Selective sensing of zinc ions with a novel magnetic resonance imaging contrast agent

Article abstract of DOI:10.1039/b100994j

Light-based microscope imaging techniques using fluorescence sensor molecules suffer from photobleaching and light scattering, but magnetic resonance imaging (MRI) can provide three-dimensional imaging without these problems. Recently, "smart" MRI contras

Full text of DOI:10.1039/b100994j

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Selective sensing of zinc ions with a novel magnetic resonance
imaging contrast agent†
Kenjiro Hanaoka, Kazuya Kikuchi, Yasuteru Urano and Tetsuo Nagano*
2
2ϩ
Graduate School of Pharmaceutical Sciences, The University of Tokyo, 3-1, 7-chome, Hongo,
Bunkyo-ku, Tokyo 113–0033, Japan. E-mail: tlong@mol.f.u-tokyo.ac.jp; Fax: ϩ81 3 5841 4855
Received (in Cambridge, UK) 29th January 2001, Accepted 20th June 2001
First published as an Advance Article on the web 31st July 2001
Light-based microscope imaging techniques using fluorescence sensor molecules suffer from photobleaching and
light scattering, but magnetic resonance imaging (MRI) can provide three-dimensional imaging without these
problems. Recently, “smart” MRI contrast agents which modulate the access of water to a chelated gadolinium
3ϩ
(
Gd ) ion in the presence or absence of a specific trigger have been reported. Zinc (Zn ) is an essential component
of many enzymes, transcription factors and synaptic vesicles in excitatory nerve terminals, so imaging of chelatable
2
ϩ
3ϩ
2ϩ
Zn is of interest. We have designed and synthesized the Gd DTPA bisamide complex 7a as a Zn -sensitive MRI
2ϩ
contrast agent. Compound 7a shows a dose-dependent change in the R relaxivity in the presence of Zn . We
investigated this relaxation behavior, and for this purpose we also synthesized the Gd DTPA amide ethyl ester
complex 7b. It was shown that binding between 7a and Zn caused a change in the relaxation time. Moreover, 7a had
high selectivity for Zn against Ca and Mg . Compound 7a may have practical problems for in vivo usage, since
the R relaxivity is reduced with increased Zn concentration. However, this report demonstrates new approaches to
the design and synthesis of Gd complexes with R values that change with variation in Zn concentration.
1
3ϩ
2ϩ
2ϩ
2ϩ
2ϩ
2ϩ
1
3ϩ
2ϩ
1
Introduction
Experimental
2ϩ
Zinc (Zn ), which is a key component of many enzymes, tran-
Materials
1
scription factors and synaptic vesicles in excitatory nerve ter-
2
DTPA bisanhydride was purchased from Aldrich Chemical Co.
Inc., USA. DTPA anhydride ethyl ester was kindly donated
by Nihon Schering K.K., Japan. All other reagents were
purchased from either Tokyo Kasei Kogyo Co., Ltd., Japan
or Wako Pure Chemical Industries, Ltd., Japan. Ethanol, tri-
ethylamine, dichloromethane and N,N-dimethylformamide were
used after distillation.
minals, is present in serum at a concentration of ca. 12 µM
2
ϩ
3
(
total Zn ). Light-based microscope imaging of chelatable
2ϩ
Zn in the extra- and intra-cellular environments or tissues
2
ϩ
with fluorescent dyes that respond dose-dependently to Zn
2
ϩ
has contributed to our understanding the roles of Zn in cells
4
and tissues. However, light-based microscope imaging tech-
niques with fluorescent dyes can be limited by photobleaching
of the dyes and also by light scattering to cells within 100 µm of
the surface. Magnetic resonance imaging (MRI) can overcome
these problems and seems to be suitable for three-dimensional
Instruments
1
13
H- and C-NMR spectra were recorded on a JEOL JNM-
5
imaging.
LA300. Mass spectra (MS) were measured with a JEOL
JMS-DX 300 and a JEOL JMS-SX 102A mass spectrometer.
UV-visible spectra were obtained on a Shimadzu UV-1600.
HPLC purification was performed on a reversed-phase column
3
ϩ
Gadolinium (Gd ) complexes are frequently chosen as MRI
contrast agents. These complexes enhance the T (spin-lattice)
and T 2 (spin–spin) relaxation rates of water protons by rapid7
exchange of inner-sphere water molecules with bulk solvent.
The enhancement is, in part, due to the direct interaction of
water molecules (inner sphere) with the unpaired electrons of
6
1
(
Gl Sciences, Inertsil Prep-ODS 30 mm × 250 mm) fitted on a
Jasco PU-1587 System.
3
ϩ
3ϩ
the paramagnetic metal ion, Gd . Recently, some Gd -based
smart” MRI contrast agents have been reported for monitor-
Synthesis
“
2ϩ
8–10
N-tert-Butoxycarbonylethylenediamine (2). To a solution of
ethylenediamine (1) (32 ml, 479 mmol) in a mixture of ethanol
ing enzyme activity, Ca or pH.
change in the relaxation rate in the presence of enzyme, Ca or
pH owing to modulation of the accessibility of water molecules
These MRI agents show a
2ϩ
(
30 ml) and triethylamine (2.8 ml) was added dropwise a solu-
tion of di-tert-butyl dicarbonate (4.7 ml, 20.4 mmol) in ethanol
10 ml) at 0 ЊC. The mixture was stirred for 1 h at room temper-
3ϩ
3ϩ
to the chelated Gd ion. Here, we report the Gd diethylene-
(
triaminepentaacetic acid (DTPA) bisamide complex 7a as a
2ϩ
ature and evaporated to dryness. The residue was dissolved in
dichloromethane (50 ml) and extracted with 1 M aqueous acetic
acid three times. The solution was basified with 2 M aqueous
NaOH and extracted with dichloromethane four times. The
combined dichloromethane extract was dried over anhydrous
Zn -sensitive MRI contrast agent. This complex was designed
on the basis that N,N,NЈ,NЈ-tetrakis(2-pyridylmethyl)ethylene-
diamine (TPEN) readily complexes with Zn , but hardly
complexes with Ca and Mg .
2
ϩ
2ϩ
2ϩ 11
potassium carbonate and evaporated to dryness, to afford 2
1
(
2.83 g, 86.7%) as a colorless oil. H-NMR (300 MHz; CDCl ;
3
†
Electronic supplementary information (ESI) available: Fig. S1 and
t
Me Si): δ 1.25 (2H, br s, NH ), 1.45 (9H, s, Bu ), 2.80 (2H, t,
Fig S2 showing HPLC confirmation of complexes 7a and 7b. See http://
www.rsc.org/suppdata/p2/b1/b100994j/
4
2
J = 5.7 Hz, NCH
), 3.17 (2H, q, J = 6.0, 5.7 Hz, CNCH ), 5.02
2
2
1
840
J. Chem. Soc., Perkin Trans. 2, 2001, 1840–1843
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b100994j

View Article Online
1
3
(
4
1H, br s, NH). C-NMR (75 MHz, CDCl ): δ 28.43, 41.92,
3
ϩ
ϩ
3.49, 79.11, 156.30. MS (EI ): m/z 160 (M ).
N-tert-Butoxycarbonyl-NЈ,NЈ-bis(2-pyridylmethyl)ethylene-
diamine (3). To a solution of 2 (2.50 g, 15.6 mmol) in ethanol
100 ml) were added anhydrous sodium carbonate (7.30 g, 68.8
mmol) and 2-(chloromethyl)pyridine hydrochloride (5.60 g,
4.4 mmol). The resulting suspension was heated under reflux
(
3
under Ar overnight (21 h) and evaporated to dryness. The
residue was suspended in 2 M aqueous NaOH (100 ml) and
extracted with dichloromethane three times. The combined
dichloromethane extract was washed with brine, dried over
anhydrous potassium carbonate and evaporated to dryness. The
residue was purified by aluminum oxide column chroma-
tography (n-hexane–dichloromethane, 1 : 1) to give 3 (4.34 g,
1
8
1.2%) as an orange oil. H-NMR (300 MHz; CDCl ; Me Si):
3
4
t
Fig. 1 The water proton relaxivity of 7a (closed triangle) and 7b (open
δ 1.45 (9H, s, Bu ), 2.71 (2H, t, J = 5.7 Hz, C NCH ), 3.23 (2H,
2
2
triangle) at pH 8.0, 25 ЊC in the presence of various concentrations of
Zn : 0, 0.1, 0.3, 0.5, 1.0, 1.5 and 2.0 equiv.
m, CONCH ), 3.87 (4H, s, NCCNCH ), 5.79 (1H, br s, NH),
2
2
2ϩ
7
.16 (2H, dd, J = 6.4, 4.1 Hz, 5-H), 7.42 (2H, d, J = 7.7 Hz,
3
-H), 7.64 (2H, dd, J = 7.7, 6.4 Hz, 4-H), 8.55 (2H, d, J = 4.1
5
8.96, 59.52, 63.11, 126.22, 127.23, 144.19, 145.43, 153.33,
ϩ
13
Hz, 6-H). C-NMR (75 MHz; CDCl ): δ 28.48, 38.53, 53.52,
6
3
172.13, 172.67, 173.77, 176.70, 177.95. MS (FAB ): m/z 646
0.22, 78.65, 122.02, 123.03, 136.39, 149.10, 156.16, 159.32. MS
ϩ
(
[M ϩ H] ).
ϩ
ϩ
(
EI ): m/z 342 (M ).
3
؉
Gd DTPA bisamide complex (7a). Compounds 7a and 7b
13
N,N-Bis(2-pyridylmethyl)ethylenediamine (4). A solution of 3
were synthesized according to the literature. Briefly, to a
solution of 6a (120 mg, 0.143 mmol) in Tris buffer (1 M; pH 8.0,
5 ml) was added 100 mM aqueous GdCl (1.57 ml). The result-
(
4.30 g, 12.6 mmol) in dichloromethane (40 ml) was added
dropwise to TFA (100 ml) at 0 ЊC. The mixture was stirred for
h at room temperature and evaporated to dryness. The residue
3
1
ing mixture was stirred at room temperature for 30 min and
was dissolved in 2 M aqueous NaOH (75 ml) and extracted with
dichloromethane. The combined dichloromethane extract was
dried over anhydrous potassium carbonate and evaporated to
dryness. The residue was purified by aluminum oxide column
purified by HPLC with methanol–H O (3 : 2) as eluent to give
2
7a (71.0 mg, 50.0%) as a colorless solid. Mp > 276 ЊC
Ϫ1
(decomp.). IR (KBr): νmax/cm 3391 (H O), 3260, 3090, 2953,
2
ϩ
ϩ
1624, 1435, 1400, 769. MS (FAB ): m/z 997 ([M ϩ H] ). Anal.
Calcd. (found) for C H N O Gdؒ7.5H O: C, 44.59 (44.65);
chromatography (dichloromethane–methanol; 95 : 5) to give 4
42
52 11
8
2
1
(
1.95 g, 64.1%) as an orange oil. H-NMR (300 MHz; CDCl ;
H, 5.97 (6.02); N, 13.62% (13.40%).
3
Me Si): δ 1.60 (2H, br s, NH ), 2.67 (2H, t, J = 5.7 Hz, CH ),
4
2
2
3
؉
2
.80 (2H, t, J = 5.7 Hz, CH ), 3.85 (4H, s, CCNCH ), 7.15 (2H,
Gd DTPA amide ethyl ester complex (7b). The reaction
mixture was purified by HPLC with methanol–H O (2 : 3) as
eluent. Yield 27.9%. Mp > 237 ЊC (decomp.). IR (KBr):
2
2
dd, J = 7.3, 5.0 Hz, 5-H), 7.49 (2H, d, J = 7.5 Hz, 3-H), 7.65
2
13
(
2H, dd, J = 7.5, 7.3 Hz, 4-H), 8.53 (2H, m, 6-H). C-NMR (75
Ϫ1
MHz; CDCl ): δ 39.57, 57.34, 60.71, 122.02, 123.05, 136.43,
ν
max/cm 3398 (H O), 3260, 3120, 2980, 1597, 1435, 1404, 1207,
2
ϩ ϩ
3
ϩ
ϩ
1
49.06, 159.58. MS (EI ): m/z 242 (M ).
769. MS (FAB ): m/z 801 ([M ϩ H] ). Anal. Calcd. (found)
for C H N O Gdؒ4.0H O: C, 41.32 (41.31); H, 5.55 (5.49);
30
40
7
9
2
DTPA bisamide (6a). Compounds 6a and 6b were synthesized
according to the literature. Briefly, DTPA bisanhydride (5a,
48 mg, 0.48 mmol) was slowly added to a solution of 4 (200
N, 11.24% (11.02%).
Relaxation time measurement
Relaxation time, T , of aqueous solutions of the Gd complex
12
1
3ϩ
mg, 0.83 mmol) in DMF (1.3 ml) at 50 ЊC and the mixture was
heated under Ar for 4 h at 70 ЊC. The solution was cooled down
and was evaporated to dryness to give a colorless oil. Com-
pound 6a was precipitated from ethanol by the addition of
acetone, washed with acetone and dried under reduced pressure
1
7
a or 7b was measured in Tris buffer (0.1 M; pH 8.0) by using
the standard inversion–recovery procedure (JEOL JNM-
LA300, 25 ЊC). The relaxivity, R , of 7a or 7b was determined
1
from the slope of the plot of 1/T vs. [7a] or [7b] (0.3, 0.5, 0.7,
1
1
3ϩ
to afford a colorless solid (127 mg, 36.5%). H-NMR (300
1
.0 mM). The buffered Gd complex (7a or 7b) solution was
MHz; D O): δ 2.95 (4H, t, J = 6.2 Hz), 3.04 (4H, t, J = 5.7 Hz),
2
allowed to equilibrate for at least 10 min after addition of
ZnCl , CaCl or MgCl aqueous stock solution.
3
.08 (4H, s), 3.17 (4H, t, J = 6.2 Hz), 3.23 (4H, s), 3.38 (4H, t,
J = 5.7 Hz), 3.64 (2H, s), 4.12 (8H, s), 7.37 (4H, t, J = 7.7, 5.3
2
2
2
Hz), 7.42 (4H, d, J = 7.9 Hz), 7.85 (4H, t, J = 7.9, 7.7 Hz), 8.40
13
Results and discussion
(
4H, d, J = 5.3 Hz). C-NMR (75 MHz; D O): δ 37.10, 51.48,
2
5
1
3.97, 55.47, 55.56, 58.72, 59.71, 59.91, 125.64, 126.52, 142.17,
It is known that N,N,NЈ,NЈ-tetrakis(2-pyridylmethyl)ethylene-
ϩ
2ϩ
46.92, 153.61, 171.35, 174.76, 179.19. MS (FAB ): m/z 842
diamine (TPEN) complexes strongly with Zn , but hardly at
ϩ
2ϩ
2ϩ
(
[M ϩ H] ).
all with Ca or Mg . Reactions between primary amines and
DTPA bisanhydride have been widely used in the synthesis of
3ϩ
12
DTPA amide ethyl ester (6b). DTPA amide ethyl ester 6b was
DTPA bisamide chelaters. Therefore, we designed the Gd
DTPA bisamide complex 7a, which contains the TPEN moiety,
prepared by reacting DTPA anhydride ethyl ester 5b with 4
1 equiv.) using the same method as described for DTPA bis-
2ϩ
(
as a Zn -sensitive MRI contrast agent. Compound 7a was
1
amide 6a. Yield 49.6%. H-NMR (300 MHz; D O): δ 1.14 (3H,
t, J = 7.3 Hz), 2.96 (2H, t, J = 5.3 Hz), 3.08 (2H, t, J = 6.0 Hz),
3
obtained according to the reaction Scheme 1. In characterizing
2
3ϩ
the Gd DTPA bisamide complex 7a, we observed that the
2ϩ
.15 (2H, t, J = 6.6 Hz), 3.25–3.30 (6H, m), 3.40 (2H, t, 5.7 Hz),
water proton R relaxivity of 7a had an unusual Zn depend-
1
3
.42 (2H, s), 3.43 (2H, s), 3.65 (2H, s), 3.70 (2H, s), 4.09 (2H,
ence. The R decreased dose-dependently between 0 and 1.0
equiv. Zn , reaching a minimum at 1.0 equiv. Zn to 7a, then
1
2ϩ
2ϩ
q, J = 7.3 Hz), 4.17 (4H, s), 7.59 (2H, dd, J = 7.5, 5.7 Hz), 7.64
(
2H, d, J = 8.0 Hz), 8.10 (2H, dd, J = 8.0, 7.5 Hz), 8.52 (2H,
R increased dose-dependently between 1.0 and 2.0 equiv. (Fig.
1
13
d, J = 5.7 Hz). C-NMR (75 MHz; D O): δ 14.17, 37.32,
5
1). The R relaxivity of the complex 7a decreased approximately
2
1
2ϩ
2ϩ
0.57, 51.90, 53.34, 53.60, 55.51, 55.99, 56.26, 57.83, 57.92,
33% when Zn (1.0 equiv. to 7a) was added to a Zn -free
J. Chem. Soc., Perkin Trans. 2, 2001, 1840–1843 1841

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Fig. 2 UV-visible spectra of 7a (150 µM) in Tris buffer (0.1 M; pH 8.0)
2
ϩ
in the presence of various concentrations of Zn : 0 equiv. (open
triangle), 0.5 equiv. (open square), 1.0 equiv. (open circle). For the
2ϩ
addition of Zn between 1.0 and 3.0 equiv., the absorbance spectra
remained at a plateau.
solution. To provide further insight into the unusual relaxation
behavior of 7a, we examined the UV-visible spectra of a 7a
solution (150 µM) at pH 8.0 (0.1 M Tris buffer) in the presence
2ϩ
of various concentrations of Zn (Fig. 2). The absorbance
2
ϩ
between 220 and 300 nm changed linearly with increase in Zn
2ϩ
concentration up to 1 : 1 Zn –7a molar ratio, with isosbestic
points at 217.5, 255.0, 268.0, 275.5 nm and remained at a
2ϩ
plateau with further increase of Zn . The reason for these
2ϩ
results is considered to be as follows. When the Zn –7a molar
2ϩ
ratio is between 0 : 1 and 1 : 1, Zn and 7a form a 1 : 1 com-
2ϩ
plex. On the other hand, when the Zn –7a molar ratio exceeds
2ϩ
one, Zn and 7a form a 2 : 1 complex (Fig. 3). We hypothesize
2ϩ
that 7a in the Zn –7a; 1 : 1 complex has fewer water molecules
3ϩ
2ϩ
binding directly to Gd than 7a in a Zn -free solution. This
3ϩ
Fig. 3 Schematic representation of Gd DTPA bisamide complex 7a
2ϩ
can be understood in terms of the Zn coordination geo-
for the proposed conformational dependence of the structure in the
2
ϩ
metry, which is proposed to be as shown in Fig. 3. Complex 7a
presence and absence of Zn .
3
ϩ
Scheme 1 Synthesis of Gd DTPA bisamide complex. Reagents and conditions: (i) di-tert-butyl dicarbonate, EtOH, rt; (ii) 2-(chloromethyl)-
pyridine hydrochloride, Na CO , EtOH, reflux, under Ar; (iii) TFA, rt; (iv) DTPA bisanhydride, DMF, 70 ЊC, under Ar; (v) GdCl ؒ6H O,
2
3
3
2
Tris buffer (pH 8.0), rt.
1
842
J. Chem. Soc., Perkin Trans. 2, 2001, 1840–1843

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a
3ϩ
Table 1 R relaxivity for solutions of the Gd DTPA bisamide com-
1
b
2ϩ
2ϩ
2ϩ
plex 7a upon addition of Zn , Ca or Mg
2
ϩ
2ϩ
2ϩ
No addition
Zn
Ca
Mg
6
.06
3.98
was measured at 25 ЊC and 100 mM Tris buffer,
pH 8.0. All cations were added as ZnCl , CaCl or MgCl (1 equiv.
6.14
6.41
a
Ϫ1 Ϫ1
R relaxivity/mM
s
1
b
2
2
2
to 7a).
showed characteristic changes of the R1 relaxivity in the
2ϩ
presence of various concentrations of Zn from 0 : 1 to 1 : 1
2ϩ
Zn –7a molar ratio, and increased gradually from 1 : 1 to
2
ϩ
2
: 1 Zn –7a molar ratio. We hypothesize that upon the add-
2ϩ 2ϩ
ition of Zn between 0 : 1 and 1 : 1 Zn –7a molar ratio, water
molecules bound directly to Gd are displaced. However, upon
the addition of Zn between 1 : 1 and 2 : 1 Zn –7a molar
ratio, 7a and Zn form a 1 : 2 complex which bears the same
3ϩ
2ϩ
2ϩ
2ϩ
number of water molecules as 7a. The R relaxivity and UV-
1
3ϩ
visible spectra of the Gd DTPA amide ethyl ester complex 7b
are also consistent with this idea. Moreover, compound 7a had
2
ϩ
2ϩ
2ϩ
high selectivity for Zn2 against Ca and Mg . This agent is
ϩ
14
the first non-ionic Zn -sensitive MRI contrast agent, and a
2ϩ
decrease of the R relaxivity upon addition of Zn has never
1
previously been reported.
2ϩ
As a means for measuring Zn , this new derivative 7a may
not differentiate the concentration in the likely desirable range
of 10–15 µM. However, this compound should be an excellent
candidate for incorporation into sensors designed for the select-
ive detection of Zn in biological applications. Such studies are
in progress. Compound 7a may also be a candidate for the
3
ϩ
Scheme 2 Synthesis of Gd DTPA amide ethyl ester complex 7b.
Reagents and conditions: (i) DTPA anhydride ethyl ester, DMF, 70 ЊC,
under Ar; (ii) GdCl ؒ6H O Tris buffer (pH 8.0), rt.
3
2
2ϩ
contains two amides, which have weak chelating ability, and
there is probably a steric repulsion effect of the four pyridines.
In the Zn –7a 2 : 1 complex, 7a has the same number of water
molecules binding directly to Gd as 7a in a Zn -free solution,
because the second Zn opens up the water-accessible space on
the surface of Gd (Fig. 3). These Zn -chelating character-
istics of 7a therefore modulate access of water to the Gd ion.
To understand further the chelating characteristics of such
compounds, we examined the Gd DTPA amide ethyl ester
complex 7b. Compound 7b has a different Zn -chelating seg-
ment from 7a, with the same Gd chelating moiety. Compound
b was synthesized according to reaction Scheme 2. The water
proton R relaxivity of 7b at pH 8.0 (0.1 M Tris buffer ) in the
presence of various concentrations of Zn did not change with
Zn concentration (Fig. 1). We also measured the UV-visible
spectra of a 7b solution (300 µM) at pH 8.0 (0.1 M Tris buffer)
upon addition of Zn (data not shown). The absorbance
spectra of a 7b solution changed similarly to that of a 7a solu-
tion. The absorbance between 220 and 300 nm changed linearly
with Zn concentration up to 1 : 1 Zn –7b molar ratio and
remained at a plateau with further increase in Zn concen-
tration. It can be considered from these results that 7b and Zn
form a 1 : 1 complex, and that the water-accessibility of the
chelated Gd ion is not changed by this complexation. These
results are consistent with the hypothesis in the case of 7a.
For a Zn -sensitive MRI contrast agent, high selectivity
against Mg and Ca is crucial. Therefore, we also examined
the effects of Ca , Mg and H on the binding of 7a to Zn .
No effect of H on the R1 relaxivity of 7a was observed
between pH 4 and 10, in the absence of Zn (data not shown).
The R relaxivity of 7a solution on the addition of one equiv-
alent of Ca or Mg at pH 8.0 (0.1 M Tris buffer) is shown in
Table 1: there was no change. The Gd DTPA bisamide com-
formation of responsive luminescent lanthanide complexes
2ϩ
15
(
Eu, Tb).
3ϩ
2ϩ
2ϩ
3ϩ
2ϩ
Acknowledgements
3ϩ
We thank Nihon Schering K.K. for advice and for providing
DTPA anhydride ethyl ester.
3ϩ
2ϩ
3ϩ
References
7
1
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2ϩ
2
2ϩ
3 M. M. Parker, F. L. Humaller and D. J. Mahler, Clin. Chem., 1967,
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2ϩ
4
M. S. Nasir, C. J. Fahrni, D. A. Suhy, K. J. Kolodsick, C. P. Singer
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2ϩ
2ϩ
2ϩ
5 A. Y. Louie, M. M. Hüber, E. T. Ahrens, U. Rothbächer, R. Moats,
R. E. Jacobs, S. E. Fraser and T. J. Meade, Nature Biotechnol., 2000,
2ϩ
1
8, 321.
3ϩ
6 P. Caravan, J. J. Ellison, T. J. McMurry and R. B. Lauffer, Chem.
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7
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2ϩ
1
2ϩ
2ϩ
8 R. A. Moats, S. E. Fraser and T. J. Meade, Angew. Chem., Int. Ed.
Engl., 1997, 36, 726.
2ϩ
2ϩ
ϩ
2ϩ
ϩ
9
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1413.
2ϩ
1
0 S. Zhang, K. Wu and A. D. Sherry, Angew. Chem., Int, Ed., 1999, 38,
192; M. Mikawa, N. Miwa, M. Braautigam, T. Akaike and
A. Maruyama, J. Biomed. Mater. Res., 2000, 49, 390.
1
3
2ϩ
2ϩ
3ϩ
11 M. P. Cuajungco and G. J. Lees, Brain Res., 1998, 799, 97;
Y. Lawrence, J. P. Ozil and K. Swann, Biochem. J., 1998, 335, 335.
2ϩ
plex 7a thus showed high selectivity for binding Zn against
2ϩ
2ϩ
1
2 S. W. A. Bligh, A. H. M. S. Chowdhury, D. Kennedy, C. Luchinat
and G. Parigi, Magn. Reson. Med., 1999, 41, 767.
Ca and Mg .
1
3 M. Li and P. R. Selvin, J. Am. Chem. Soc., 1995, 117, 8132.
Conclusion
14 O. Reany, T. Gunnlaugsson and D. Parker, Chem. Commun., 2000,
4
73.
3ϩ
We designed and synthesized the Gd DTPA bisamide com-
plex 7a as a Zn -sensitive MRI contrast agent. The 7a solution
1
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