19F CEST imaging probes for metal ion detection

Article abstract of DOI:10.1039/c7ob01068k

For detecting metal ions with ¹⁹F chemical exchange saturation transfer magnetic resonance imaging (¹⁹F CEST MRI), a class of novel fluorinated chelators with diverse fluorine contents and chelation properties were conveniently synthesized on gram scales. Among them, a DTPA-derived chelator with high sensitivity and selectivity was identified as a novel ¹⁹F CEST imaging probe for simultaneously detecting multiple metal ions.

Full text of DOI:10.1039/c7ob01068k

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Organic & Biomolecular Chemistry
PAPER
1
9
F CEST imaging probes for metal ions detection
a
b
a
a
a
b
a
Qiaoli Peng, Yaping Yuan, Huaibin Zhang, Shaowei Bo, Yu Li, Shizhen Chen, Zhigang Yang, Xin
b
abc
Zhou, and Zhong-Xing Jiang*
Received 00th January 20xx,
Accepted 00th January 20xx
19
19
To detecting metal ions with F chemical exchange saturation transfer magnetic resonance imaging ( F CEST MRI), a class
DOI: 10.1039/x0xx00000x
of novel fluorinated chelators with diverse fluorine contents and chelation properties were conveniently synthesized on
1
9
gram scales. From them, a DTPA derived chelator with high sensitivity and selectivity was identified as a novel F CEST
www.rsc.org/
imaging probe for simutaneously detecting multiple metal ions.
Introduction
Metal ions are involved in numerous biological events and,
therefore, monitoring the presence of certain metal ions and
their local concentrations with novel imaging technology is of
great importance for accurately understanding these biological
1
19
events. Among the many imaging technologies,
F MRI is
very attractive because it provides quantitative images without
2
,3
ionizing radiation, tissue depth limit, and background signal.
To this end, some fluorinated chelators, such as 5,5’ꢀdifluoroꢀ
,2ꢀbis(oꢀaminophenoxy) ethaneꢀN,N,N’,N’ꢀtetraacetic acid
5FꢀBAPTA) and 5,5’,6,6’ꢀtetrafluoroꢀ1,2ꢀbis(oꢀaminophenoxy)
ethaneꢀN,N,N’,N’ꢀtetraacetic acid (TFꢀBAPTA), were
employed to monitor metal ions with either F NMR by G. A.
1
(
Figure 1. Structures of fluorinated chelators 1ꢀ4.
aminetetraacetic acid (EDTA) and diethylenetriaminepentaacetic
acid (DTPA) were selected as the backbones for chelators 1ꢀ4 due to
their low price, easy availability, and high chelation selectivity on
1
9
1
c
1d
19
Smith et al and F. A. X. Schanne et al, or F CEST by M. T.
6
19
McMahon et al. Although F CEST MRI can dramatically
amplify the signal of metal ionꢀbound fluorinated chelator, it is
still very challenging to detect metal ions of low concentration
4
19
metal ions. In order to achieve metal ion selectivity in F CEST
MRI, the number of the chelating groups in chelators 1ꢀ4 were tuned
by the monoamide and diamideꢀderivatization of EDTA and DTPA,
respectively. Besides the carboxylic groups in chelators 1ꢀ4, the
hydroxyl and amide groups are also available for metal ion
1
9
with 5FꢀBAPTA or TFꢀBAPTA due to their limited F MRI
1
9
sensitivity. Without using high resolution F MRI and
1
9
extending the scan time, only a very weak F signal can be
generated from the two fluorine atoms in 5FꢀBAPTA and TFꢀ
BAPTA. In addition, it is very tedious to prepare these
fluorinated chelators, especially on gram scales. Therefore, it is
5
chelation. Because of the strong electronꢀwithdraw effect of two
adjacent trifluoromethyl groups, the hydroxyl groups in chelators 1ꢀ4
are actually very acidic which are good chelation groups for metal
5c,d
ion chelation. Once the hydroxyl and amide groups in chelators 1ꢀ
1
9
of great importance to develop novel easily available F CEST
imaging probes with high sensitivity and selectivity for
monitoring metal ions at low concentration.
4
chelated with metal ions, the electron environment of the fluorines
19
changes accordingly which induces a F NMR response, e.g.,
chemical shift change (ꢁω) and line broadening. If the exchange rate
Herein, a class of fluorinated chelators 1ꢀ4 with multiple
between metal ionꢀbound chelator and free chelator is slow enough
1
9
symmetric fluorines, single F NMR peak, and controllable
19
(
k
ex < ꢁω), a new peak will show up on the F NMR spectra of
1
9
chelation property were designed as novel F CEST imaging probes
Fig. 1). Multiple symmetrical fluorines, which collectively give a
19
6
chelators 1ꢀ4. According to the mechanism of F ionꢀCEST MRI,
(
1
9
19
i.e., using a radiofrequency to saturate the new peak from metal ionꢀ
bound chelator and detecting the saturation transfer to the free
chelators, the signal of bound metal ions can be dramatically
amplified.
single F NMR peak, were employed as a strong F NMR/MRI
a.
Hubei Province Engineering and Technology Research Center for Fluorinated
Pharmaceuticals and School of Pharmaceutical Sciences, Wuhan University,
Wuhan 430071, China.
State Key Laboratory for Magnetic Resonance and Atomic and Molecular Physics,
Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences,
Wuhan 430071, China.
State Key Laboratory for Modification of Chemical Fibers and Polymer Materials,
b.
Results and discussion
A convenient and scalable synthesis of fluorinated chelators 1ꢀ4
was developed, in which the amidation reaction between
dianhydrides 5, 6 and fluorinated aniline 7, respectively, was
employed as the key step (Scheme 1). Fresh dianhydrides 5 and 6,
which are also commercially available, were prepared by
dehydration of EDTA and DTPA with acetic anhydride in the
c.
Dong Hua University, Shanghai 201620, China.
†
Emails of corresponding authors: zgyang@whu.edu.cn; zxjiang@whu.edu.cn.
19
Electronic Supplementary Information (ESI) available: F NMR of chelators in the
2+
3+
presence of Cu , Fe , EDTA and DTPA, association constant of chelators with
1
13
metal ions, copies of H/ C NMR, MS/HRMS spectra, HPLC chromatograms. See
DOI: 10.1039/x0xx00000x
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Scheme 1. Synthesis of fluorinated chelators 1ꢀ4
1
9
2+
2+
2+
7
Figure 2. F NMR of free chelators and Mg , Ca , Zn ꢀbound
chelators 1 (a), 2 (b), 3 (c), and 4 (d). Each sample contained 0.8
mM metal ions and 4.0 mM chelators in 40 mM Hepes buffer at pH
presence of pyridine, respectively. Fluorinated amine 7 with a
bis(trifluoromethyl)ꢀcarbinol moiety was then prepared through a
FriedelꢀCrafts reaction of hexafluoroacetone trihydrate on 4ꢀ
7
.2.
8
methylaniline in the presence of pꢀtoluenesulfonic acid. Monoazides
1
, 3 and diamides 2, 4 were selectively prepared by tuning the ion boundꢀchelators is less than 1 ppm. The small ꢁω values indicate
relative amount of amine 7 and dianhydride 5, 6 during the that metal ion chelation has limited influence on the electron
amidation reaction, respectively. It is noteworthy that the hydroxyl environment of the fluorines because multiple CꢀC single bonds
group in 7 is very inert during the amidation reactions because the significantly blocked the chelation effect. As a comparison, ꢁω of
2
+
two adjacent trifluoromethyl groups dramatically lowered the free TFꢀBAPTA and Zn ꢀbound TFꢀBAPTA reaches 10.5 ppm
reactivity by increasing its acidity and steric hindrance. To guarantee because the aromatic system can efficiently transfer the chelation
6
19
high purity of the fluorinated chelators 1ꢀ4, the amidation reaction effect to fluorines. The appearance of wellꢀdefined new F NMR
mixtures were purified with preparative HPLC. Finally, fluorinated peaks also indicates that the exchange between metal ionꢀbound
chelators 1ꢀ4 were prepared on 0.61ꢀ2.25 gram scales with good chelator and free chelator is very slow (kex < ꢁω), which is fit for
1
9
yields and high purities, respectively.
generating
F CEST contrast. Chelator 4 was chosen for
1
9
downstream F CEST study for two reasons. On one hand, chelator
As poor water solubility of fluorinated compounds always
limited their application in biological systems, the water solubility of
chelators 1-4 was then investigated. It was found that the water
solubility of chelators 1-4 is closely related to their fluorine contents
1
9
4
has a singlet F NMR peak from twelve fluorines which can
dramatically improve the F MRI sensitivity and lower the
detectable concentration. On the other hand, a single F NMR peak
1
9
19
from free and metal ionꢀbound chelator 4, respectively, can simplify
the F CEST process by avoiding the suppression of nearby peaks
(
F%). Low F% chelators 1 (21%) and 3 (18%) are soluble in water at
a wide pH range of 0 to 14 while high F% chelators 2 (28%) and 4
25%) are soluble in water only at pH above 6 and 5, respectively.
Therefore, all the fluorinated chelators 1-4 have no solubility issue
1
9
(
Fig. 2d).
(
The influence factors, including metal ion concentration, pH,
1
9
1
9
temperature, and addition of fast exchange metal ions, on the
F
for downstream F NMR/MRI study around physiological pH.
2
+
19
NMR of free and ionsꢀbound chelator 4 were also studied. The Ca
concentrationꢀdependent F NMR showed that chelator 4 and Ca
formed a complex with a broad F NMR peak around ꢀ74.9 ppm. A
wellꢀdefined peak can be observed when the Ca /chelator ratio is
larger than 1/10 (Fig. 3a). The pHꢀdependent F NMR showed that
pH 6.0ꢀ7.5, which is within physiological pH, is the best range for
detecting the Ca ꢀbound chelator 4 with F NMR (Fig. 3b). It is
As expected, each chelator produces a single F NMR peak
from multiple symmetrical fluorines which is ideal for detecting the
chelator at low concentration (red peaks in Fig. 1). To study the
chelation effects on F NMR, F NMR spectra of chelators 1-4 in
the presence of Mg , Ca , Fe , Cu , and Zn were collected.
After chelating with Fe and Cu , no observable new F NMR
peaks can be detected on the F NMR spectra of chelators 1-4 (Fig.
S1 in ESI†). In contrast, wellꢀdefined new F NMR peaks on
NMR spectra of chelators 1ꢀ4 were detected in the presence of Mg ,
1
9
2+
19
2
+
1
9
19
1
9
2
+
2+
3+
2+
2+
3
+
2+
19
2+
19
1
9
1
9
19
noteworthy that very little chemical shift change was found in the
F
2+
2
+
range of pH 6.0ꢀ7.5, i.e. ꢁω= 0.07 for Ca ꢀbound and free chelator
+
2
+
2+
4
, respectively. The addition of fast exchanging ions K resulted in
Ca and Zn (Fig. 2). For chelators 1ꢀ3, multiple chelation peaks
were detected in the presence of Ca and Zn which is correspond
to multiple species formed between the chelator and metal ion. It is
19
2
+
2+
no observable F NMR line broadening or chemical shift change,
while the addition of Mg resulted in observable F NMR line
2
+
19
9
broadening of free chelator 4 peak (Fig. 3c). Temperatureꢀdependent
very important to point out that each free chelator 1ꢀ4 produces a
1
9
1
9
F NMR showed no line broaden for free chelator 4 and, while,
singlet F NMR peak around ꢀ74.4 ppm, respectively, with a small
ω of 0.2 ppm despite their structural difference. Even after
chelation with metal ions, the ꢁω between free chelators and metal
2
+
obvious line sharpens for Ca ꢀchelator 4 at elevated temperature
ꢁ
1
0
(
Fig. 3d). It was also found that, in the range of 283 K to 310 K,
2
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2
+
2+
Figure 3. Concentration (a, Ca % = [Ca ]/[chelator 4] × 100%),
+
2+
pH (b), additive (c, 100 mM K and 0.8 mM Mg ), and
19
temperatureꢀdependent (d) F NMR of chelators 4. Each sample
contained 4.0 mM chelators 4 and 0.4 mM Ca (b, c) or 0.8 mM
Ca (d) in 40 mM Hepes buffer at pH 7.2.
2+
2
+
observable chemical shift change (ꢁω = 0.2 ppm) was observed for
2
+
both free and Ca ꢀbound chelator 4. In contrast to 5FꢀBAPTA of
19
which the F NMR chemical shift is very sensitive to pH,
temperature, and fast exchanging ions, chelator 4 shows little
response to these influence factors but to temperature. Therefore,
1
9
19
using chelator 4 in F NMR and F MRI can actually avoid the
image artifact and chemical shift calibration, which dramatically
1
9
19
2
+
simplifies the F NMR and F CEST MRI process.
Figure 4. pH (aꢀd) and metal ionꢀdependent (e, none; f, Mg ; g,
1
9
2+
2+
2+
To evaluate the F metal ion induced CEST (iCEST) effect of Ca ; h, Zn ) Zꢀspectra of chelator 4 and Ca . Each sample
metal ions, the pH and metal ionꢀdependent Zꢀspectra of chelator 4 contained 2.0 mM chelator 4 and 0.2 mM Ca in 40 mM Hepes
were collected. On one hand, an iCEST effect for Ca at ꢁω = 0.4 buffer at the indicated pH.
2
+
2
+
ppm was observed from the pHꢀdependent Zꢀspectra (Fig. 4aꢀd).
Because Ca ꢀbound chelator 4 dissociates under weak acidic
conditions, a pronounced iCEST effect can only be found when pH
2+
2+
Clear contrast for Mg at ꢁω = 0.6 ppm can still be observed in the
2+ 2+
presence of coexisting ions Ca and Zn (Fig. 5f). In the
meanwhile, it can also selectively detect Ca and Zn in the metal
ionꢀbound chelator 4 comparing to that of EDTA was evaluated with
F iCEST MRI. In the presence of 1 mM EDTA, competing
2+
2+
>
6.0. On the other hand, an iCEST effect can be found for other
2+
selected metal ions. For Mg , a wellꢀdefined iCEST effect on
19
chelator 4 at ꢁω = 0.6 ppm was found (Fig. 4f), while an iCEST
chelating reactions for metal ions between EDTA and chelator 4
2
+
2+
2+
2+
effect was found at ꢁω = 0.4 ppm for Zn (Fig. 4h). Thus, by tuning took place. It turned out that Mg , Ca , and Zn form much more
1
9
the frequency of saturation pulse, it is feasible for chelator 4 to stable complex with EDTA than these of chelator 4 because no
F
2
+
2+
2+
19
selectively detecting Mg , Ca , and Zn using F CEST MRI.
iCEST MRI was observed at either 0.4 ppm or 0.6 ppm. Association
1
9
2+ 2+ 2+
F iCEST MRI on metal ions was carried out on a 9.4 T scanner. constant measurement of chelators 1ꢀ4 with Mg , Ca , and Zn
Firstly, selective F iCEST MRI of chelator 4 on Mg , Ca , and through a titration method indicated that the EDTA and DTPA
Zn was studied (Fig. 5). F MRI of four tubes containing 5 mM
1
9
2+
2+
11
2
+
19
have higher binding affinities towards these metal ions than that of
the corresponding fluorinated chelators (Fig. S3 and Table S1 in
ESI†). Thus, addition of EDTA can turn off the F iCEST MRI by
preventing the formation of metal ion boundꢀchelator 4. In this way,
an on and offꢀ F iCEST MRI strategy for selectively detecting metal
ions can be developed. Because each metal ion has quite unique
stability constant with commercially available chelators, such as
EDTA, DTPA, DOTA, etc., it is possible to selectively turn off
iCEST MRI by addition of a commercially available chelator to a
metal ions mixture and selectively chelating the F iCEST MRIꢀ
generating metal ion(s). Based on these observation, Mg , Ca , and
Zn can be sensitively and selectively detected with chelator 4 by
F iCEST MRI at a concentration as low as 50 ꢂM.
To investigate the sensitivity of using chelator 4 in detecting
chelator 4 and 50 ꢂM each metal ions showed no noticeable contrast
19
(
Fig. 5a). However, when a saturation pulse was applied at ꢁω = 0.4
1
9
ppm, F iCEST images showed clear contrast for the tubes
containing Ca and Zn with contrast percentile of 20% and 19%,
respectively (Fig. 5b). When a saturation pulse was applied at ꢁω =
19
2
+
2+
2
+
19
0
.6 ppm, only the tube containing Mg showed clear contrast
F
19
F
iCEST images with contrast percentile of 17% (Fig. 5c). Therefore,
2+
using chelator 4, it is feasible to selectively detect Mg at ꢁω = 0.6
19
2
+
2+
ppm and simultaneously detect both Ca and Zn at ꢁω = 0.4 ppm
2+
2+
19
2
+
using F iCEST MRI. Although the ꢁω is pretty small, the chemical
1
9
shift insensitive nature of free and metal ion boundꢀchelator 4 to pH,
1
9
temperature, and fast exchanging ions makes the F iCEST MRI
1
9
2+
19
2+
19
process straightforward. Secondly, the selective F iCEST MRI of Mg with F iCEST MRI, a Mg concentrationꢀdependent
F
2
+
2+
2+
2+
Mg in the presence of Ca , Zn and vice versa were studied.
iCEST MRI was then carried out (Fig. 6). It was found that Mg can
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2
+
19
be detected at a concentration 10 ꢂM. In this case, a magnetization multiple ions, especially for Mg , can be selectively detected by
transfer ratio (MTR) of 11% was observed at a Mg /4 ratio of 1:500
with a data collection time of 6.5 minutes.
F
iCEST MRI with one of these probes. Because of the symmetrical
distribution of twelve fluorines, the probe exhibit enhanced
sensitivity for detecting metal ions at low concentration. To imaging
2+
1
9
metal ions in vivo with F iCEST MRI, some fine tunes on
sensitivity, selectivity, chelation property, and physicochemical
properties of the existing probes are always required. The study here
has illustrated a fine tune strategy to enhance the sensitivity of the
1
9
F imaging probe while maintain its metal ion selectivity by
constructing novel fluorinated chelators. Further fine tunes on the
structure of the probes to improve the selectivity on multiple metal
ions are actively going on in this group.
Experimental
General information
1
9
Figure 5. F iCEST MRI of chelator 4 in the presence of metal ions
1
19
13
2
+
2+
2+
H, F and C NMR spectra of chelators were recorded on a 400
Mg , Ca , Zn , and EDTA. Each sample contained 5 mM chelator
and 50 ꢂM indicated metal ion, respectively. Sample dꢀf were
MHz. Chemical shifts are in ppm and coupling constants (J) are in
4
1
2
+
Hertz (Hz). H NMR spectra were referenced to solvent hydrogens
added extra 50 ꢂM of Mg or 1 mM EDTA, respectively.
1
3
(
3.31 ppm) using CD
referenced to solvent carbons (m, 49.64ꢀ48.36 ppm for CD
NMR spectra were referenced to 2% perfluorobenzene (s, ꢀ164.90
ppm) in CD OD or sodium trifluomethanesulfonate (s, ꢀ79.61 ppm)
in D O. The splitting patterns for H NMR spectra are denoted as
3
OD as solvent. C NMR spectra were
1
9
3
OD). F
3
1
2
follows: s (singlet), d (doublet), q (quartet), m (multiplet). Unless
otherwise indicated, all reagents were obtained from commercial
supplier and used without prior purification. DMF, Et
3
N and
Pyridine were dried and freshly distilled prior to use.
19
F MRI experiments were performed on a 9.4 T microꢀimaging
19
system with a 10 mm inner diameter F coil (376.4 MHz) for both
radiofrequency transmission and reception. The RARE sequence was
employed for all MRI acquisitions with single average. RARE
factor=4, TR=6000 ms, TE=5.37 ms, matrix=32*32, Number of
average=4, FOV=30 mm*30 mm, slice thickness=20 mm, Saturation
pulse strength =1 uT, Saturation time=3 s.
General procedure for the preparation of compounds 1-4
compound 5. A slurry of EDTA (2.92 g, 10.00 mmol), dry pyridine
(4.75 g, 60.00 mmol), and acetic anhydride (4.08 g, 40.00 mmol)
Figure 6. (a) Plot of magnetization transfer ratio (MTR) vs XMg. (b)
19
2+
F iCEST MRI of chelator 4 in the presence of Mg . Alignment of
four tubes containing 5 mM of 4 and different concentrations of
o
was heat to 80 C and stirred at this temperature for 24 h. After
2
+
Mg (XMg =1:100, 1:250, 1:500, 1:1000).
cooled to room temperature, the reaction mixture was filtered. The
solid residue was washed with acetic anhydride and diethyl ether to
1
yield 5 as white solid (2.56 g, yield 99%). H NMR (400 MHz, dꢀ
Conclusions and prospects
DMSO) δ 3.70 (s, 8H), 2.66 (s, 4H).
1
9
In this study, we have developed a class of novel F CEST
imaging probes and applied them in sensitively and selectively
Compound 6. Diethylenetriaminepentaacetic acid bisanhydride 6
was prepared by following the same procedure for 5 as white solid
19
detecting metal ions with F iCEST MRI. These imaging probes
1
(
3.93 g, yield 99%). H NMR (400 MHz, dꢀDMSO): δ 2.60 (t, J =
1
9
with a strong and singlet F NMR peak from multiple fluorines can
be conveniently prepared on gram scales from commercially
available chelators. Comparing to known fluorinated metal ion
chelators, F NMR chemical shift of these chelators is not sensitive
to environment, e.g. pH and temperature, but sensitive of certain
metal ions, which turns these chelators into selective F imaging
probes for these metal ions. However, the chemical shift changes
induced by the metal ions in quite low, < 1 ppm, comparing to 10.5
ppm from TFꢀBAPTA. By tuning the frequency of saturation pulse,
6
.2 Hz, 4H), 2.76 (t, J = 6.2 Hz, 4H), 3.44 (s, 2H), 3.72 (s, 8H).
Compound 7. To a sealed vessel was added 4ꢀmethylaniline (4.00 g,
37.33 mmol), 4ꢀtoluene sulfonic acid monohydrate (0.71 g, 3.73
mmol), 1,1,1,3,3,3ꢀhexafluoroacetone trihydrate (12.32 g, 55.99
mmol) and toluene (25 mL). The vessel was sealed up and stirred for
1
9
1
9
8
h at 110 °C. The solution was concentrated under vacuum and the
residue was purified by flash chromatography on silica gel
(
petroleum ether/ethyl acetate = 9/1) to give compound 7 as white
4
| J. Name., 2012, 00, 1-3
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1
needles (6.93 g, yield 68%). H NMR (400 MHz, CDCl
3
) δ 7.38 (s,
Acknowledgements
1
H), 7.17 (d, J = 8.0 Hz, 1H), 6.97 (d, J = 8.0 Hz, 1H), 2.36 (s, 3H);
19
We are thankful for financial support from the National Key
Research and Development Program of China
Compound 1. To a stirring solution of EDTA dianhydride 5 (1.02 g, (2016YFC1304704), the National Natural Science Foundation
.00 mmol) in DMF (35 mL) and triethylamine (1.62 g, 16.00 mmol) of China (21372181, 21402144 and 21572168), and State Key
3
F NMR (376 MHz, CDCl ) δ ꢀ78.41.
4
was added a solution of amine 7 (0.55 g, 2.00 mmol) in 8 mL DMF Laboratory for Magnetic Resonance and Atomic and Molecular
dropwise under an atmosphere of nitrogen at 0 °C. After the Physics (Wuhan Institute of Physics and Mathematics).
addition, the mixture was stirred at 0 °C for 1 h and room
temperature for another 8 h. The reaction was quenched with H O
2
Notes and references
and the solvent was evaporated under vacuum. The residue was
1
(a) K. P. Carter, A. M. Young, A. E. Palmer, Chem. Rev.,
2014, 114, 4564ꢀ4601; (b) R. Y. Tsien, Biochemistry, 1980,
19, 2396ꢀ2404; (c) G. A. Smith, R. T. Hesketh, J. C.
Metcalfe, J. Feeney, P. G. Morris, Proc. Natl. Acad. Sci. USA,
1983, 80, 7178ꢀ7182; (d) F. A. X. Schanne, T. L. Dowd, R.
K. Gupta, J. F. Rosen, Proc. Natl. Acad. Sci. USA, 1989, 86,
5133ꢀ5135; (e) H. Komatsu, T. Miki, D. Citterio, T. Kuboba,
Y. Shindo, Y. Kitamura, K. Oka, K. Suzuki, J. Am. Chem.
Soc., 2005, 127, 10798ꢀ10799; (f) T. Nishihara, Y.
Kameyama, H. Nonaka, Y. Takakusagi, F. Hyodo, K.
Ichikawa, S. Sando, Chem. Asian J., 2017, 12, 949ꢀ953.
purified with preparative RPꢀHPLC to give 1 (0.61 g, yield 56%) as
1
white solid. H NMR (400 MHz, CD
3
OD) δ 8.07 (d, J = 8.2 Hz, 1H),
7.38 (s, 1H), 7.32 (d, J = 8.5 Hz, 1H), 4.11 (s, 4H), 3.96 (s, 2H), 3.90
(
3
s, 2H), 3.44 (t, J = 5.4 Hz, 2H), 3.35 (t, J = 5.5 Hz, 2H), 2.35 (s,
13
3
H); C NMR (100 MHz, CD OD) δ 172.3, 170.9, 168.7, 136.2,
1
35.9, 132.2, 129.4, 126.8, 124.4 (q, J = 289.6 Hz), 120.1, 82.0ꢀ80.9
19
(
m), 58.5, 55.8, 53.5, 52.2, 21.0; F NMR (376 MHz, CD
3
OD) δ ꢀ
ꢀ
ꢀ
74.69; HRMS (ESI) calcd for C20H F N O ([MꢀH] ), 546.1317,
22 6 3 8
found, 546.1328.
Compound 2. Chelator 2 was prepared by following the same
1
2. (a) E. T. Ahrens, R. Flores, H. Xu, P. A. Morel, Nat.
Biotechnol., 2005, 23, 983ꢀ987; (b) J. M. Janjic, M. Srinivas,
D. K. Kadayakkara, E. T. Ahrens, J. Am. Chem. Soc., 2008,
130, 2832ꢀ2841; (c) Z.ꢀX. Jiang, X. Liu, E. K. Jeong, Y. B.
Yu, Angew. Chem., Int. Ed., 2009, 48, 4755ꢀ4758; (d) J. Ruizꢀ
Cabell, B. P. Barnett, P. A. Bottomley, J. W. M. Bulte, NMR
Biomed., 2011, 24, 114ꢀ129; (e) D. Vivian, K. Cheng, S.
Khuranan, S. Xu, E. H. Kriel, P. A. Dawson, J. P. Raufman, J.
E. Polli, Mol. Pharm., 2014, 11, 1575ꢀ1582; (f) S. Langereis,
J. Keupp, J. L. J. van Velthoven, I. H. C. de Roos, D.
Burdinski, J. A. Pikkemaat, H. Grüll, J. Am. Chem. Soc.,
2009, 131, 1380ꢀ1381.
procedure for chelator 1 as white solid (1.30 g, yield 81%). H NMR
3
(400 MHz, CD OD) δ 8.08 (d, J = 8.1 Hz, 2H), 7.36 (s, 2H), 7.22 (d,
J = 8.1 Hz, 2H), 3.84 (s, 4H), 3.80 (s, 4H), 3.25 (s, 4H), 2.33 (s, 6H).
13
3
C NMR (100 MHz, CD OD) δ 171.6, 167.7, 136.0, 132.1, 129.4,
1
26.9, 124.3 (q, J = 288.4 Hz), 121.2, 82.0ꢀ80.8(m), 58.4, 55.9, 53.1,
1
9
21.1; F NMR (376 MHz, CD
3
OD) δ ꢀ74.69; HRMS (ESI) calcd for
H F N O ([MꢀH] ), 801.1799, found, 801.1798.
ꢀ
ꢀ
C
30 29 12 4 8
Compound 3. To a stirring solution of DTPA dianhydride 6 (1.43 g,
.00 mmol) and triethylamine (1.62 g, 16.00 mmol) in DMF (35 mL)
4
was dropwise added amine 7 (0.55 g, 2.00 mmol) in DMF (8 mL)
under an atmosphere of nitrogen at 0 °C and the mixture was stirred
at this temperature for 1 h. After warm to room temperature, the 3. (a) C. Zhang, S. S. Moonshi, H. Peng, S. Puttick, J. Reid, S.
mixture was stirred at room temperature for additional 8 h. The
Bernardi, D. J. Searles, A. K. Whittaker, ACS Sens., 2016, 1,
757ꢀ765; (b) D. Xie, S. Kim, V. Kohli, A. Banerjee, M. Yu, J.
S. Enriquez, J. J. Luci, E. L. Que, Inorg. Chem., 2017, 56,
6429ꢀ6437; (c)T. Nakamura, H. Matsushita, F.Sugihara, Y.
reaction was quenched with H O and the solvent was evaporated
2
under vacuum. The residue was purified with preparative RPꢀHPLC
1
to give 3 (0.79 g, yield 61%) as white solid. H NMR (400 MHz,
CD
3
OD) δ 8.14 (d, J = 8.6 Hz, 1H), 7.38 (s, 1H), 7.33 (d, J = 8.5 Hz,
Yoshioka, S. Mizukami, K. Kikuchi, Angew. Chem.
, Int. Ed.,
1
H), 4.17 (s, 2H), 3.85 (s, 2H), 3.80 (s, 6H), 3.52ꢀ3.35 (m, 8H), 2.35
2015, 54, 1007–1010.
13
3
(s, 3H); C NMR (100 MHz, CD OD) δ 174.4, 173.7, 170.8, 169.4, 4. (a) R. Hagen, J. P. Warren, D. H. Hunter, J. D. Roberts, J.
1
36.6, 135.4, 132.2, 129.4, 125.9, 123.0, 120.2, 82.0ꢀ80.9 (m), 59.0,
Am. Chem. Soc., 1973, 95, 5712ꢀ5716; (b) N. Illy, D.
Majonis, I. Herrera, O. Ornatsky, M. A. Winnik,
Biomacromolecules, 2012, 13, 2359ꢀ2369; (c) J. S. Summers,
J. B. Baker, D. Meyerstein, A. Mizrahi, I. Zilbermann, H.
Cohen, C. M. Wilson, J. R. Jones, J. Am. Chem. Soc., 2008,
130, 1727ꢀ1734.
. (a) G. E. Fryxell, W. Chouyyok, R. D. Rutledge, Inorg. Chem.
Commun., 2011, 14 , 971ꢀ974; (b) J. C. Joyner, L. Hocharoen,
J. A. Cowan, J. Am. Chem. Soc., 2012, 134, 3396ꢀ3410; (c) L.
Tahsini, S. E. Specht, J. S. Lum, J. J. M. Nelson, A. F. Long,
19
56.0, 54.9, 54.7, 54.3, 51.3, 50.9, 21.0; F NMR (376 MHz,
ꢀ
ꢀ
3 29 6 4
CD OD) δ ꢀ74.64; HRMS (ESI) calcd for C24H F N O10 ([MꢀH] ),
647.1793, found, 647.1796.
Compound 4. Chelator 4 was prepared by following the same
procedure for chelator 3 as white solid (2.25 g, yield 83%). H NMR
1
5
(400 MHz, CD
3
OD) δ 8.09 (d, J = 8.3 Hz, 2H), 7.35 (s, 2H), 7.26 (d,
J = 8.5 Hz, 2H), 4.19 (s, 2H), 3.78 (s, 4H), 3.73 (s, 4H), 3.48 (t, J =
13
6
.1 Hz, 4H), 3.33(t, J = 6.2 Hz, 4H), 2.33 (s, 6H); C NMR (100
MHz, CD OD) δ 173.1, 169.9, 169.8, 136.3, 135.6, 132.2, 129.3,
26.3, 124.4 (q, J = 288.7 Hz), 120.6, 82.1ꢀ80.9 (m), 58.9, 56.0,
3
J. A. Golen, A. L. Rheingold, L. H. Doerrer, Inorg. Chem.
,
1
1
9
2013, 52, 14050–14063; (d) S. A. Cantalupo, S. R. Fiedler,
M. P. Shores, A. L. Rheingold, L. H. Doerrer, Angew. Chem.
Int. Ed., 2012, 51, 1000–1005.
54.8, 54.1, 51.6, 21.0; F NMR (376 MHz, CD
3
OD) δ ꢀ74.69;
10 ([MꢀH] ), 902.2276, found,
ꢀ
ꢀ
HRMS (ESI) calcd for C34
H
36
F
12
N
5
O
902.2259.
6
. (a) A. BarꢀShir, A. A. Gilad, K. W. Y. Chan, G. Liu, P. C. M.
van Zijl, J. W. M. Bulte, M. T. McMahon, J. Am. Chem. Soc.
,
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DOI: 10.1039/C7OB01068K
ARTICLE
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Journal Name
2
Gilad, P. C. M. van Zijl, M. T. McMahon, J. W. M. Bulte, J.
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S. S. Kelkar, L. Xue, S. R. Turner, T. M. Reineke,
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(a) W. Yu, Y. Yang, S. Bo, Y. Li, S. Chen, Z. Yang, X. Zheng,
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(a) M. Peana, S. Medici, V. M. Nurchi, J. I. Lachowicz, G.
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1
1. (a) J. Bjerrum, "Metal Ammine Formation in Aqueous
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Organic & Biomolecular Chemistry
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DOI: 10.1039/C7OB01068K
19
F CEST imaging probes for metal ions detection
a
b
a
a
a
b
a
b
Qiaoli Peng, Yaping Yuan, Huaibin Zhang, Shaowei Bo, Yu Li, Shizhen Chen, Zhigang Yang, Xin Zhou, and
,
a,b
Zhong-Xing Jiang*
Graphical Abstract
1
9
An easily available fluorinated chelator was developed as a highly sensitive F iCEST MRI probe
for selectively detecting of metal ions.