Highly selective detection of Hg2+ ion by push-pull-type purine nucleoside-based fluorescent sensor

Article abstract of DOI:10.1016/j.tet.2014.05.050

Highly selective detection of Hg²⁺ ion has been achieved using the push-pull-type purine nucleoside-based fluorescent sensor L1. The sensor L1 incorporating aza-18-crown-6 at C6 position of purine nucleoside, is highly sensitive and selective t

Full text of DOI:10.1016/j.tet.2014.05.050

Tetrahedron xxx (2014) 1e5
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Highly selective detection of Hg2þ ion by pushepull-type purine
nucleoside-based fluorescent sensor
,
Shao-Hua Gao a, Ming-Sheng Xie a, Hai-Xia Wang a, Hong-Ying Niu b, Gui-Rong Qu a
,
*
,
Hai-Ming Guo a
*
a Collaborative Innovation Center of Henan Province for Green Manufacturing of Fine Chemicals, Key Laboratory of Green Chemical Media and
Reactions, Ministry of Education, School of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang, Henan 453007, PR China
b School of Chemistry and Chemical Engineering, Henan Institute of Science and Technology, Xinxiang 453003, China
a r t i c l e i n f o
a b s t r a c t
Highly selective detection of Hg2þ ion has been achieved using the pushepull-type purine nucleoside-
based fluorescent sensor L1. The sensor L1 incorporating aza-18-crown-6 at C6 position of purine
nucleoside, is highly sensitive and selective toward Hg2þ ion in CH3CNeH2O mixture (92/8, v/v). The
Article history:
Received 18 January 2014
Received in revised form 7 May 2014
Accepted 15 May 2014
Available online xxx
detection limit for the fluorescent sensor L1 toward Hg2þ ion is 7.8 ꢀ 10ꢁ8
.
Ó 2014 Elsevier Ltd. All rights reserved.
Keywords:
Purine nucleoside
Fluorescent sensor
Hg2þ ion
Pushepull type
Aza-18-crown-6
1. Introduction
weak fluorescent properties.9 Herein, we designed and synthesized
the new purine nucleoside-based fluorescent sensors to enhance
their fluorescent properties via pushepull strategy10. As shown in
Fig. 1, the purine part was used as the chromophore, the electron
donating amino derivative was attached to the C6-position of pu-
rine to exert a push influence, and the electron-withdrawing aryl
group was incorporated to the C8-position of purine to increase the
conjugated system and offer a pull effect. By this pushepull effect,
the electron flowed across the fluorescent sensor, and its fluores-
cent properties were improved. In the pushepull-type purine
nucleoside-based fluorescent sensor, if a chelating group was in-
troduced into the C6-position of purine, the fluorescent sensor may
show high selectivity towards Hg2þ ion. Herein, we wish to report
The Hg2þ ion has attracted much more concerns for its delete-
rious effects on human health and natural ecosystem.1 The toxicity
of Hg2þ ion, even at very low concentration, can cause several
human healthy problems including vision lose, serious cognitive,
motion disorders, and prenatal brain injury.2 In spite of the high
toxicity of Hg2þ ion, mercury and mercuric salts are still applied
extensively in industrial production.3 Duo to their widely use, the
highly sensitive and selective detection analytical method of Hg2þ
ion is still in great demand.4 Among numerous sensing methods,
fluorescence technology is especially appealing.5 Up to now, great
endeavors have been devoted to develop various outstanding
fluorescent sensors.6 However, most of them were designed based
on the thiophilic affinity of the Hg2þ ion,7 and purine nucleosides
had never been used as chromophore. Considering the better bi-
ological compatibility of purine nucleosides and our systematic
work on modification of purine nucleosides,8 we wish to report
a new fluorescent sensor with purine nucleoside as the chromo-
phore and no-sulfur chelator as the receptor for the selective de-
tection of Hg2þ ion.
Hg2+
Push
Pull
N
N
Hg2+
N
N
N
N
N
N
O2N
AcO
O2N
AcO
N
N
O
O
Purine nucleosides are the important subunit of RNA and DNA,8
however, they are hard to serve as fluorescent sensors due to their
H
H
H
H
H
H
H
H
OAc OAc
OAc OAc
L1-Hg2+
L1
* Corresponding authors. E-mail addresses: quguir@sina.com (G.-R. Qu),
guohm518@hotmail.com (H.-M. Guo).
Fig. 1. Pushepull-type purine nucleoside-based fluorescent sensors.
0040-4020/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2014.05.050
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S.-H. Gao et al. / Tetrahedron xxx (2014) 1e5
a new pushepull-type purine nucleoside-based fluorescent sensor
quench was observed for Pd2þ and Fe3þ ions. When Zn2þ ions were
added, slightly fluorescence enhancements were detected. In ad-
dition, other metal ions including Cr3þ, Mn2þ, Co2þ, Ni2þ, Cu2þ, Agþ,
Cd2þ, Pb2þ did not result in any noticeable changes. However, when
these metal ions were added to fluorescent sensor L2, there was no
distinctive fluorescence change (Fig. S1). The effect of various metal
ions on the fluorescence intensity of L1 was shown in Fig. 2B, which
indicated that the selectivity of L1 toward Hg2þ ion over other
metal ions is extremely high.
In order to understand the coordination effect of fluorescent
sensor L1 with Hg2þ ion in detail, the fluorescence titration ex-
periments of the sensor L1 with Hg2þ ion were performed in
CH3CNeH2O mixture (92/8, v/v). As shown in Fig. 3A, when the
amount of Hg2þ ion was increased from 0 to 30 equiv, the in-
tensities of the fluorescent band of L1 centered at 403 nm de-
creased gradually, and the fluorescence was almost quenched
completely when 3.0 equiv of Hg2þ was added. The change of ab-
sorption spectrum of L1 with different concentrations of Hg2þ ion
were shown in Fig. S2. Furthermore, the fluorescence intensities of
L1 is linearly proportional to the concentration of Hg2þ ions ranging
from 0 to 4ꢀ10ꢁ6 mol Lꢁ1 (Fig. 3B), which indicated that the fluo-
rescent sensor L1 could be applied for the Hg2þ-polluted analysis.
The detection limit of the fluorescent sensor L1 for Hg2þ ion is
estimated to be 7.8ꢀ10ꢁ8 M.
for the selective detection of Hg2þ ion.
2. Results and discussion
Initially, the fluorescent sensors L1eL2 were synthesized as
follow: the 4-nitrophenyl group was chosen as the electron-
withdrawing group attaching to the C8 position of the adenosine
to form a push effect, and the aza-18-crown-6 or aza-15-crown-5
were incorporated to the C6 position of purine nucleosides as
chelators (Scheme 1, L1eL2).
Scheme 1. Synthesis route of L1eL2. a) 1-Iodo-4-nitrobenzene, Pd(OAc)2, CuI, Cs2CO3,
N2; b) Ac2O, pyridine; c) isoamyl nitrite, CHBr3; d) RH, EtOH, Et3N.
Subsequently, the fluorescence spectra of L1eL2 (8.0 mM) in the
presence of various mental ions (6.0 equiv) were conducted in the
solution of CH3CNeH2O mixture (92/8, v/v). As shown in Fig. 2A,
the addition of 6.0 equiv of Hg2þ ion results in an obviously quench
fluorescence of L1 at 403 nm. Meanwhile, a slightly fluorescence
Fig. 3. (A) Fluorescence spectra of L1 (8.0 mM) in the presence of different concen-
trations of Hg2þ ion (0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 1.0, 1.5, 2.0, 3.0, 4.0, 5.0, 6.0, 8.0, 10,
15, 20, 30 equiv) in CH3CNeH2O mixture (92/8, v/v) solution. (B) Plot of fluorescence
intensities of L1 (8.0 m
M) in the presence of different concentrations of Hg2þ ion (0, 0.1,
0.2, 0.3, 0.4, 0.5 equiv) at 403 nm.
Subsequently, the TPEN titration experiments of L1eHg2þ sys-
tem were tried to testify the coordination between L1 and Hg2þ
.
When excess amount of TPEN were added, the fluorescence of L1
recovered gradually (Fig. 4). The fluorescence value of L1 recovered
to the baseline value when the concentrations of TPEN reached to
1.2 equiv of Hg2þ ion. It suggested that the binding of Hg2þ ion with
L1 was the reason of the fluorescence quenching. To our delight, the
Fig. 2. (A) Fluorescence responses of L1 (8.0
mM) toward various metal ions (6.0 equiv).
(B) Fluorescence responses of L1 (8.0 M) upon addition of different metal cations
m
(6.0 equiv), and the bars are relative fluorescence intensities (I0/I). All the experiments
were carried out in CH3CNeH2O mixture (92:8, v/v) (lex¼310 nm).
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S.-H. Gao et al. / Tetrahedron xxx (2014) 1e5
3
Then, the 1H NMR titration experiments were carried out in
CD3CN to verify the binding mode between L1 and Hg2þ ion. The
partial 1H NMR spectra of L1 in the presence of different concen-
tration of Hg2þ ion were shown in Fig. 6. When the concentration of
Hg2þ ion increased, an obvious downfield shift of proton Ha at the
C2 position of purine nucleoside was observed. Meanwhile, the
signals of protons Ha, Hb, and Hc all became broad. These results
indicated that there was a coordination effect between Hg2þ and
the aza-18-crown-6 moiety of sensor L1. Based on an overall con-
sideration of various evidences, a proposed binding mode was
shown in Fig. 7. The Hg2þ ion was fixed by two moleculars of
fluorescent sensor L1, in which the aza-18-crown-6 moieties were
functioned as the binding units.
Fig. 4. Fluorescence spectra of the solutions of L1 (8.0 m mM) in the
M) and Hg2þ (32
presence of different concentrations of TPEN (0, 0.2, 0.5, 0.8, 1.0, 1.2, 1.5, 2.0, 4.0,
8.0 equiv of Hg2þ) in CH3CNeH2O mixture (92:8, v/v).
binding process is reversible, which indicated that the sensor L1 is
the first ‘OneOff’ type purine nucleoside-based fluorescent sensor
for Hg2þ ion.
In order to examine the potential applicability of L1 for the se-
lective detection of Hg2þ ion, the ions competition experiments
were conducted. In CH3CNeH2O system (92/8, v/v), various metal
ions, including Cr3þ, Mn2þ, Fe3þ, Co2þ, Ni2þ, Cu2þ, Zn2þ, Pd2þ, Agþ,
Cd2þ, Pb2þ ions were investigated. As shown in Fig. 5A, the sensor
L1 showed almost unchanged responses to Hg2þ after adding other
competing metal ions. Next, the absorption-based Job plot curve
was conducted (Fig. 5B) to investigate the coordination ratio of L1
with Hg2þ ion. The absorption value at 300 nm reached the maxi-
mum when X¼0.33 [X¼[Hg2þ]/([Hg2þ]þ[L1])], which indicated
that L1 formed a 2:1 complex with Hg2þ ion.
Fig. 6. Partial 1H NMR (400 MHz) spectra of L1 in CD3CN. (1) [Hg2þ ]/[L1]¼0; (2) [Hg2þ
]/[L1]¼0.2 equiv; (3) [Hg2þ ]/[L1]¼0.5 equiv; (4) [Hg2þ ]/[L1]¼0.7 equiv; (5) [Hg2þ
]/[L1]¼1.0 equiv.
Fig. 7. Proposed interaction model between L1 and Hg2þ ion.
3. Conclusion
A novel pushepull purine nucleoside-based fluorescent sensor
L1, incorporating aza-18-crown-6 as the binding units, was
designed and synthesized. The sensor L1 displays specific selec-
tivity and sensitivity to Hg2þ ion with a low detection limit.
Meanwhile, the fluorescent sensor L1 is linearly proportional to the
concentration of Hg2þ ions ranging from 0 to 4ꢀ10ꢁ6 mol Lꢁ1
,
which could be applied for the Hg2þ-polluted analysis. Further
application of the fluorescent sensor L1 is currently underway.
4. Experimental section
4.1. General information
Fig. 5. (A) Fluorescence response of L1 (8.0
mixture (92/8, v/v) containing 240 M of various mental ions. (B) The change of the
mM) to 48 m
M of Hg2þ in CH3CNeH2O
All reagents and solvents were commercially available and used
without further purification. The 1H NMR spectra were recorded at
400 MHz NMR spectroscopy and 13C NMR spectra were recorded at
m
absorption value at 300 nm when the total concentration of Hg2þ ions and L1 is 40
mM
and the ratio of Hg2þ increased (0.05, 0.1, 0.15, 0.2, 0.25, 0.28, 0.3, 0.33, 0.4, 0.5, 0.6, 0.7,
0.8, 0.9).
100 MHz NMR spectroscopy. Proton chemical shifts d were given in
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4
S.-H. Gao et al. / Tetrahedron xxx (2014) 1e5
parts per million relative to tetramethylsilane (Si(CH3)4¼0.00 ppm)
in CDCl3. 1H NMR coupling constants (J) are reported in Hertz (Hz).
Multiplicity are indicated as the following: s (singlet), d (doublet),
dd (doublet doublet), m (multi). High resolution mass spectra are
taken using Q-TOF system, with Electrospray Ionization (ESI) as the
ionization method used for the HRMS measurement. Absorption
spectra were measured on UV-1700 spectrophotometer. Fluores-
cence emission spectra were measured FP-6500. All reactions were
monitored by thin layer chromatography (TLC). For column chro-
matography 200e300 mesh silica gel (GF254) was used as the sta-
tionary phase.
temperature. After completion of the reaction, the solvent was
evaporated under reduced pressure and the residue was purified by
chromatography over silica gel eluting with petroleum: ethyl ace-
tate to give the orange oil compound 4. The yield of Compound 4
was 55%. 1H NMR (400 MHz, CDCl3):
d 8.79 (s, 1H), 8.45 (m, 2H),
8.08 (d, 2H), 6.46 (dd, J¼6.0, 4.0 Hz, 1H), 5.98 (t, 1H), 5.87
(d, J¼4.0 Hz, 1H), 4.54e4.51 (m, 1H), 4.43e4.40 (m, 1H), 4.37e4.29
(m, 1H), 2.13 (s, 3H), 2.05 (s, 3H), 2.02 (s, 3H).
4.3.4. (2R,3R,4R,5R)-2-(Acetoxymethyl)-5-(8-(4-nitrophenyl)-6-
(1,4,7,10,13-pentaoxa-16-azacyclooctadecan-16-yl)-9H-purin-9-yl)
tetrahydrofuran-3,4-diyl diacetate (L1). To a round-bottom flask
compound 4 (0.1154 g, 0.2 mmol) and aza-18-crown-6 (0.0631 g,
4.2. Fluorescent response experiments
0.24 mmol) was dissolved in alcohol. Then, triethylamine (31 mL,
Stock solutions of L1, L2 in CH2Cl2 and each metal salt in H2O
were prepared. Test solutions were prepared by placing
0.1 mLe2.0 mL of the probe’s stock solution into test tubes. Then,
the CH2Cl2 solvent was removed. Subsequently the test samples
were dissolved by adding appropriate amount of CH3CN solution.
After that, appropriate amount of each metal stock solution was
added. At last, supplement defined amount of H2O to give the final
concentration. After complete mixing, measurements of UVevis
absorption and fluorescent emission were carried out with a 1.0 cm
standard quartz cell. For compound L1, excitation, 310 nm; emis-
sion, 350e500; slit widths for excitation and emission were 5.0 nm.
For compound L2, excitation, 310 nm; emission, 350e500; slit
widths for excitation and emission were 5.0 nm and 10.0 nm.
0.22 mmol) was added, after that the reaction mixture was reflux at
80 ꢂC for 8 h. After completion of the reaction, the solvent was
evaporated under reduced pressure, and the residue purified by
column chromatography on silica gel with petroleum ether:ethyl
acetate to give the desired yellow oil product L1 in 65% yield. 1H
NMR (400 MHz, CDCl3):
d
8.38 (d, J¼8.8 Hz, 2H), 8.33 (s, 1H), 7.97
(d, J¼8.8 Hz, 2H), 6.51 (dd, J¼4.0, 6.0 Hz, 1H), 6.07 (t, J¼5.8 Hz, 1H),
5.88 (d, J¼4 Hz, 1H), 4.56e4.52 (m, 1H), 4.40e4.35 (m, 2H), 3.83 (br,
4H), 3.69 (s, 8H), 3.67 (s, 12H), 2.11 (s, 3H), 2.07 (s, 3H), 2.01 (s, 3H).
13C NMR (100 MHz, CDCl3):
d 170.5, 169.4, 169.4, 148.9, 130.6, 124.1,
120.1, 88.3, 80.2, 80.2, 72.3, 70.9, 70.6, 62.8, 20.8, 20.5, 20.4. HRMS
(ESI): m/z [MþH]þ calcd for C34H45N6O14
: 761.2988; found:
761.2982.
4.3. General procedure for the synthesis of L1 and L2
4.3.5. (2R,3R,4R,5R)-2-(Acetoxymethyl)-5-(8-(4-nitrophenyl)-6-
(1,4,7,10-tetraoxa-13-azacyclopentadecan-13-yl)-9H-purin-9-yl)tet-
rahydrofuran-3,4-diyl diacetate (L2). Follow the procedure of
compound L1, aza-15-crown-5 (0.0631 g, 0.24 mmol) was dissolved
4.3.1. (2R,3R,4S,5R)-2-(6-Amino-8-(4-nitrophenyl)-9H-purin-9-yl)-
5-(hydroxymethyl)tetrahydrofuran-3,4-diol (2). To a Schlenk tube
containing a stirbar, a mixture of adenosine (0.1335 g, 0.5 mmol), 4-
iodonitrobenzene (0.2489 g, 1.0 mmol), Pd(OAc)2 (0.0112 g,
0.05 mmol), CuI (0.2857 g, 1.5 mmol), Cs2CO3 (0.4073 g, 1.25 mmol)
were dissolved in DMF (3.0 mL). Then the Schlenk tube was fitted
with a rubber cap, evacuated and back-filled with nitrogen (this
sequence was repeated for three times). The reaction mixture was
stirred at 120 ꢂC for 24 h. After completion of the reaction, the DMF
was evaporated and the residue was purified by chromatography
over silica gel eluting with CH2Cl2:MeOH, affording compound 2
in alcohol then triethylamine (31 mL, 0.22 mmol) was added in, after
that the reaction mixture was reflux at 80 ꢂC for 8 h. After com-
pletion of the reaction, the solvent was evaporated under reduced
pressure, and the residue purified by column chromatography on
silica gel with petroleum ether:ethyl acetate to give the desired
yellow oil product L2 in 68% yield. 1H NMR (400 MHz, CDCl3):
d 8.37
(d, J¼8.8 Hz, 2H), 8.337 (s, 1H), 7.97 (d, J¼8.8 Hz, 2H), 6.53 (dd, J¼4.0
6.0 Hz, 1H), 6.06 (t, J¼5.8 Hz, 1H), 5.88 (d, J¼4 Hz, 1H), 4.52e4.57
(m, 1H), 4.36e4.39 (m, 1H), 3.88e3.99 (br, 6H), 3.67 (s, 8H), 3.65
(s, 5H), 2.10 (s, 3H), 2.07 (s, 3H), 2.01 (s, 3H). 13C NMR (100 MHz,
with orange solid (33%. yield). 1H NMR (400 MHz, DMSO-d6):
d 8.43
(d, J¼8.4 Hz, 2H), 8.19 (s, 1H), 8.03 (d, J¼8.4 Hz, 2H), 7.62 (s, br, 2H),
5.82 (d, J¼8.4 Hz,1H), 5.75e5.72 (m,1H), 5.53 (d, J¼6.4 Hz,1H), 5.23
(s, 1H), 5.15e5.13 (m, 1H), 4.17 (s, 1H), 3.97 (s, 1H), 3.71
(d, J¼12.4 Hz, 1H), 3.57 (t, J¼10.2 Hz, 1H).
CDCl3): d 170.6, 169.4, 169.35, 154.5, 152.86, 151.9, 148.6, 146.9,
135.8, 130.4, 124.0, 120.4, 88.1, 80.2, 72.2, 71.2, 71.2, 71.1, 70.8, 70.4,
70.4, 70.4, 70.2, 63.0, 20.7, 20.5, 20.4. HRMS (ESI): m/z [MþNa]þ
calcd for C34H45N6NaO14: 739.2546; found: 739.2551.
4.3.2. (2R,3R,4R,5R)-2-(Acetoxymethyl)-5-(6-amino-8-(4-
nitrophenyl)-9H-purin-9-yl)tetrahydrofuran-3,4-diyl diacetate (3).
Compound 2 (194.2 mg, 0.5 mmol) and DMAP (6.1 mg, 0.05 mmol,
10 mol %) was dissolved in pyridine and then acetic anhydride
Acknowledgements
We are grateful for financial support from the National Natural
Science Foundation of China (Nos. 21172059, 21272059, 21202039,
and 21372066), Excellent Youth Foundation of Henan Scientific
Committee (No. 114100510012), the Program for Innovative
Research Team from the University of Henan Province
(2012IRTSTHN006), the Program for Changjiang Scholars and
Innovative Research Team in University (IRT1061), Research Fund
for the Doctoral Program of Higher Education of China (No.
20124104110006) and the Program for Science & Technology In-
novation Talents in Universities of Henan Province (No.
13HASTIT013).
(188 mL, 2.0 mmol) was added in drop by drop. The mixture was
stirred for 24 h at room temperature. After completion of the re-
action, the solvent was evaporated under reduced pressure and the
residue was purified by chromatography over silica gel eluting with
CH2Cl2:MeOH, affording compound 3 with orange solid (51%. yield).
1H NMR (400 MHz, CDCl3):
(d, J¼8.4 Hz, 2H), 6.49 (dd, J¼5.8 4.4 Hz, 1H), 6.07 (t, J¼6.0, 1H), 5.85
(d, J¼4.0 Hz, 1H), 5.70 (s, br, 2H), 4.57e4.52 (m, 1H), 4.40e4.35 (m,
1H), 2.11 (s, 3H), 2.06 (s, 3H), 2.02 (s, 3H).
d
8.41 (s,1H), 8.42 (d, J¼9.2 Hz, 2H), 8.00
4.3.3. (2R,3R,4R,5R)-2-(Acetoxymethyl)-5-(6-bromo-8-(4-
nitrophenyl)-9H-purin-9-yl)tetrahydrofuran-3,4-diyl diacetate (4).
Compound 3 (257.2 mg, 0.5 mmol) was dissolved in tribromo-
methane, and then isoamyl nitrite (1.35 mL, 10.0 mmol) was
dropped slowly. The mixture was stirred for 3.5 h at room
Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.tet.2014.05.050.
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5
Chem. 2007, 72, 3550; (p) Kim, S. H.; Kim, J. S.; Park, S. M.; Chang, S.-K. Org. Lett.
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Please cite this article in press as: Gao, S.-H.; et al., Tetrahedron (2014), http://dx.doi.org/10.1016/j.tet.2014.05.050