Naphthalimide-based fluorescent off/on probes for the detection of thiols

Article abstract of DOI:10.1016/j.tet.2012.04.110

Herein, we report the design and synthesis of two naphthalimide-based fluorescent probes. These probes provided high on/off signal ratios and exhibited good selectivity towards thiols over other analytes. Thus, they were identified as good sensors for the

Full text of DOI:10.1016/j.tet.2012.04.110

Tetrahedron 68 (2012) 5363e5367
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Naphthalimide-based fluorescent off/on probes for the detection of thiols
,
Wei Sun a, Wenhua Li a, Jing Li a, Jian Zhang b, Lupei Du a, Minyong Li a
*
a Department of Medicinal Chemistry, Key Laboratory of Chemical Biology of Natural Products (MOE), School of Pharmacy, Shandong University, Jinan, Shandong 250012, China
b Institute of Immunopharmacology & Immunotherapy, School of Pharmacy, Shandong University, Jinan, Shandong 250012, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Herein, we report the design and synthesis of two naphthalimide-based fluorescent probes. These probes
provided high on/off signal ratios and exhibited good selectivity towards thiols over other analytes. Thus,
they were identified as good sensors for the detection of thiols both in living cells and in rabbit plasma
samples.
Received 16 March 2012
Received in revised form 23 April 2012
Accepted 28 April 2012
Available online 7 May 2012
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Naphthalimide
Fluorescent probes
Thiols
Cell imaging
1. Introduction
fluorescent chromophore.5 Notably, the naphthalimide fluorophore
could be quenched by electrophilic group, such as 2,4-
Biological molecules containing thiol groups, such as cysteine,
homocysteine and glutathione, played an essential role in the an-
tioxidant related processes. Generally, alternations in the level of
cellular thiols have been amalgamated with various diseases, such
as slowed growth, leucocyte loss, psoriasis, liver damage, cancer and
AIDS.1 Meanwhile, some aromatic thiols are poisonous to living life,
which have been considered as highly pollutant markers.2 Conse-
quently, selective detection of thiols will provide critical insight into
biological and environmental sciences. Because of the simplicity,
low detection limit, wide range of dynamic response and feasibility
of intracellular detection, fluorescent markers have attracted much
attention in recent years.3 An ideal fluorescent probe should not
only have high response, good selectivity and high stability, but also
be convenient to synthesize and fast response under mild condi-
tions. Herein, we report the design and synthesis of two fluorescent
thiol probes that have most of the above-mentioned desirable fea-
tures. These fluorescent probes could be particularly applicable for
the in vitro and in vivo detection of thiols.
dinitrobenzenesulfonyl (DNS) ester. Upon the treatment with
thiols, the DNS group could be removed, resulting in the formation
of fluorescent active products (4a and 4b in this case, Scheme 1).6
Our design strategy was based on the nucleophilic aromatic
substitution of thiols and thiol-mediated cleavage.4 In this design,
the naphthalimide moiety, which exhibits large Stokes’ shift
and long emission wavelength, was selected as the
* Corresponding author. Tel./fax: þ86 531 8838 2076; e-mail address: mli@
Scheme 1. Synthesis of probes 1a and 1b.
sdu.edu.cn (M. Li).
0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2012.04.110

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W. Sun et al. / Tetrahedron 68 (2012) 5363e5367
2. Results and discussion
2.1. Synthesis
The synthesis of the proposed probes 1a and 1b started from 4-
bromo-1,8-naphthalic anhydride (Scheme 1). After conversion to
the corresponding naphthalimide 2a and 2b in over 90% yield,7
compound 4a was received in two steps with over 75% yield.8
Probe 1a was achieved by reacting 4a with 2,4-
dinitrobenzenesulfonyl chloride in DCM (77% yield). In the case of
1b, compound 2b was first converted to the boronate 3b in 69%
yield. After oxidation to the phenol 4b (51% yield), the sulfonation
with 2,4-dinitrobenzenesulfonyl chloride gave the probe 1b in
about 73% yield.
2.2. Fluorometeric analysis
These two probes were initially evaluated for their ability to
detect PhSH under near physiological conditions (0.1 M phosphate
buffer, pH 7.4). Fig. 1 manifests the fluorescent intensity changes
according to the reaction time. Notably, these molecules present
very weak fluorescence. However, with the presence of PhSH, sig-
nificant fluorescence increase (more than 60-fold) was observed.
The probe 1b exemplified the quicker response towards PhSH, in
which the fluorescent intensity of probe 1b reached the plateau at
100 s after the addition of PhSH, and almost appeared no change
when longer reaction time was examined. For probes 1a, this
procedure took longer time to reach the maximum intensity (ca.
200 s) under the identical conditions. These results clearly rec-
ommend that the reactivity of probe 1b is higher than 1a.
Fig. 2. Fluorescence responses of probes 1a (a) and 1b (b) (10
mM) to different analytes
(25 M). Signals were acquired in 0.1 M phosphate buffer (0.1% DMSO), pH 7.4, and all
m
data were obtained at 560 nm (lex¼450 nm) after incubation with analytes at room
temperature.
40
30
20
feedback. Considering that 2,4-dinitrobenzenesulfonyl-based fluo-
rescent probes possibly react with various oxygen species (ROS),9
the fluorescence response to ROS was detected additionally. The
experimental outputs substantially disclosed that probe 1a has no
response to KO2 and H2O2.
Compared with probe 1a, probe 1b exhibited higher response
towards aliphatic thiols (Cys, GSH and TGA). This molecule en-
courages significant response towards thiophenols and aliphatic
thiols, as well as faint fluorescent comeback for other analytes,
including reactive sulfur species, nucleophiles, amino acid and ROS.
Probe 1b is three-fold more responsive to thiols over reactive sulfur
species (NaHS, Na2S and NaHSO3) and reveals no response to the
other analytes (KI, aniline, Gly, Glu, KO2 and H2O2).
10
0
probe1a
probe1b
The sensitivities of the probes were then deliberated by fluo-
rescence response towards different PhSH concentrations. The
concentration-dependent fluorescence responses of probe 1a and
1b were both examined at 15 min after the addition of PhSH (Fig. 3).
The fluorescent intensities of probes increased more than four-fold
0
100
200
300
400
Time (s)
Fig. 1. Time courses of the reaction of probes 1a and 1b (10
mM) with thiophenol in
PBS (pH 7.4, 0.1% DMSO). The concentration of thiophenol is 15
mM. Fluorescence in-
after the addition of 0.78
m
M probes (I[0.78 M]/I[0 M]ꢀ4). Further-
m
m
tensity was recorded at 560 nm with lex¼450 nm.
more, the quenching constants of the probes reacting with PhSH
were calculated by SterneVolmer equation,10 with Ksv¼5.4ꢁ106
(for 1a) and 10.1ꢁ106 (for 1b).
The comebacks of probes towards relevant aliphatic thiols,
reactive sulfur species, common nucleophiles and amino
acids were considered as well. As a result, probe 1a exhibits an
applicable fluorescent response to aromatic thiols, thiophenol
(PhSH) and 4-chlorothiophenol (4-Cl-PhSH) (Fig. 2a). Moreover, 2-
aminothiophenol (2-NH2ePhSH), sodium hydrosulfide (NaHS,
2.3. Cell imaging and fluorescent sensing of thiols in
commercial rabbit plasma
a donor of H2S), sulfide anion and aliphatic thiols, such as
L-gluta-
The implementation of these two probes on fluorescent imaging
thione (GSH), -cysteine (Cys) and thioglycolic acid (TGA) that could
L
was explored in living cells. After incubated with two probes
cleave benzenesulfonamide, were able to induce weak responses
(which are 12e31% of the responses of aromatic thiols); while other
analytes, including bisulfite, nucleophiles (aniline and KI) and
(20 mM) for 20 min, human ovarian clear cell carcinoma cell (ES-2)
expressed distinctive intracellular fluorescence in the cytosol
(Fig. 4a and e). Almost no fluorescence was found with the addition
of 1a, while significant fluorescence was detected on the addition of
amino acids (L-Gly and L-Glu), almostly did not trigger fluorescent

W. Sun et al. / Tetrahedron 68 (2012) 5363e5367
5365
another 20 min. Low fluorescence response was detected as
depicted in Fig. 4c and h. All these results demonstrated that probe
1b could be facilitated for ratiometric fluorescent imaging of thiols
in living cells.
In view of the significant response of probe 1b to thiols, this
molecule was as well applied to fluorescent sensing of thiols in
commercial rabbit plasma pretreated with triphenylphosphine,
which can reduce the disulfides to free thiols.11 As shown in Fig. 5,
the reasonable linear relationship (R¼0.99) between fluorescent
response and the concentration of plasma confirms that probe 1b
could quantitatively detect free aliphatic thiols in plasma samples.
Fig. 3. Concentration-dependent emission intensity changes of probes 1a (a) and 1b
(b) (10 M) at room temperature: experiments were conducted in 0.1 M phosphate
m
buffer (pH 7.4, 0.1% DMSO). The emission spectra were obtained at 15 min after the
addition of PhSH with lex¼450 nm.
Fig. 5. Emission spectral traces of 1b (10 mM) in PBS buffer with increasing volume
percentage of commercial rabbit plasma (from 0 to 2.4%) at room temperature. Inset:
fluorescence intensity at 550 nm versus the volume percentage of plasma [plasma]¼0,
0.2%, 0.4%, 0.8%, 1.6% and 2.4%.
3. Conclusion
In conclusion, this report describes two naphthalimide-based
fluorescent probes for the selective detection of thiols. The probe
1a has indicated significant response to thiophenols over other
aliphatic thiols, amino acids, common nucleophiles and ROS.
Compared with 1a, the probe 1b has the enhanced reactivity, and
exhibits the appreciable response to thiols, including thiophenols
and aliphatic thiols, over other competitive species. In the sub-
sequent experiment, probe 1b is satisfactory for determining cel-
lular thiols and free thiols in commercial rabbit plasma qualitatively
and quantitatively. These interesting results disclose that such
a fast, efficient and economical detection method for thiols has
considerable potential for the detection and differentiation of ar-
omatic and aliphatic thiols, and therefore would be very useful in
the new emerging biomedical area of reactive sulfur species.
Fig. 4. Fluorescence microscopic images of living ES-2 cells. Cells were incubated with
probes for 20 min at 37 ꢃC before imaging (a) is the fluorescence images of cells in-
cubated with probe 1a (20
(e), (f) and (g) are the fluorescence images of cells incubated with different concen-
trations of 1b: (e) 20 M, (f) 10 M, (g) 5 M; (i), (j) and (k) present the corresponding
bright field images of (e), (f) and (g); (c) and (h) present the fluorescence images of
cells incubated with probe 1a (20 M) and 1b (20 M), after preincubation with NEM
(100 M) for 30 min; (d) and (l) demonstrate the corresponding bright field images of
mM), and (b) is the corresponding bright field images of (a);
4. Experimental section
4.1. General remarks
m
m
m
m
m
m
All reagents were purchased from Acros and Aldrich and used
without purification. Mill-Q water was used throughout all exper-
iments. 1H NMR and 13C NMR were recorded on a Bruker 300 NMR
spectrometer. Mass spectra were performed by the analytical and
the mass spectrometry facilities at Shandong University. Infrared
spectrum was recorded with a Nicolet-470 spectrophotometer us-
ing KBr pellets in the 4000e400 cmꢂ1 region. Quartz cuvettes were
used in fluorometric studies. A Shimadzu UV-1700 UVevisible
spectrometer and Nanodrop ND3300 fluorospectrometer was
employed for the absorbance and fluorescent studies, respectively.
In brief, solutions of compounds 1a and 1b (10ꢁ10ꢂ6 M) were
(c) and (h); (m) exhibits the mean fluorescent photons after addition of different
concentrations of 1b processed by ImageJ software.
1b, and the signal intensity decreased as the concentration of 1b
reduced from 20 to 5 mM (Fig. 4eeg and m). It’s thought provoking
that 1b gave a higher turn-on response than 1a for the detection of
thiol in cells, which may be caused by the increased hydrophilicity
of 1b with the introduction of a hydroxyl group.
Furthermore, ES-2 cells were pretreated with 200
methylmaleimide (NEM) for 0.5 h to quench the content of bi-
ological thiols, and were incubated with two probes (20 M) for
mM N-
m

5366
W. Sun et al. / Tetrahedron 68 (2012) 5363e5367
prepared in 0.1 M phosphate buffer at pH 7.4 (0.1% DMSO). Sub-
sequently, the solutions were transferred into a 1 cm quartz cell and
UV absorbance data were recorded immediately. The fluorescence
excitation wavelengths used herein were the UV lmax values.
Human ovarian clear cell carcinoma cell (ES-2) was grown in
RPMI-1640 media with 5% CO2 and 95% air at 37 ꢃC. Cells were then
transferred on 24-well plate and allowed to adhere for 12 h. After
the culture medium was removed, the cells were carefully washed
with PBS (phosphate buffered saline), and then incubated with
bis(pinacolato)diboron (0.52 g, 2.34 mmol), Pd(dppf)Cl2 (34 mg,
0.08 mmol) and potassium acetate (0.46 g, 4.68 mmol) in dioxane
(15 mL) was stirred at 90 ꢃC overnight under nitrogen atmosphere.
After cooled to room temperature, the reaction mixture was diluted
with CH2Cl2, washed with H2O and brine, and dried over Na2SO4.
After removal of solvent, the reaction mixture was purified by flash
chromatography on silica gel (PE/methylene chloride¼2/1) to af-
ford 3b as a white powder, yield 69.2%, mp 202e204 ꢃC, 1H NMR
(300 MHz, CDCl3):
d
1.419 (s, 12H), 3.637 (m, 2H), 4.160 (t, J¼6.6 Hz,
probe 1a or 1b (20
mM, 10
mM and 5
m
M; 0.5:100 DMSO/PBS, v/v, pH
2H), 4.798 (s, 1H), 7.921 (t, J¼7.8 Hz, 1H), 8.242 (d, J¼7.2 Hz, 1H),
8.497 (m, 2H), 9.017 (d, J¼8.4 Hz, 1H); HRMS m/z calcd for
C20H23BNO5 [(MþH)þ] 368.1665; found 368.1670.
7.4) at 37 ꢃC for 20 min. Fluorescence imaging was performed after
washing the cells three times with PBS buffer. The fluorescence
images were taken in optical windows between 540 and 580 nm
(green) using an OLMPUS CK30-F200 fluorescent microscope. The
images for the corresponding N-methylmaleimide controls were
taken with identical settings.
4.3.3. 6-Hydroxy-2-(2-hydroxyethyl)-1H-benzo[de]isoquinoline-
1,3(2H)-dione (4b). To a solution of 3b (150 mg, 0.41 mmol) in 3 mL
DMF, 3 mL H2O2 (30%) was added, and the mixture was then stirred
at room temperature for 2 h, diluted with ethyl acetate, extracted
with 1 M HCl and brine, and dried over MgSO4. After solvent re-
moval, the crude product was purified by flash chromatography
(methylene chloride/methanol¼70:1) on silica gel to give 53 mg
4.2. Preparation of probe 1a
4.2.1. N-Ethyl-4-bromo-1,8-naphthalimide (2a). Compound 2a was
synthesized according to literature procedures.7
yellow product 4b, yield 51.9%, 1H NMR (300 MHz, DMSO):
d 3.598
(m, 2H), 4.132 (t, J¼6.6 Hz, 2H), 4.806 (t, J¼5.4 Hz, 1H), 7.160 (d,
J¼8.4 Hz, 1H), 7.770 (t, J¼7.8 Hz, 1H), 8.362 (d, J¼7.8 Hz, 1H), 8.479
(d, J¼7.8 Hz, 1H), 8.539 (d, J¼8.4 Hz, 1H), 11.867 (s, 1H).
4.2.2. 2-Ethyl-6-hydroxy-1H-benzo[de]isoquinoline-1,3(2H)-dione
(4a).8 A mixture of compound 2a (0.65 g, 2.14 mmol), CH3ONa
(0.92 g, 17 mmol) and CuSO4$5H2O (64 mg, 0.25 mmol) were
refluxed in 10 mL CH3OH for 12 h. Subsequently, 20 mL cold water
was added in the above mixture to give a yellow precipitate. After
filtration, compound 3a was obtained in 91% yield. A mixture of
compound 3a (0.15 g, 0.59 mmol) and 5 mL HI (57%) was refluxed
by 6 h. After cooling and filtration, compound 4a was obtained as
4.3.4. 2-(2-Hydroxyethyl)-1,3-dioxo-2,3-dihydro-1H-benzo[de]iso-
quinolin-6-yl 2,4-dinitrobenzenesulfonate (1b). The synthesis is
analogous to 1a with compound 4b as the starting material. The
product was obtained as a yellow solid in 72.9% yield, mp
201e203 ꢃC. 1H NMR (300 MHz, DMSO):
d 3.621 (d, 2H), 4.148 (t,
a yellow needle in 83% yield. 1H NMR (300 MHz, DMSO):
d
1.182 (t,
J¼6.3 Hz, 2H), 4.792 (t, J¼6.0 Hz, 1H), 7.674 (d, J¼8.4 Hz, 1H), 7.958
(t, J¼7.8 Hz, 1H), 8.380 (m, 2H), 8.477 (d, J¼8.4 Hz, 1H), 8.572 (d,
J¼7.5 Hz, 1H), 8.625 (dd, J1¼2.4 Hz, J2¼8.7 Hz, 1H), 9.162 (d,
J¼6.6 Hz, 3H), 4.064 (m, 2H), 7.166 (d, J¼8.4 Hz, 1H), 7.776 (t,
J¼7.2 Hz, 1H), 8.373 (d, J¼8.4 Hz, 1H), 8.491 (d, J¼7.2 Hz, 1H), 8.545
(d, J¼7.2 Hz, 1H), 11.883 (s, 1H).
J¼2.1 Hz, 1H); 13C NMR (DMSO, 75 MHz):
d 163.11, 162.50, 151.71,
148.31, 148.07, 133.74, 131.59, 131.05, 130.62, 128.84, 128.69, 127.77,
127.21, 124.71, 122.69, 122.13, 121.32, 119.97, 57.69, 41.97; IR (KBr,
cmꢂ1): 3514, 3103,1702, 1659, 1557, 1543, 1383, 1350, 1232, 1191,
4.2.3. 2-Ethyl-1,3-dioxo-2,3-dihydro-1H-benzo[de]isoquinolin-6-yl
2,4-dinitrobenzenesulfonate (1a). To a suspension of 4a (0.10 g,
0.41 mmol) in 5 mL dichloromethane, Et3N (71
m
L, 0.51 mmol) was
1069, 82,946, 795, 785, 755, 731; HRMS m/z calcd for C20H13N3O10
S
added dropwise at 0 ꢃC. The mixture was stirred for 5 min, and
treated with 2,4-dinitrobenzenesulfonyl chloride (0.13 g,
0.49 mmol). The resulting mixture was stirred at room temperature
for 4 h, diluted with CH2Cl2 (20 mL), washed with 1 M HCl (20 mL)
and brine (20 mL), and dried over MgSO4. After solvent removal, the
crude product was purified by flash chromatography (methylene
chloride) on silica gel to give 0.15 g yellow product 1a (76.9%), mp
[(MþH)þ] 488.0394; found 488.0389.
Acknowledgements
We acknowledge Professor Xiaodong Shi (Department of
Chemistry, West Virginia University, USA) for his assistance in
manuscript writing. The present work was supported by grants
from Shandong Natural Science Foundation (No. JQ201019) and
Independent Innovation Foundation of Shandong University,
IIFSDU (No. 2010JQ005).
200e202 ꢃC. 1H NMR (300 MHz, DMSO):
d
1.210 (t, J¼7.2 Hz, 3H),
4.078 (m, 2H), 7.678 (d, J¼8.1 Hz, 1H), 7.956 (m, 1H), 8.381 (m, 2H),
8.486 (d, J¼8.1 Hz, 1H), 8.607 (m, 2H), 9.161 (d, J¼2.4 Hz, 1H); 13C
NMR (DMSO, 75 MHz):
d 162.78, 162.17, 151.71, 148.37, 148.07,
133.76, 131.61, 131.09, 130.58, 128.80, 128.70, 127.76, 127.28, 124.74,
122.63, 122.06, 121.31, 120.00, 34.90, 12.96; IR (KBr, cmꢂ1): 3108,
1701, 1663, 1556, 1541, 1386, 1349, 1246, 1191, 1072, 846, 796, 783,
753, 731; HRMS m/z calcd for C20H13N3O9S [(MþH)þ] 472.0445;
found 472.0449.
Supplementary data
Spectroscopic data related to this article can be found online at
doi:10.1016/j.tet.2012.04.110. These data include MOL files and
InChiKeys of the most important compounds described in this
article.
4.3. Preparation of probe 1b
References and notes
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1,3(2H)-dione (2b). The synthesis is analogous to 2a with etha-
nolamine as the starting material. The product was obtained as
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