A highly selective and sensitive fluorescent thiol probe through dual-reactive and dual-quenching groups

Article abstract of DOI:10.1039/c4cc08156k

A new fluorescent probe installed with dual-reactive and dual-quenching groups was rationally designed for highly selective and sensitive sensing of biothiols. The sensitivity of the probe toward thiols was significantly improved by dual-quenching effects. Furthermore the selectivity of the probe was also greatly enhanced by installation of dual-reactive groups. This journal is

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A highly selective and sensitive fluorescent thiol probe through dual-
reactive and dual-quenching groups
Huatang Zhang,‡a Changyu Zhang,‡b Ruochuan Liu,a Long Yi*,b,c and Hongyan Sun*,a
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
quenching effects. The background fluorescence will be
50 minimized and as a result, the fluorescent turnꢀon fold can be
significantly increased upon reaction with thiols. This will finally
lead to the enhancement of the sensitivity of the probe. On the
other hand, we also envisage that selectivity of the probe will be
greatly increased due to different reaction mechanisms
55 (nucleophilic substitution and Michael addition) toward thiols. A
singleꢀreactive substance can only react with one of the reactive
(quenching) groups, and another reactive (quenching) group can
still partially quench the fluorescence (Scheme 1A). In another
word, the fluorescence will only gain maximal turnꢀon effect with
60 the molecules that can undergo two different reaction events to
release both quenching groups. Such a strategy should greatly
improve the selectivity of the fluorescent probe. Based on our
design strategy (Scheme 1), we hypothesize that both the
sensitivity and selectivity of the dualꢀquenching probe (DQ-
65 probe) will be improved. To our delight, herein we successfully
developed such a fluorescent probe to detect biothiols.
A new fluorescent probe installed with dual-reactive and
dual-quenching groups was rationally designed for highly
selective and sensitive sensing of biothiols. Sensitivity of the
probe toward thiols was significantly improved by dual-
10 quenching effects. Furthermore selectivity of the probe was
also greatly enhanced by installation of dual-reactive groups.
Smallꢀmolecule thiols are essential for maintaining cellular redox
environment and play important roles in regulating various
cellular functions.1 Abnormal thiol concentration has been
15 implicated with the formation of a number of important diseases.
For instance, thiol deficiency is associated with liver damage,
AIDS, etc.2 High concentrations of homocysteine (Hcy) are
linked with cardiovascular disease and Alzheimer’s disease.3
Because altered levels of thiols in various physiological media
20 have been linked to specific pathological conditions, researchers
have shown keen interest to develop methods to detect thiols
quantitatively and selectively in living organisms.4 In recent
years, fluorescenceꢀbased method has become as a popular
approach for detection of biothiols due to its high sensitivity, easy
25 implementation and nonꢀinvasiveness.5 A number of organic
reactions have been utilized to design various fluorescent thiol
probes, including cyclization reactions between aldehyde and
aminothiols,6 Michael addition reactions,7 cleavage reactions of
2,4ꢀdinitrobenzenesulfonyl (DNBS) with thiols,8 nucleophilic
30 substitution,9 disulfide exchange reaction,10 and others.11 These
work greatly advanced the research on the fluorescent detection
of biothiols; however, many of them suffer from relatively low
sensitivity due to the inefficiency of the quenching
group6a,7e,7f,9d,10a,10e or inadequate selectivity due to the cross
35 reactivity of the reactive unit utilized.6bꢀ6d,8d,9g,11d Thus, new thiol
probes with improved properties are still needed to be developed.
To obtain highly selective and sensitive fluorescent probes for
biothiols, we herein introduce a novel design strategy through
installation of dualꢀquenching and dualꢀreactive groups onto a
40 fluorophore. The designing principle is illustrated in Scheme 1. A
fluorophore is attached with two reactive groups, DNBS and a
highly activated Michael acceptor. DNBS has been reported as a
useful reactive unit for thiol detection in previous study,8 and the
DNBS moiety can quench the fluorescence via intramolecular
45 charge transfer (ICT) effect. On the other hand, the Michael
acceptor can quench the fluorescence via a photoinduced electron
transfer (PET) effect.7b We envisage that the fluorescence will be
heavily quenched through combined usage of PETꢀICT dualꢀ
A
70
B
O2N
NO2
75
O2
N
O
NO2
O
S
O
O
O
HO
O
O
O
S
O
O
O
O
O
O
O
O
O
NH
NH
O
O
DQ-probe 1
MQ-probe 2
MQ-probe 3
Scheme 1. (A) Schematic illustration of a design strategy for dualꢀ
quenching probe (DQ-probe) based on the combination of two monoꢀ
80 quenching probes (MQꢀprobe). ICT, intramolecular charge transfer; PET,
photoinduced electron transfer. (B) Chemical structures of fluorescent
probes used in this work.
As a proofꢀofꢀconcept study, we chose 3ꢀaminoꢀ7ꢀhydroxylꢀ
coumarin as a fluorophore due to its high fluorescence intensity,
85 excellent solubility, good cell permeability, and easeꢀofꢀ
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synthesis. The chemical structures of DQ-probe 1 and its control
A
B
monoꢀquenching probes (MQ-probe 2 and 3) were shown in
Scheme 1B. All probes were well characterized by 1H NMR, 13C
NMR and HRꢀMS (see ESI).
5
We firstly examined the absorption spectra of probes in PBS
buffer (10 mM, pH 7.4). As shown in Fig 1A and 1B, 1 and 2
displayed a maximum absorbance at 330 nm and 295 nm,
respectively. After reacting with cysteine, the maximum
absorbance was shifted to 355 and 400 nm for 1 and 2,
10 respectively. Thiolꢀmediated cleavage of the electronꢀ
withdrawing DNBS group releases oxygen donor at pH 7.4,
increasing the pushꢀpull character of the dye and resulting in
large bathochromic shifts in the absorption.
50 Fig. 2 Timeꢀcourse experiments of 1 µM DQ-probe 1 (A) or MQ-probe
2 (B) with Cys (10 µM) in PBS buffer (10 mM, pH 7.4).
To gain detailed information about the sensitivity of DQ-probe
1, the fluorescence intensity change was closely monitored by
addition of various concentrations of Cys into the probe (Fig. 3,
55 S1ꢀS4). The fluorescence intensity was linearly related to the
concentration of Cys from 0 to 1 µM (Fig. 3B). Specifically, the
detection limit of 1 was determined as low as 20 nM based on
3σ/slope method.7g These results clearly indicated that DQ-probe
1 has shown significantly improved sensitivity compared to MQ-
60 probe 2 due to the installation of dualꢀquenching groups.
The fluorescent experiments were carried out in PBS buffer
15 (10 mM, pH 7.4). Probe 1 showed almost no fluorescence (nearly
the same as that of buffer only) in PBS buffer without cysteine
(Fig. 1C), while 2 and 3 showed weak background fluorescence
under similar condition (Fig 1D).7b These results are consistent
with our design that the coumarin fluorescence in 1 is heavily
20 quenched through combined usage of PETꢀICT dualꢀquenching
effects. After reacting with Cys, a significant fluorescent increase
was observed for both 1 and 2, with the maximum emission at
473 nm and 440 nm, respectively. The increase of maximum
fluorescent intensity is more than 400ꢀfold for 1 and about 13ꢀ
25 fold for 2. From these results, we can conclude that the
fluorescence turnꢀon fold upon reacting with thiols for DQ-probe
1 is indeed greatly increased, which agrees well with our initial
hypothesis.
A
B
C
D
A
C
B
Fig. 3 Fluorescence response of DQ-probe 1 (A) and MQ-probe 2 (C)
65 with different concentration of Cys (0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8,
0.9, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 µM) in PBS buffer and linear relationship
between fluorescence intensity (1 at 470 nm and 2 at 440 nm) and Cys
concentration for 1 (B) and 2 (D).
D
70 In order to examine whether DQ-probe 1 possess better
selectivity than MQ-probe 2, they were incubated with 20
different amino acids in PBS buffer and the fluorescent intensity
was measured accordingly (Fig. 4, S5ꢀS8). The fluorescent
intensity of 1 with addition of different thiol species (Cys,
75 homocysteine, reduced glutathione (GSH)) could result in more
than 400ꢀfold increase. For 2, only 12ꢀfold fluorescent increase
was observed under the same conditions. The other 19 amino
acids samples exhibited no noticeable increase of fluorescence
signal. These experimental results implied that the DQ-probe 1 is
80 highly selective toward thiol group but not toward other
biologically relevant nucleophiles such as amino and imidazole
group. In addition, we also performed competitive selectivity
studies with some representative amino acids (glycine, arginine,
lysine and histidine). The results indicated that 1 or 2 can
85 selectively detect thiols even though the concentration of other
competitive amino acids is 100 equiv. higher than thiol’s
concentration. It is noted that the selectivity of DQ-probe 1
toward thiols is more than 200 fold compared to other amino
30
Fig.
1
The spectra response of fluorescent probes toward Cys.
Absorption spectra of 10 µM 1 (A) or 2 (B) in the absence and presence
of Cys (100 µM) in PBS buffer (10 mM, pH 7.4). The emission spectra of
1 µM 1 (C) or 2 (D) in the absence and presence of Cys in PBS.
35
Encouraged by these preliminary results, we moved forward to
study the reaction kinetics of the probes (Fig. 2). Probe 1 or 2 (1
µM) was incubated with 10 µM Cys in PBS buffer at 37 ºC. Both
probes were shown with fast fluorescent increase at the
beginning. Subsequently, the fluorescence intensity went through
40 a gradual increase and finally reached a plateau. Both 1 and 2
showed fairly fast reaction rate. A control experiment with only 1
or 2 was incubated in PBS buffer for the stability test. The results
indicated that 2 is prone to slow hydrolysis at pH 7.4 (Fig. 2B),
which is similar with that of 3 in the previous report.7b However,
45 the fluorescence increase due to hydrolysis in 1 can be almost
negligible (Fig 2A) because of the dualꢀquenching and dualꢀ
reactive effects.
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DOI: 10.1039/C4CC08156K
acids, while that of MQ-probes 2 and 3 is lower than 10 and 60
fold,7b respectively. Taken together, the selectivity of DQ-probe
is indeed greatly increased due to the usage of dualꢀreactive and
dualꢀquenching groups. This can be explained by that the
fluorescence of a competitive substance will only gain maximal
turnꢀon effect when it can undergo two different reaction events,
i.e. both nucleophilic substitution and Michael addition. H2S was
recently identified as an important signaling molecule,12 and its
reactivity is similar to that of thiols.13 We also tested DQꢀprobe 1
C
F
B
E
A
D
5
10 towards H2S, and results indicated that our probe showed high
fluorescence signal after addition of H2S (Fig. S9).
B
A
C
45 Fig. 5 Confocal bioimages of living HeLa cells. Fluorescence (A),
brightfield (B) and overlay (C) images of HeLa cells incubated with 1 (1
ꢁM) for 20 min; fluorescence (D), brightfield (E) and overlay (F) images
of HeLa cells which were preꢀincubated with 2 mM Nꢀethylmaleimide for
30 min and then treated with 1 ꢁM of 1 for 20 min.
50
D
thiol activation is much larger which leads to high sensitivity of
the probe toward thiols; 2) based on the dualꢀreactive groups, the
selectivity of the probe toward thiols is greatly improved, because
the thiol detection can only be obviously interfered by molecules
55 that can react with both different reactive groups. As a proofꢀofꢀ
concept study, we prepared a new DQ-probe 1, which was
shown to detect thiols with high sensitivity with 20 nM detection
limit. The selectivity of DQ-probe 1 is also superior to many
existing thiol probes and MQ-probes (2 and 3).6ꢀ11 Furthermore,
60 our novel design strategy could be employed as a general method
for preparation of other kinds of fluorescent probes. Progress for
highly sensitive and selective fluorescent probes for endogenous
H2S14 based on dualꢀquenching and dualꢀreactive effects will be
reported recently.
15 Fig. 4 Selectivity and competitive selectivity studies of fluorescent
probes toward different species in PBS buffer. Relative fluorescence
intensity at 470 nm for 1 µM 1 (A) and at 440 nm for 1 µM 2 (B) assayed
with 19 natural amino acids (1 mM), Cys (10 ꢁM), GSH (10 ꢁM) and
Hcy (10 ꢁM); Relative fluorescence intensity at 470 nm for 1 µM 1 (C)
20 and at 440 nm for 1 µM 2 (D) with Cys (10 ꢁM) in the presence of
several represented amino acids (1 mM).
65
Sun and Yi thank Prof. Zhen Xi at Nankai University for general
support. We would like to thank the financial support of City
University of Hong Kong Grant (7004025, 9667091), NSFC
(21402007), the Fundamental Research Funds for the Central
70 Universities of BUCT (YS1401).
Finally, we examined whether DQ-probe 1 can be used to
detect intracellular thiols in living cells. As shown in Fig. 5, after
incubating HeLa cells with 1 for 20 min, strong blue fluorescence
25 could be readily observed inside the cells under confocal
fluorescence microscope (Fig. 5Aꢀ5C). The control experiments
were performed by preincubating cells with Nꢀethylmaleimide
(NEM) and subsequently adding of 1. Nꢀethylmaleimide is
commonly used as a thiolꢀdepleting agent for cells. No significant
30 fluorescence signal was observed under similar experimental
conditions (Fig. 5Dꢀ5F). These results suggest that DQ-probe 1
is cellꢀpermeable and can react with intracellular thiols efficiently
and selectively. Considering the relationship between the
cytosolic biothiols level with many diseases,2 this probe may
35 offer a simple and viable way to monitor the cytosolic thiol level.
Notes and references
a
Department of Biology and Chemistry, City University of Hong Kong,
83 Tat Chee Avenue, Kowloon, Hong Kong, P. R. China. Fax: (852)
75 34420522; Tel: (852) 3442 9537; E-mail: hongysun@cityu.edu.hk
b
State Key Laboratory of Organic-Inorganic Composites and Beijing
University of Chemical Technology, Beijing University of Chemical
Technology, 15 Beisanhuan East Road, Chaoyang District, Beijing
100029, China. E-mail: yilong@mail.buct.edu.cn
80 c Collaborative Innovation Center of Chemical Science and Engineering
(Tianjin)
† Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/b000000x/
In summary, we report a novel strategy for rational design of
highly selective and sensitive fluorescent thiol probe, namely, a
fluorescent probe installed with dualꢀreactive and dualꢀquenching
40 groups. The principle of our strategy has two aspects: 1) based on
the dualꢀquenching effects, the fluorescence of the probe can be
maximally inhibited, and the turnꢀon response upon
85 ‡ The authors pay equal contributions to this work.
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