A novel near-infrared fluorescent probe for cysteine in living cells based on a push-pull dicyanoisophorone system

Article abstract of DOI:10.1016/j.jphotochem.2017.06.001

The near-infrared (NIR) fluorescent probe for rapid, highly sensitive and selective detection of cysteine (Cys) has great significance in both biological and environment sciences. In this work, a novel and specific dicyanoisophorone-based NIR probe for Cy

Full text of DOI:10.1016/j.jphotochem.2017.06.001

Journal of Photochemistry and Photobiology A: Chemistry 346 (2017) 215–220
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Short note
A novel near-infrared fluorescent probe for cysteine in living cells based
on a push-pull dicyanoisophorone system
Yuewen Yu , Hongping Xu , Wang Zhang , Qiaorong Hana,b, Bingxiang Wanga,b,*,
a
a
a
a,b,
Yuliang Jiang
*
a
College of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210023, PR China
Jiangsu Collaborative Innovation Center of Biomedical Functional Materials and Jiangsu Key Laboratory of Biofunctional Materials, PR China
b
A R T I C L E I N F O
A B S T R A C T
Article history:
Received 4 May 2017
Received in revised form 31 May 2017
Accepted 1 June 2017
The near-infrared (NIR) fluorescent probe for rapid, highly sensitive and selective detection of cysteine
(Cys) has great significance in both biological and environment sciences. In this work, a novel and specific
dicyanoisophorone-based NIR probe for Cys was developed. This probe features remarkably large Stokes
shift, with turn-on fluorescence property for highly sensitive and selective detection of Cys over other
amino acids including the similar structured homocysteine (Hcy) and glutathione (GSH). The probe based
on the conjugate addition-cyclization reaction, and has low detection limit of Cys. Furthermore, this
probe shows great potential for Cys detection, which was successfully applied to bioimage Cys in living
cells with low cytotoxicity.
Available online 2 June 2017
Keywords:
Dicyanoisophorone
NIR
Large stokes shift
Cysteine
©
2017 Elsevier B.V. All rights reserved.
Cell imaging
1. Introduction
cyclization reaction with aldehyde [27–30], cleavage of disulfide
and sulfonamide [31–33] and others [34–38].
Detection of sulfydryl-containing amino acids is being watched
by more and more researchers, and has become a popular research
nowadays. The sulfydryl-containing amino acids are also known as
biothiols such as cysteine (Cys), homocysteine (Hcy) and
glutathione (GSH). They are commonly found in biological systems
and possess different crucial roles in cellular processes [1–6].
Among these biothiols, Cys is the least abundant thiol inside the
cell, its concentration being in the micromolar range [7,8]. It has
been reported that the abnormal levels of Cys could induce many
diseases such as decreased hematopoiesis, psoriasis, leucocyte
loss, edema, neurotoxicity, liver damage, and Parkinson’s disease
In recent years, several excellent Cys-specific probes based on
various traditional fluorophores have been developed [39–47]. For
example, Strongin’s group reported a HMBT-based probe for Cys
detection, which can undergo an excited-state intramolecular
photontransfer (ESIPT) process [44]. Chen’s group developed a
fluorescein-based probe, which displays excellent selectivity and
sensitivity for Cys over Hcy and GSH [46]. However, most of them
have emission wavelength and absorption wavelength within the
ultraviolet or visible region (300–650 nm), which will be
detrimental to its use in biological imaging. Near-infrared (NIR)
probes are widely accepted as an ideal candidate in biological
system studies, because it works at 650–900 nm (NIR region), and
possesses several advantages, such as lower energy, better tissue
penetration as well as minimum background interference from
autofluorescence [48,49]. Although NIR probes have made some
progress in rescent years, but some limitations including low
sensitivity, complex synthesis and/or low yield were still existed
(Table S1) which could hinder its further application [50–52].
Therefore, it is still imperative to develop novel near-infrared
fluorescent probes for rapid, highly selective and sensitive
detection of Cys.
[9–12]. Thus, discrimination of Cys is of great interest and
importance in biochemistry and biomedicine fields.
Over the past decades, several methods have been utilized to
detect Cys [13–17]. Among them, fluoresce-based methods are
usually preferred due to their high sensitivity, ease of implemen-
tation, fast-response, cost-effectiveness, noninvasiveness, real-
time detection capability and good biocompatibility [18–21]. Until
now, a few fluorescent probes have been developed for Cys through
various mechanisms, including Michael addition [22–26],
Herein we report a noticeable NIR fluorescent probe DP-NIR for
the first time as shown in Scheme 1. The probe DP-NIR is composed
of a novel fluorophore (dicyanoisophorone) as a reporter and
acrylate moieties as a triggered moiety for Cys, and has high
*
Corresponding authors at: College of Chemistry and Materials Science, Nanjing
Normal University, Nanjing 210023, PR China
E-mail address: wangbingxiang@njnu.edu.cn (B. Wang).
http://dx.doi.org/10.1016/j.jphotochem.2017.06.001
1010-6030/© 2017 Elsevier B.V. All rights reserved.

216
Y. Yu et al. / Journal of Photochemistry and Photobiology A: Chemistry 346 (2017) 215–220
Scheme 1. The synthetic routes of probe DP-NIR. Reagents and conditions: (a) malononitrile, ethanol, piperidine; (b) p-hydroxybenzaldehyde, acetonitrile, piperidine; (c)
ꢀ
acryloyl chloride, CH
2
Cl
2
, Et
3
N, 0 C.
selectivity and sensitivity for Cys over Hcy and GSH. In addition,
this probe can be easily synthesized from cheap commercial raw
materials with good yield. It shows almost none background
fluorescence, and a rapid fluorescent turn-on response for Cys with
a large stokes shift. Moreover, the probe was successfully applied
for imaging Cys in living cells. The synthetic route of probe DP-NIR
is shown in Scheme 1.
138.79, 130.35, 127.58, 126.72, 121.83, 116.33, 114.62, 113.80, 75.22,
42.78, 38.64, 32.15, 27.91.
2.4. Synthesis of probe DP-NIR
3
Compound 1 (0.29 g, 1 mmol) and Et N (0.23 mL, 2 mmol) were
dissolved in 20 mL anhydrous dichloromethane, the solution
became dark red. The resulting mixture was stirred for 30 min
ꢀ
2. Experiment
in the ice bath at 0 C. Then, the solution of acryloyl chloride
(
0.16 mL, 2 mmol) in 3 mL dichloromethane was added dropwise.
2
.1. Materials and instruments
The resulting mixture was stirred for 1 h, and then was allowed to
react at room temperature until the reaction was completed
(monitored by TLC). The reaction mixture was added with 25 mL
water to wash the solution three times, and then the dichloro-
All chemical reagents and solvents were purchased from
commercial suppliers and used without further purification.
Double distilled water was used throughout the work. All reactions
2 4
methane phase was dried over anhydrous Na SO . The solvent was
1
were monitored using thin-layer chromatography (TLC). H NMR
evaporated under reduced pressure, the crude solid product was
further purified by silica column chromatography (petroleum
ether: ethyl acetate = 10/1, v/v) to afford the pure product DP-NIR
13
and C NMR spectra were recorded on the Bruker AN-400
spectrometer with chemical shifts reported in ppm (TMS as
internal standard). Electron impact mass spectra were run on MAT-
ꢀ
1
(0.15 g, 91% yield). Melting point: 166–168 C. H NMR (400 MHz,
DMSO-d ): 7.78 (d, J = 8.7 Hz,2H), 7.45 (d, J = 16.2 Hz, 1H), 7.32 (d,
212 spectrometer. Ultraviolet-visible (UV-vis) absorption spectra
6
d
were measured with a Varian Cary 50 spectrophotometer at 1 cm
of the light path length. Fluorescence spectra were recorded on
Varian Cary Eclipse fluorescence spectrophotometer with an
excitation wavelength of 557 nm. High-resolution EI mass spectra
were recorded on a MALDI-TOF/TOF Ultrafle Xtreme (Bruker USA)
mass spectrometer.
J = 16.2 Hz, 1H), 7.25 (d, J = 8.7 Hz, 2H), 6.91 (s, 1H), 6.56 (dd, J = 17.3,
1.4 Hz, 1H), 6.43 (dd, J = 17.3, 10.2 Hz, 1H), 6.18 (dd, J = 10.2, 1.4 Hz,
13
1H), 2.63 (s, 2H), 2.56 (s, 2H), 1.03 (s, 6H). C NMR (100 MHz,
DMSO-d ): 170.85, 164.46, 156.18, 151.46, 136.91, 134.35, 128.01,
123.42, 122.78, 114.31, 113.48, 76.99, 42.76, 38.61, 32.17, 27.91.
6
d
+
HRMS (ESI): (C22
found 367.1411.
20 2 2
H N O ) m/z: calculated for [M + Na] : 367.1422;
2.2. Synthesis of compound 2
2
.5. Sensing mechanism of probe DP-NIR with Cys
A solution of malononitrile (1.98 g, 30 mmol) in dry ethanol
(
25 mL) was added to a mixture of piperidine (0.25 mL, 2.5 mmol)
Cys (78 mg, 0.5 mmol) was added to a solution of DP-NIR
172 mg, 0.5 mmol) in 10 mL DMSO-PBS(v/v = 1/1). Then the
ꢀ
and isophorone (3.46 g, 25 mmol). The solution was stirred at 50 C
for 10 h. After cooling to room temperature, the black solution was
slowly poured into water (200 mL) and the precipitated solid was
filtered. Recrystallization from n-hexane afforded a brown solid.
(
reaction mixture was stirred at room temperature until the
reaction was completed (monitored by TLC). The mixture was
extracted with dichloromethane for three times. The combined
organic layers were removed under reduced pressure. The residue
was purified by silica column chromatography (petroleum ether:
ethyl acetate = 20/1, v/v) to afford compound A.
ꢀ
1
Mp: 71–73 C. H NMR (400 MHz, DMSO-d
H), 2.53 (s, 2H), 2.24 (s, 2H), 2.05 (s, 3H), 0.96 (s, 6H). C NMR
100 MHz, DMSO-d 171.82, 162.87, 119.87, 113.88, 113.09, 76.54,
6
) d 6.56 (d, J = 1.3 Hz,
13
1
(
4
6
) d
5.26, 42.41, 32.33, 27.81, 25.43.
2.3. Synthesis of compound 1
2.6. UV–vis and fluorescence measurements
To a stirred solution of Compound 2 (1.0 g, 5.4 mmol) and
p-hydroxybenzaldehyde (0.7 g, 5.4 mmol) in dry acetonitrile
Stock solution of DP-NIR (1 mM) was prepared in analytical
grade DMSO. Other analytes including Alanine (Ala), arginine
(Arg), asparagine (Asn), aspartic(Asp), glutamine (Gln), glutamic
acid (Glu), glycine (Gly), histidine (His), isoleucine (Ile), leucine
(Leu), lysine (Lys), methionine (Met), cysteine (Cys), homocysteine
(Hcy), glutathione (GSH), phenylalanine (Phe), proline (Pro), serine
(Ser), tyrosine (Trp), tyrosine (Tyr) were dissolved in deionized
water to afford 10 mM aqueous solution. The stock solutions were
made freshly and were diluted to desired concentrations. For a
(
60 mL) was added several drops of piperidine. The mixture was
ꢀ
stirred at 80 C for 6 h. The resulting residue was dissolved in 40 mL
dichloromethane, washed with water (3 Â 40 mL), The organic
2 4
phase was dried over anhydrous Na SO , filtered and evaporated to
dryness in vacuo. The residue was purified by flash column
chromatography on silica gel (petroleum ether: ethyl acetate, 20:1,
1
v/v) to afford compound 1 as an orange solid (1.34 g, 86%). H NMR
(
(
(
400 MHz, DMSO-d
6
)
d
10.01 (s, 1H), 7.56 (d, J = 8.6 Hz, 2H), 7.22
typical optical study, a DP-NIR (10 mM) solution in PBS buffer
(10 mM, pH 7.4, with 50% DMSO) was prepared. One-time glue
dropper was used to absorb 3.0 mL of DP-NIR solution to a quartz
d, J = 2.7 Hz, 2H), 6.79-6.81 (m, 3H), 2.60 (s, 2H), 2.53 (s, 2H), 1.01
13
s, 6H). C NMR (100 MHz, DMSO-d
6
):
d
170.79, 159.79, 157.23,

Y. Yu et al. / Journal of Photochemistry and Photobiology A: Chemistry 346 (2017) 215–220
217
ꢀ
cuvette, the temperature controlled at 25 C. The fluorescence
in mind, thus the new probe DP-NIR was designed and developed
spectra were measured with the excitation at 557 nm.
for detecting Cys.
3
.2. UV–vis and fluorescence measurements
2.7. Confocal microscopy imaging
In this study, the Uv–vis absorption spectral and fluorescence
spectral of probe DP-NIR to various analytes were investigated in
DMSO-PBS buffer (10 mM, pH 7.4, 1:1, v/v). As shown in Fig. 1a, DP-
HeLa cells were cultured in Dulbecco's Modified Eagle's
Medium (DMEM) including 10% FBS (Fetal Bovine Serum),
1
00 mg/mL penicillin and 100
water saturated incubator at 37 C. Cells were plated in 96-well
m
g/mL streptomycin in a 5% CO
2
,
NIR (10
02 nm. With the addition of Cys (25
red-shift ( max = 557 nm) was observed, which revealed the
m
M) exhibits an absorption peak with maximum at
ꢀ
4
m
M), a large Uv–vis spectral
plates and incubated with probe DP-NIR concentration of 10
m
M
l
ꢀ
for 24 h at 37 C. Before observing the cells with the help of the
confocal laser scanning microscopy (Nikon A1R), the cells were
washed with PBS buffer solution three times.
deacylation of the probe. At the same time, color of the solution
changed from pale yellow to brown as the inset of Fig. 1a. This
phenomenon made it feasible for Cys detection in the naked-eye
model.
As we all know, NIR emission is very favorable in biological
system studies, because it works at 650–900 nm (NIR region), and
possesses several merits, such as lower energy, better tissue
penetration as well as minimum background interference from
autofluorescence and so on. In order to study the sensing behavior
3
. Results and discussion
3.1. Design and synthesis of the probe DP-NIR
To generate an NIR fluorescent probe for biological system
of DP-NIR to Cys (25 mM) better, fluorescence titration experi-
imaging, a novel fluorescent probe (DP-NIR) was first designed and
synthesized through acylation of compound 1 with acryloyl
chloride in 91% yield. Structure of probe DP-NIR was confirmed
by H NMR, C NMR and mass spectrometry (Figs. S5–S7). The
design of this probe was based on the following rationale: (1) large
ments of the probe with Cys were implemented and shown in
Fig. 2a. It shows that the fluorescence intensity is gradually
increased and saturated at 673 nm under the excitation at 557 nm.
1
13
As shown in Fig. 2b, an excellent linear relationship of F/F
0
2
(
Y = 0.9533 + 0.9971[Cys], R = 0.9942) at 673 nm versus Cys con-
p
-conjugation system and long emission wavelength in the NIR
region of fluorophore dicyanomethylene-4H-pyran derivatives
53]; (2) photostability and hypotoxicity; (3) acrylate group as a
centration in the range of 0–20 M was also obtained (F and F are
m
0
the fluorescence intensities of probe DP-NIR in the absence and
presence of Cys respectively). The above results show that the
excitation and emission of the probe (DP-NIR) are both in NIR
range with a large Stokes shifts (ꢁ116 nm). In addition, the
[
fluorescent quencher and a reliable Cys selective trigger. Dicya-
noisophorone was selected as fluorophore due to it’s character-
istics of push-pull electrons which usually showed intra-molecular
charge transfer (ICT) at excited state [54]. In addition, the acrylate
group is linked to the fluorophore not only used as the reaction site
for Cys, but also for the blockage of the ICT process, which could
lead to the drastic decrease in quantum yield. Uppon addition of
Cys, probe DP-NIR could react with Cys and obtain compound 1
through conjugated addition and then deacylation of the probe,
and resulted in the elimination of ICT process with the obvious
fluorescence recovery. With these theoretical facts and hypotheses
detection limit of DP-NIR for Cys was as low as 0.16 mM calculated
from the calibration curve based on the definition by IUPAC
CDL = 3S /K ) [55], which indicates that probe DP-NIR has good
sensitivity to detect Cys in biological systems.
(
a
b
3
.3. The selectivity and sensitivity of probe DP-NIR
To evaluate the selectivity of probe DP-NIR for Cys, a range of
amino acids was examined. As shown in Fig. 3a, after the addition
of Cys (25 M), obvious fluorescent enhancement was observed.
While the other analytes including the similar-structured biothiols
Hcy, GSH) and 17 amino acids (Ala, Arg, Asn, Asp, Gln, Glu, Gly, His,
Ile, Leu, Lys, Met, Phe, Pro, Ser, Trp, Tyr), even at high concentration
1 mM), showed almost no effect on the fluorescence of the probe.
m
(
(
The result indicates that DP-NIR has high selectivity toward Cys,
and can discriminate Cys from Hcy and GSH. The selective
detection of Cys over other analytes can be expediently observed
by the naked-eye both under room light and a portable 365 nm
light (Fig. S8). Competition experiments were also investigated
that there were almost no change response in which DP-NIR was
first mixed with various analytes, respectively. After added Cys,
remarkable fluorescent enhancement was observed. All these
results clearly show that probe DP-NIR exhibits a high selectivity
and sensitivity for Cys.
3
.4. Effects of pH on Cys sensing
In order to obtain information on the pH effects, we also
investigated the effect of DP-NIR and of DP-NIR with Cys in the pH
range of 4.0–10.0. As shown in Fig. 4, the fluorescence intensity of
DP-NIR is maintained in the above pH range. This result shows that
Fig. 1. Uv–vis spectra changes of DP-NIR (10
mM) with addition of different
probe DP-NIR is stable. However, after the addition of Cys (25
the fluorescence intensity at 673 nm increase in the pH range of
.0–10.0, indicating that the probe is suitable for monitoring Cys in
physiological pH circumstances.
mM),
concentrations of Cys (0–25 M) in PBS buffer solution (10 mM, pH 7.4, 50% DMSO).
m
Insert: color changes from pale yellow to brown. (For interpretation of the
references to colour in this figure legend, the reader is referred to the web version of
this article.)
7

218
Y. Yu et al. / Journal of Photochemistry and Photobiology A: Chemistry 346 (2017) 215–220
Fig. 2. (a) Fluorescence spectra of DP-NIR (10
mM) after the addition of Cys (0–25
mM) in PBS buffer solution (10 mM, pH 7.4, 50% DMSO). Ex = 557 nm. (b) The linear
relationship of F/F versus the concentration of Cys (0–20
0
mM).
Fig. 3. (a) Fluorescence spectra of DP-NIR(10
response of probe DP-NIR (10 M) to various amino acids (the black bars) and to Cys in the presence of other analytes (the red bars) in PBS buffer solution (10 mM, pH = 7.4,
0% DMSO). Ex = 557 nm (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
mM) in the presence of Cys (10 mM), other analytes (1 mM) in PBS buffer solution (10 mM, pH = 7.4, 50% DMSO). (b) Fluorescence
m
5
chromatography as described above (in content of 2.5). The target
1
product was performed by TLC and H NMR analyses. The results of
1
TLC and H NMR show that product A is compound 1 (Fig. S9–
Fig. S10). Furthermore, the comparative experiments on UV
absorption spectra and fluorescence spectra of compounds 1,
compound A, and probe DP-NIR were also carried out, which
further confirmed the result that product A is compound 1
(
Fig. S11). With these results and the knowledge learned from
Strongin and other researchers on the conjugate addition-cycliza-
tion reaction process reported for acrylate-functionalized fluores-
cent probes with Cys [44–46], thereby, the sensing mechanism of
probe DP-NIR for Cys was proposed as shown in Scheme 2. Probe
DP-NIR has almost no fluorescence, which can be attributed to the
decrease of donating ability of acrylate group and called “ICT-OFF”.
After Cys was added, the terminal acrylate group of probe would
react with Cys and lead to deacylase, resulted in the “ICT-ON” with
the obvious NIR fluorescence recovery, which was in good
agreement with the design strategy.
Fig. 4. Fluorescence intensity of DP-NIR (10
or presence (red) of Cys (25 M) at various pH values in DMSO-PBS (10 mM, pH 4–
0, 1:1, v/v) at room temperature. E = 557 nm (For interpretation of the references
mM) at 673 nm in the absence (blank)
m
1
X
to colour in this figure legend, the reader is referred to the web version of this
article.)
3
.6. Cell imaging
In order to prove that our probe has a potential application in
3
.5. Reaction mechanism
living system, the fluorescent imaging of intracelluar Cys in living
cells was further studied. Before imaging in living cells, an MTT
assay with Hela cells was used to investigate the cytotoxicity of DP-
In this work, the sensing mechanism of DP-NIR to Cys was
further investigated. DP-NIR and Cys were used as the raw material
to react, and then separated the target product (A) by column
NIR (10, 20, 30, 40, 50
mM), the cell viability is greater than 85%
after incubated 24 h even at a high concentration of probe DP-NIR

Y. Yu et al. / Journal of Photochemistry and Photobiology A: Chemistry 346 (2017) 215–220
219
Scheme 2. The proposed response mechanism of probe DP-NIR to Cys.
ꢀ
Fig. 5. Confocal fluorescence images of living HeLa cells. (a) HeLa cells incubated with DP-NIR (10
m
M) in PBS buffer solution (10 mM, pH 7.4, 50% DMSO) for 20 min at 37 C.
ꢀ
(
b) HeLa cells were pretreated with NEM (1 mM) for 30 min and then further incubated with DP-NIR (10
m
M) for 30 min at 37 C under excitation with 561 nm laser. Scale
bar = 20 m.
m
(
50
m
M) (Fig. S12), which indicates that probe DP-NIR has low
reaction site for Cys. This probe has rapid discrimination, high
selectivity and sensitivity of Cys from other structurally and
functionally similar thiols and amino acids, such as Hcy, GSH. It is
found that DP-NIR can offer particular NIR fluorescence turn-on
cytotoxicity. Next, the practical utility of DP-NIR for Cys detection
in confocal luminescence imaging was performed. As shown in
Fig. 5a, upon incubation with probe DP-NIR (10 mM) for 30 min,
remarkable intracellular NIR fluorescence was observed. However,
as the control experiments, when Hela cells were pretreated with
N-ethylmaleimide (NEM, a well-known thiols trapping reagent),
then incubated with probe DP-NIR, the cells showed almost no
fluorescence (Fig. 5b). These results denoted that probe DP-NIR has
good membrane permeability and it can be used as a new NIR
fluorescent probe for imaging intracellular Cys.
means for Cys detection with a low limit of detection (0.16 mM) in
DMSO-PBS solution (10 mM, pH 7.4, 1:1, v/v). In addition, the
results of bioimaging in living cells suggest that the probe DP-NIR
could be applied for sensing intracellular Cys with low cytotoxicity
and satisfactory cell membrane permeability. Therefore, this new
probe has great potential for applications involving detection of
Cys both in vivo and in vitro.
4. Conclusions
Acknowledgments
In summary, we have successfully synthesized
a
novel
This work was financially supported by the Priority Academic
isophorone-based probe DP-NIR, the acrylate group used as a
Program Development of Jiangsu Higher Education Institutions of

2
20
Y. Yu et al. / Journal of Photochemistry and Photobiology A: Chemistry 346 (2017) 215–220
China, and the China Postdoctoral Fund Surface Project
2017M610335).
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