Fluorescent intensity ratio (I/I0) of complex 6-Fe3+ (1 ꢀ 10ꢁ5 M) in
distilled water (pH 5.69) at 298 K in the presence of 10.0 equiv of different metal
Figure 2. Fluorescent intensity ratio (I0/I) of 6 (1 ꢀ 10ꢁ5 M) in distilled water (pH
ions. Inset: Photograph of 6-Fe3+ after adding different metal ions. A: 6-Fe3+
(1.0 ꢀ 10ꢁ5 M); B–L: 6-Fe3++Hg2+, Mg2+, Cu2+, Ba2+, Ca2+, Zn2+, Ag+, Pb2+, Cd2+
,
5.69) at 298 K upon addition of 10.0 equiv of different metal ions. Inset: photograph
of 6 after adding different metal ions. A: 6 (1.0 ꢀ 10ꢁ5 M); B–K: 6+Fe3+, Ca2+, Cd2+
Mg2+, Cu2+, Zn2+, Ba2+, Ag+, and Pb2+ (1.0 ꢀ 10ꢁ4 M).
,
Fe3+(1.0 ꢀ 10ꢁ4 M), and 4 (1.0 ꢀ 10ꢁ5 M).
Figure 3. Fluorescence titration of 6 (1 ꢀ 10ꢁ5 M) with Fe(ClO4)3 in distilled water
(pH 5.69) at 298 K. [Fe3+]: 0.25, 0.5, 0.75, 1.0, 1.25, 1.5, 1.75, 2.0, 2.5 ꢀ 10ꢁ5 M.
Figure 6. Fluorescence titration of compound 6-Fe3+ (1 ꢀ 10ꢁ5 M) with Hg(ClO4)2
in distilled water (pH 5.69) at 298 K. [Hg2+]: 0.25, 0. 5, 0.75, 1.0, 1.25, 1.5, 1.75, 2.0,
3.0, 5.0, 10 ꢀ 10ꢁ5 M. The excitation wavelength was 320 nm and the emission
intensity was measured at 474 nm.
tum yield of complex 6-Fe3+ with Hg2+ (2 equiv) was 0.02718. The
time of complex 6-Fe3+ (1 ꢀ 10ꢁ5 M) towards Hg2+ (2 equiv) in
water for the reaction time curve reaching a stable plateau was
needed for 5 min (Fig. S17). The titration reaction curve of complex
6-Fe3+ toward Hg2+ was investigated as Fig. 6. The fluorescent
intensity of complex 6-Fe3+ increased in response to the increases
in the concentration of the added Hg2+ in water, and the titration
reaction curve showed a steady increase until a plateau was
reached when 2 equiv of Hg2+ ion was added.
To examine the sensitivity of complex 6-Fe3+ towards Hg2+, its
detection limit was evaluated as Fig. 7. The fluorescence titration
profile of complex 6-Fe3+ (5 ꢀ 10ꢁ7 M) with Hg2+ demonstrated
that Hg2+ could be detected at least down to 2 ppb,9 and the fluo-
rescent intensity of complex 6-Fe3+ increased linearly with the
concentration of Hg2+ ion (0–10) ꢀ 10ꢁ9 M (R2 = 0.9977). To inves-
tigate the utility of complex 6-Fe3+ as an ion-selective fluorescent
chemosensor for Hg2+ ion, the cross-contamination experiments
were conducted in the presence of Hg2+ at 2 ꢀ 10ꢁ5 M mixed with
other metal ions such as Ag+, Zn2+, Fe3+, Ca2+, Mg2+, Ba2+, Pb2+, Cd2+
Figure 4. Job0s plots according to the method for continuous variations, indicating
the 1:1 stoichiometry for 6-Fe3+ (the total concentration of
2.446 ꢀ 10ꢁ5 M).
6
and Fe3+ is

Y. Yan et al. / Bioorg. Med. Chem. 21 (2013) 508–513
511
which would let it to be a single-use Hg2+ sensor for practical appli-
cations. More significantly, the strategy described in this paper at
least provides a new way for the design of novel fluorescent
sensors.
4. Experimental
4.1. General remarks
All solvents and reagents were used as obtained from commer-
cial sources without further purification. Zn(ClO4)2ꢂ6H2O, Fe(-
ClO4)2ꢂ6H2O, Ca(ClO4)2ꢂ4H2O, Mg(ClO4)2, Ba(ClO4)2, Pb(ClO4)2,
Cd(ClO4)2, Cu(ClO4)2, Hg(ClO4)2 and AgPF4 were investigated as
sources for metal ions. Analytical thin-layer chromatography
(TLC) and preparative thin-layer chromatography (PTLC) were per-
formed with silica gel plates using silica gel 60 GF254 (Qingdao
Haiyang Chemical Co., Ltd). Melting points are uncorrected. Infra-
red spectra (IR) were recorded on a Bruker TENSOR 27 spectrome-
ter. Nuclear magnetic resonance spectra (NMR) were recorded on a
Bruker Avance III 500 MHz instrument in CDCl3 or DMSO-d6 (1H at
500 MHz and 13C at 125 MHz) using TMS (tetramethylsilane) as the
internal standard. High-resolution mass spectra (HR-MS) and elec-
tron ionization mass spectrometry (EI-MS) were carried out with
APEX II Bruker 4.7T AS instrument and HP 5988 mass spectrometry
instrument, respectively. Fluorescent spectra were determined on
a Hitachi F-4500 spectrophotometer. UV–Visible spectra were
determined on a Hitachi U-3310 spectrophotometer.
Figure 7. Fluorescence titration of complex 6-Fe3+ (5 ꢀ 10ꢁ7 M) with Hg(ClO4)2 in
distilled water at 298 K. [Hg2+]: 0, 2, 2.5, 4, 5, 6, 7.5, 8, 10, 20, 30, 40, 50, 100 ppb.
The excitation wavelength was 320 nm.
4.2. Synthesis of 2
A mixture of 4, 40-dihydroxybenzophenone (4.3 g, 20 mmol), 1,
3-dibromopropane (10.0 g, 50 mmol), and K2CO3 (13.8 g,
100 mmol) in DMF (20 mL) under N2 was stirred at room temper-
ature for 24 h. Then the resulting mixture was extracted with
CH2Cl2. The CH2Cl2 extract was dried over anhydrous Na2SO4,
and concentrated under reduced pressure to give a residue, which
was purified by column chromatography over silica gel eluting
with PE/EtOAc (20:1, v/v) to give 2 (4.06 g, 44% yield) as a white so-
lid: CAS: 288248-49-7; mp 62–63 °C (lit. 62–64 °C) 8b; IR (cmꢁ1):
2931, 2868, 1731, 1637, 1601, 1504, 1416, 1244, 838, 763; 1H
NMR (500 MHz, CDCl3) d: 7.77 (d, J = 8.5 Hz, 4H), 6.96 (d,
J = 8.5 Hz, 4H), 4.18 (t, J = 5.5 Hz, 4H), 3.61 (t, J = 6.0 Hz, 4H),
2.33–2.38 (m, 4H); 13C NMR (125 MHz, CDCl3) d: 194.4, 162.0,
132.3, 131.0, 114.0, 65.5, 32.2, 29.8; HRMS (ESI) m/z Calcd for
Figure 8. Results of the competition experiments between Hg2+ and selected metal
ions in distilled water at 298 K; the concentration of Hg2+ was 2 ꢀ 10ꢁ5 M and that
of each competing metal ions was 1 ꢀ 10ꢁ4 M.
C
19H21O3Br2 [M+H]+ 454.9852. Found 454.9844.
and Cu2+ at 1 ꢀ 10ꢁ4 M, respectively (Fig. 8). It demonstrated that
the selectivity of complex 6-Fe3+ towards Hg2+ ion was almost
not affected by other competitive ions. As described in the partial
1H NMR spectra of Fig. 9a, when complex 6-Fe3+ was treated with
2 equiv of Hg2+, the resonance of the aldehyde proton appeared
with the advance of time as compared with 4. After 5 min, the
whole aldehyde proton appeared at 9.86 ppm. Based on the above
1H NMR data, the proposed sensing mechanism was described as
Fig. 9b.
4.3. Synthesis of 3
TiCl4 (2.25 mL, 18.8 mmol) was added dropwise to a stirred sus-
pension of Zn powder (2.6 g, 40.0 mmol) in dry THF (70 mL) under
N2 below ꢁ10 °C. After adding, the reaction was allowed to proceed
at reflux for 2 h. Then a solution of 2 (2.28 g, 5 mmol) and diphenyl
ketone (0.91 g, 5 mmol) in THF (65 mL) was added to the suspen-
sion of the titanium reagent. After adding, the reaction was al-
lowed to proceed at reflux for 4 h. The reaction mixture was
poured into 10% aq K2CO3 (200 mL), and after vigorous stirring
for 5 min, the dispersed insoluble material was removed by vac-
uum filtration. The organic layer was separated, and the aqueous
layer was extracted with CH2Cl2 (3 ꢀ 150 mL). The combined or-
ganic fractions were washed with water and dried over anhydrous
Na2SO4. The mixture was concentrated and purified by column
chromatography over silica gel eluting with PE/EtOAc (50:1, v/v)
to give 3 (906 mg, 30% yield) as a white solid: CAS: 1067211-23-
7; mp 119–121 °C (lit. 123–125 °C) 8b; IR (cmꢁ1): 2936, 2931,
1698, 1607, 1508, 1469, 1243, 756, 698; 1H NMR (500 MHz, CDCl3,)
3. Conclusions
In conclusion, we have developed a new selective and sensitive
fluorescence ‘on-off-on’ probe 6 for sequential recognition of Fe3+
and Hg2+ in water. Especially the complex 6-Fe3+ could behave as
a
‘turn on’ fluorescent sensor over a wide-range pH value
(pH >4.0) for detection of Hg2+. The selectivity of this complex for
Hg2+ over other heavy and transition metal ions is excellent, and
its sensitivity for Hg2+ is at 2 ppb in water. Therefore, in reality
probe 6 would be most useful when pre-equilibrated with Fe3+
,

512
Y. Yan et al. / Bioorg. Med. Chem. 21 (2013) 508–513
(a)
(b)
O
O
O
O
CHO
CHO
O
O
O
S
OH
S
S
O
HO
HO
O
O
Fe3+
Hg2+
O
OH
S
O
6-Fe3+
4
Figure 9. Partial spectra of complex (6+1 equiv of Fe3+), and complex (6+1 equiv of Fe3+ + 2 equiv of Hg2+ in DMSO-d6) (a); proposed sensing mechanism for complex
(6+1 equiv of Fe3+) with Hg2+ (b).
d: 7.05–7.12 (m, 6H), 7.01 (d, J = 7.0 Hz, 4H), 6.92 (d, J = 8.5 Hz, 4H),
6.62 (d, J = 8.5 Hz, 4H), 4.00 (t, J = 6.0 Hz,4H), 3.56 (t, J = 6.5 Hz,4H),
2.24–2.29 (m, 4H); 13C NMR (125 MHz, CDCl3) d: 157.2, 144.2,
140.0, 136.6, 132.6, 131.4, 127.7, 126.1, 113.6, 65.1, 32.5, 30.1;
HRMS (ESI) m/z Calcd for C32H30O2Br2 [M]+ 604.0607. Found
604.0591.
(20 mL) was stirred at 0 °C for 5 h, and then quenched with the
addition of water (50 mL). The mixture was extracted with CH2Cl2
(3 ꢀ 80 mL). The combined organic layers were washed with brine
and dried over anhydrous Na2SO4. The mixture was concentrated
and purified by PTLC using PE/EtOAc (2:1, v/v) as the eluent to give
5 (506 mg, 94% yield) as a yellow liquid: IR (cmꢁ1): 2913, 2878,
1735, 1605, 1508, 1471, 1435, 1241, 834, 700; 1H NMR
(500 MHz, CDCl3,) d: 7.36 (d, J = 8.5 Hz, 4H), 7.05–7.11 (m, 6H),
7.00 (d, J = 6.5 Hz, 4H), 6.90 (d, J = 9.0 Hz, 4H), 6.85 (d, J = 9.0 Hz,
4H), 6.61 (d, J = 8.5 Hz, 4H), 5.29 (s, 2H), 4.10 (t, J = 6.0 Hz, 4H),
4.05 (t, J = 6.0 Hz, 4H), 3.70 (s, 12H), 3.44 (s, 2H), 3.41 (s, 2H),
3.19 (s, 2H), 3.16 (s, 2H), 2.18–2.23 (m, 4H); 13C NMR (125 MHz,
CDCl3) d: 170.5, 158.9, 157.3, 144.3, 136.5, 132.6, 131.4, 130.2,
129.5, 129.2, 127.7, 126.1, 115.5, 114.6, 113.5, 64.5, 64.1, 53.0,
4.4. Synthesis of 4
A mixture of 3 (606 mg, 1 mmol), 4-hydroxybenzaldehyde
(366 mg, 3 mmol), K2CO3 (414 mg, 3 mmol) and KI (10 mg) in
MeCN (20 mL) was refluxed for 8 h under N2. After cooling, the
mixture was filtered, and the filtrate was collected, concentrated
and purified by preparative thin-layer chromatography (PTLC)
using PE/EtOAc (2:1, v/v) as the eluent to give 4 (461 mg, 67%
yield) as a yellow solid: mp 66–68 °C; IR (cmꢁ1): 2927, 2745,
1688, 1599, 1507, 1240, 1057, 832, 700; 1H NMR (500 MHz, CDCl3,)
d: 9.88 (s, 2H), 7.81 (d, J = 8.0 Hz, 4H), 7.06 (t, J = 7.5 Hz, 6H), 6.99 (t,
J = 8.0 Hz, 8H), 6.91 (d, J = 8.5 Hz, 4H), 6.62 (d, J = 8.5 Hz, 4H), 4.21
(t, J = 6.0 Hz, 4H), 4.06 (t, J = 5.5 Hz, 4H), 2.22-2.27 (M, 4H); 13C
NMR (125 MHz, CDCl3) d: 191.1, 164.2, 157.5, 144.5, 140.2, 139.7,
136.9, 132.9, 132.3, 131.6, 130.3, 128.0, 126.4, 115.1, 113.8, 65.2,
52.5, 33.7, 29.3; HRMS (ESI) m/z Calcd for
C58H60O12S4Na
[M+Na]+ 1099.2860. Found 1099.2854.
4.6. Synthesis of 6
A mixture of 5 (366 mg, 0.34 mmol) and aq. NaOH solution
(13.6 mL, 0.1 M) in methanol/THF (1:1, v/v, 15 mL) was stirred at
rt for 24 h. Then the pH value of the mixture was adjusted to 2–
3 by aq HCl solution (0.1 M), and the mixture was extracted with
CH2Cl2 (3 ꢀ 50 mL). The combined organic layers were dried over
anhydrous Na2SO4, and concentrated under reduced pressure to
give a residue, which was recrystallized from methanol/CH2Cl2 to
afford 6 (152 mg, 44% yield) as a white solid: mp 127–128 °C; IR
(cmꢁ1): 3371, 2919, 1703, 1605, 1508, 1243, 1173,837, 651; 1H
64.2, 29.4; HRMS (ESI) m/z Calcd for
C
46H40O6Na [M+Na]+
711.2717. Found 711.2707.
4.5. Synthesis of 5
A
mixture of
4 (344 mg, 0.5 mmol), methyl thioglycolate
(0.27 ml, 3 mmol) and BF3ꢂEt2O (0.38 ml, 3 mmol) in dry CH2Cl2

Y. Yan et al. / Bioorg. Med. Chem. 21 (2013) 508–513
513
A. J.; Bichenkova, E. V.; Bryce, R. A.; Barker, C. J.; Diaz, A.; Kremer, C.; Freeman, S.
Chem. Commun. 2008, 5167.
NMR (500 MHz, CDCl3,) d: 12.65 (s, 4H), 7.30 (d, J = 8.0 Hz, 4H), 7.12
(t, J = 7.5 Hz, 4H), 7.07 (t, J = 6.5 Hz, 2H), 6.94 (t, J = 6.0 Hz, 8H), 6.85
(d, J = 8.0 Hz, 4H), 6.70 (d, J = 8.0 Hz, 4H), 5.24 (s, 2H), 4.09 (t,
J = 5.5 Hz, 4H), 4.03 (t, J = 5.5 Hz, 4H), 3.39 (s, 4H), 3.22(s, 2H),
3.19 (s, 2H), 2.12–2.14 (m, 4H); 13C NMR (125 MHz, DMSO) d:
171.2, 158.7, 157.5, 144.3, 136.2, 132.5, 131.4, 131.2, 129.3,
128.3, 126.7, 115.0, 114.2, 64.8, 64.5, 52.7, 34.5, 29.1; HRMS (ESI)
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Acknowledgments
The present research was partly supported by National Natural
Science Foundation of China (No.31071737, 31171896), and Spe-
cial Funds of Central Colleges Basic Scientific Research Operating
Expenses (QN2009045).
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.bmc.2012.11.005.
References and notes
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