Modulation of water exchange in europium(III) DOTA-tetraamide complexes via electronic substituent effects

Article abstract of DOI:10.1021/ja076325y

The chemical exchange saturation transfer (CEST) efficiency for a series Eu³⁺-based tetraamide complexes bearing p-substituents on a single coordinating pendant arm is highly sensitive to water exchange rates. The CEST effect increases in the order Me < MeO < F_～ CO₂^tBu < CN_? H. These results show that CEST contrast can be modulated by changes in electron density at a single ligating atom, and this forms the basis of creating imaging agents that respond to chemical oxidation and reduction. Copyright

Full text of DOI:10.1021/ja076325y

Published on Web 12/08/2007
Modulation of Water Exchange in Europium(III) DOTA-Tetraamide Complexes
via Electronic Substituent Effects
S. James Ratnakar,† Mark Woods,‡,§ Angelo J. M. Lubag,† Zolta´n Kova´cs,†,§ and A. Dean Sherry*,†,§
AdVanced Imaging Research Center, UT Southwestern Medical Center, 5323 Harry Hines BouleVard, Dallas, Texas
75390, Macrocyclics, 2110 Research Row, Suite 425, Dallas, Texas 75235, and Department of Chemistry,
UniVersity of Texas at Dallas, 800 West Campbell Road, Richardson, Texas 75080
Received August 22, 2007; E-mail: dean.sherry@utsouthwestern.edu
Chart 1. The EuDOTA-Tetraamide Complexes Prepared for This
Work
Magnetic resonance image (MRI) contrast originates from
differences in tissue proton densities in water and fat and from
inherent differences in water relaxation rates. Image contrast can
be altered further by administration of a paramagnetic contrast agent,
typically a chelated form of Gd3+, that shortens the longitudinal
relaxation time (T1) of bulk water protons.1 An alternative type of
MRI contrast agent has recently been proposed2 that alters image
contrast by transfer of selectively saturated spins from one chemical
pool to another, a process called chemical exchange saturation
transfer (CEST).2,3 In order to observe CEST, the rate of exchange
between the two pools (kex) must be slow relative to the frequency
difference between the two pools (∆ω). Thus, larger shift differ-
ences between the two pools permit larger values of kex. Large ∆ω
values have the added benefit of eliminating the problem of direct
off-resonance saturation of the solvent water. These distinct
advantages naturally led to the use of paramagnetic lanthanide ions
to induce large hyperfine shifts in the proximate exchangeable
protons of ligands. The metal-bound water molecule of Ln3+
DOTA-tetraamide complexes has been shown to exhibit unusually
slow water exchange rate and large hyperfine shifts, such that these
complexes meet the ∆ω g kex condition. These paramagnetic CEST
agents, often referred to as PARACEST agents, have been widely
studied as responsive agents that can report changes in tissue pH,4
temperature,5 and glucose,6 lactate,7 and Zn2+ ion8 concentrations.
Since the magnitude of CEST is critically dependent upon the
water exchange rate in such complexes, modulation of the water
exchange rate provides a platform upon which responsive PARAC-
EST agents may be based. It has been established that the water
exchange process occurs through a dissociative pathway,9 and the
rate of exchange is related to the free energy difference, ∆G*,
between the transition and the ground states. The parameters that
affect water exchange rates in lanthanide complexes have been the
subject of intense study over the past decade and have been found
to depend upon the coordination geometry,10 steric crowding,11 size
of the central Ln3+ ion,12 and the electronic properties of the
ligand.13 The idea that water exchange rates could be modulated
by changes in the electronic properties of the ligand is an attractive
prospect since this could provide the basis of a generalized approach
to the design of responsive PARACEST agents. To investigate this
hypothesis, a series of complexes were prepared that incorporated
either electron-donating (Me, OMe) or electron-withdrawing groups
complexes of each ligand were obtained by reaction with Eu(OTf)3
in acetonitrile. The complexes of DOTA-tetraamide ligand systems
can adopt two coordination geometries that exhibit different water
exchange kinetics.10b,c Highly shifted resonances, characteristic of
axial protons of the macrocyclic ring from the square antiprismatic
1
isomer,10b were observed between +22 and +28 ppm in each H
NMR spectrum, suggesting that all of these complexes exist as a
single isomer in solution.
It was anticipated that electron-donating substituents would push
electron density through the coordinating amide and onto the central
metal ion, weakening the metal-water interaction and accelerating
water exchange. Conversely, electron-withdrawing groups were
expected to decelerate water exchange. To examine this hypothesis
and establish whether these effects would translate in observable
changes, CEST spectra were recorded at 9.4 T. A CEST exchange
peak corresponding to the coordinated water molecule was observed
in each spectrum near 47 ppm (Figure 1). The magnitude of the
CEST peak differed in each complex, demonstrating that CEST
can be altered by small differences in electronic properties. To
assess the origins of these differences, the CEST spectra were fitted
to the Bloch equations (modified to obtain water exchange rates)15
by use of a nonlinear fitting algorithm written in MATLAB 7
(Mathworks Inc., Natick, MA). The results are summarized in
Table 1.
The water exchange data are more easily understood for those
substituents operating through mesomeric effects. As expected, the
water residence lifetime (τM) was found to be longer in complexes
t
bearing an electron-withdrawing group (CO2 Bu, CN) and shorter
t
(CO2 Bu, CN, F) in the para-position (Chart 1).
in those with an electron-donating group (OMe). This trend is
reversed for groups considered inductive; electron-donating groups
(Me) lengthen τM, while electron-withdrawing groups (F) shorten
τM. This suggests that the relationship between water exchange and
the electron properties of the amide substituents is more complex
than suggested by our initial hypothesis. For example, altering the
strength of the metal-water interactions may result in a change in
the acidity of the bound water protons such that a stronger metal-
Cyclen was selectively trialkylated with 3 equiv of ethyl-N-
bromoacetylglycinate, using NaHCO3 as base.14 The remaining
nitrogen was subsequently alkylated with various p-substituted
bromoacetyl aniline derivatives (CH3CN/K2CO3/60 °C). The Eu3+
† UT Southwestern Medical Center.
‡ Macrocyclics.
§ University of Texas at Dallas.
9
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J. AM. CHEM. SOC. 2008, 130, 6-7
10.1021/ja076325y CCC: $40.75 © 2008 American Chemical Society

C O M M U N I C A T I O N S
Figure 2. CEST difference images showing that the oxidized and reduced
forms of the complexes can be discriminated by MR imaging.
These results clearly demonstrate that the water exchange
kinetics, and ultimately the CEST properties, of EuDOTA-
tetraamide complexes are acutely sensitive to changes in the
electronic properties of the ligand, even at a relatively remote site,
providing these sites can communicate electronically with a donor
atom coordinated to the metal ion. This sensitivity to changes in
electronic effects could provide the basis for a general approach to
the design of new responsive PARACEST agents. Our success
generating differences in CEST in a chemically reducible system
suggests that it should prove possible to design redox-responsive
systems applicable to biology.
Figure 1. CEST arising from the coordinated water molecules of the Eu3+
complexes recorded at 7 T, 296 K, B1 ) 450 Hz, concn ) 20 mM (50%
H2O in CD3CN) and irradiation time ) 2 s (left). Spin echo CEST differ-
ence images of the complexes acquired at 9.4 T and 298 K (right), TR )
10 s, TE ) 8.2 ms, irradiation time ) 3 s, B1 ) 24 µT at -47.5 and
+47.5 ppm.
Table 1. Fitting Parameters from the CEST Spectra
X
)
effect
τM
(
µ
s)
∆ω (ppm)
1− (MS/M0)
Acknowledgment. This research was supported by grants from
the NIH (EB-04285, CA-115531, DK-058398, and RR-02584) and
the Robert A. Welch Foundation (AT-584).
Mesomeric
352 ( 17
324 ( 33
269 ( 32
198 ( 19
t
CO2 Bu
CN
-M
-M
0
47.69
47.61
46.72
47.25
0.415
0.477
0.611
0.194
H
Supporting Information Available: CEST spectra as a function
of applied B1; CEST fitting; imaging protocols; experimental details.
This material is available free of charge via the Internet at http://
pubs.acs.org.
OMe
+M
InductiVe
144 ( 8
269 ( 32
310 ( 55
F
H
-I
0
+I
44.98
46.72
46.89
0.347
0.611
0.113
Me
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