1H and17O NMR relaxometric study in aqueous solution of Gd(III) complexes of EGTA-like derivatives bearing methylenephosphonic groups

Article abstract of DOI:10.1002/mrc.2316

The Gd(III) complexes of three new octadentate chelators, prepared by substitution of four, two, and one carboxylate groups of EGTA with phosphonate groups, have been investigated by ¹H and ¹⁷O NMR relaxometric techniques in aqueous solutions. The analysis of the solvent proton relaxivity data as a function of pH, temperature, and magnetic field strength (nuclear magnetic relaxation dispersion (NMRD) profiles) in combination with the ¹⁷O transverse relaxation rate data at variable temperature allowed assessing the hydration state of the complexes, the occurrence of pH-dependent oligomerization processes for the tetraphosphonate derivative, the presence of a well-defined second sphere of hydration that markedly contributes to the relaxivity, and the values of the structural and dynamic relaxation parameters. In addition, in the case of themonophosphonate derivative the presence of a coordinated water molecule has allowed evaluation of the kinetic parameters of the exchange process, highly relevant for the possible use of this Gd(III) complex as anMRI probe. The rate of exchange of thewatermolecule, ²⁹⁸k_ex = 4.2 × 10⁸_s ^-1, is one of the highest measured so far for a nonacoordinate Gd(III) chelate and optimal for developing contrast-enhancing probes of high efficacy at high magnetic fields. Copyright

Full text of DOI:10.1002/mrc.2316

Research Article
Received: 13 June 2008
Revised: 21 July 2008
Accepted: 28 July 2008
Published online in Wiley Interscience
(www.interscience.com) DOI 10.1002/mrc.2316
1H and 17O NMR relaxometric study
in aqueous solution of Gd(III) complexes
of EGTA-like derivatives bearing
methylenephosphonic groups†
Lorenzo Tei,a Mauro Botta,a∗ Clara Lovazzano,a Alessandro Barge,b
Luciano Milonec and Silvio Aimec∗
The Gd(III) complexes of three new octadentate chelators, prepared by substitution of four, two, and one carboxylate groups of
EGTA with phosphonate groups, have been investigated by 1H and 17O NMR relaxometric techniques in aqueous solutions. The
analysis of the solvent proton relaxivity data as a function of pH, temperature, and magnetic field strength (nuclear magnetic
relaxation dispersion (NMRD) profiles) in combination with the 17O transverse relaxation rate data at variable temperature
allowed assessing the hydration state of the complexes, the occurrence of pH-dependent oligomerization processes for the
tetraphosphonate derivative, the presence of a well-defined second sphere of hydration that markedly contributes to the
relaxivity, and the values of the structural and dynamic relaxation parameters. In addition, in the case of the monophosphonate
derivative the presence of a coordinated water molecule has allowed evaluation of the kinetic parameters of the exchange
process, highly relevant for the possible use of this Gd(III) complex as an MRI probe. The rate of exchange of the water molecule,
298
k
ex
= 4.2 × 108s−1, is one of the highest measured so far for a nonacoordinate Gd(III) chelate and optimal for developing
c
contrast-enhancing probes of high efficacy at high magnetic fields. Copyright ꢀ 2008 John Wiley & Sons, Ltd.
Keywords: relaxivity; NMRD; 17O NMR; Gd(III) complex; paramagnetic NMR; phosphonate
Introduction
(diethylenetriamine-N,Nꢁ,Nꢁꢁ-pentaacetic acid) and macrocyclic
H4DOTA (1,4,7,10-tetraazacyclododecane-N,Nꢁ,Nꢁꢁ,Nꢁꢁꢁ-tetraacetic
acid) chelators are both octadentate and form very stable mono
aqua complexes with Gd(III) with tricapped trigonal prismatic
and monocapped square antiprismatic coordination geometries,
respectively.[7] Both [Gd(DTPA)(H2O)]2− and [Gd(DOTA)(H2O)]−
are approved CA currently used in the clinical practice and present
similar chemical and pharmacokinetic properties and relaxation-
enhancing ability.
For several decades, polyaminocarboxylate ligands have been
extensively used for the coordination of metal ions in an extremely
broad range of applications.[1–5] More recently, complexes of
lanthanide(III) cations with this class of chelators have been the
focusofgreatinterestbecauseoftheirincreasinguseintherapeutic
and diagnostic clinical protocols.[6] This large attention has been
stimulated by the possibility to exploit the different properties
of the various members of the lanthanide series: optical (Eu, Tb,
Yb), magnetic (Gd, Dy), and radionuclear (153Sm, 166Ho, 177Lu). The
coordination number, CN, of trivalent Ln cations typically being
between10(lightermembers)and8(heaviermembers), theligand
denticity is generally higher than 6 and most commonly 7/8. The
higher denticity ensures a greater thermodynamic stability of the
resulting metal chelates, a prerequisite of primary importance
for biomedical studies.[6] In magnetic resonance imaging (MRI)
applications as contrast-enhancing agents (CA), Gd(III) is the
ion of choice because of the well-known favorable properties:
seven unpaired electrons isotropically distributed, high magnetic
moment, and long electronic relaxation times. Gd-based CA
function by shortening the nuclear magnetic relaxation times of
the water protons through a dipolar interaction mediated by the
chemical exchange between the inner coordination sphere and
the bulk water, as well as indirectly by the interaction involving the
solvent molecules diffusing near the complex (outer coordination
sphere).[7,8] The presence of at least one inner-space (IS) water
molecule (q = 1) in fast exchange is essential to provide the
complex with a good relaxation efficiency. The acyclic H5DTPA
The replacement of one or more acetic arms by methylenephos-
phonic groups has been achieved and investigated in several
linear and cyclic ligands. This substitution may markedly influence
several properties of the corresponding Ln(III) complexes: thermo-
∗
Correspondenceto:Mauro Botta,DipartimentodiScienzedell’Ambienteedella
Vita, Universit‘a del Piemonte Orientale ‘‘A. Avogadro’’, Via Bellini 25/G, I-15100
Alessandria, Italy. E-mail: mauro.botta@mfn.unipmn.it
Silvio Aime, Dipartimento di Chimica IFM, Universit‘a degli Studi di Torino, Via
P.Giuria 7, I-10125 Torino, Italy. E-mail: silvio.aime@unito.it
†
a
Dedicated to Prof. P. Pregosin on his 65th birthday.
`
Dipartimento di Scienze dell’Ambiente e della Vita, Universita del Piemonte
Orientale ‘‘A. Avogadro’’, Via Bellini 25/G, I-15100 Alessandria, Italy
`
b
c
Dipartimento di Scienza e Tecnologia del Farmaco, Universita degli Studi di
Torino, Via P. Giuria 7, I-10125 Torino, Italy
`
Dipartimento di Chimica IFM, Universita degli Studi di Torino, Via P. Giuria 7,
I-10125 Torino, Italy
c
Magn. Reson. Chem. 2008, 46, S86–S93
Copyright ꢀ 2008 John Wiley & Sons, Ltd.

1H and 17O relaxometric study of Gd(III) complexes of EGTA derivatives
Scheme 1. Chemical structure of the ligands.
dynamic stability, overall electric charge, solution structure, rate of
coordinated water exchange (kex), and hydration number (q). The
phosphonate derivatives of the Gd-based CA also show significant
changes in the relaxometric properties: (i) the bulkier phospho-
nate groups may preclude the access of water to the coordination
site of the metal ion, thus reducing the q value; (ii) for a similar rea-
son the rate of exchange of the bound water molecule(s), kex, may
be accelerated; and (iii) the ability of the phosphonate moieties to
involve water molecules in H-bonding interactions often results in
the formation of a well-defined second coordination sphere that
enhances the relaxation efficiency of the Gd complex.
In the present work we have considered three novel
phosphonate derivatives of H4EGTA (ethyleneglycol-bis(2-
aminoethylether)-N,N,Nꢁ,Nꢁ-tetraacetic acid), an octadentate
acyclic ligand that forms stable complexes with Ln(III) cations
whose solution structures and relaxometric properties have been
studied in detail (Scheme 1).[9] In particular, the most relevant fea-
ture of [Gd(EGTA)(H2O)]− with respect to other related complexes
is the presence of a fast rate of water exchange, a property of great
relevance for the development of highly effective macromolecular
MRI CA. The value of kex, determined by 17O NMR techniques, is
about an order of magnitude higher than for [Gd(DTPA)(H2O)]2−
and [Gd(DOTA)(H2O)]−, and it is explained by the steric constraint
oftheoxoethylenicbridgeonthecoordinatedwatermoleculethat
lowers the activation energy of the dissociative exchange process.
We have synthesized and characterized with NMR relaxometric
techniques the solution behavior of three derivatives of EGTA in
which four,[10] two, and one carboxylates have been replaced by
phosphonate groups.
correlation time, τR) and has a little dependence from the other
parameters: the electronic relaxation times T1,2e (dependent on
the trace of the square of the zero-field splitting, ꢀ2, and on
the correlation time describing the modulation of the zero-field
splitting, τV), the residence lifetime of the bound water molecule
τM (τM = 1/kex), and the Gd–Hw distance r.
Characterization of GdEGT4P
The relaxivity of the tetraphosphonate Gd(III) complex has a value
of 5.3 mM−1 s−1 at 20 MHz, 298 K, and pH = 7.5. This value is
similar to that measured for GdEGTA (r1p = 4.7) and several
others q = 1 complexes of similar molecular size.[7,8] On the
otherhand,GdDOTP(H8DOTP = 1,4,7,10-tetraazacyclododecane-
1,4,7,10-tetrakis(methylenephosphonic acid), a complex that has
no water molecules in its inner coordination sphere (q = 0), has
a relaxivity (r1p = 4.6) closely related to that of the carboxylate
analogue GdDOTA (r1p = 4.7). This was explained by the presence
of a well-defined second sphere of hydration, favored by the
presence of the phosphonic groups which are able to promote
strong H-bonding interactions with the water molecules.[11–13]
Clearly, the assessment of the hydration of the complex needs
further investigation. Unlike GdEGTA, whose relaxivity is constant
in the pH range 3–12, the relaxivity of GdEGT4P has a pronounced
pH dependence. In the pH interval 12–8, r1p assumes a constant
value of 4.6 mM−1 s−1, and then it increases monotonically on
lowering the pH to reach a value of about 23 mM−1 s−1 at pH = 3
(Fig. 1). The constant value of relaxivity at basic pH, where the
unbound oxygen atom of the phosphonic groups in the complex
are fully deprotonated, suggests the presence in solution of a
monomeric species characterized by q = 0. As for GdDOTP, the
absence of a coordinated water molecule is compensated by the
presence of a number of water molecules hydrogen-bonded to
Results and Discussion
Relaxometric properties
The ability of a Gd(III) chelate to enhance the water proton nuclear
magnetic relaxation rates is evaluated in terms of the parameter
[8]
relaxivity, r1p
,
which refers to the increase of the longitudinal
relaxation rate of the solvent observed in a 1 mM aqueous solution
of the paramagnetic complex (Eqn (1)) at a given temperature and
magnetic field strength (typically 298 K and 20 MHz, respectively):
obs
r1p = (R1 − R1w)/(CA)
(1)
In Eqn (1), R1w is the relaxation rate of a corresponding
diamagnetic solution, which for pure water assumes the value
of 0.38 s−1 at 20 MHz and 298 K. The r1p values of monomeric
Gd-based CA is given by the additive IS and outer-sphere (OS)
contributions, which are roughly the same (2.5 mM−1 s−1, 20 MHz,
and 298 K) for a monoaquo complex. At high frequencies, the IS
term increases proportionally with the increase of the molecular
size of the complex (slower molecular tumbling, longer rotational
Figure 1. Plot of proton relaxivity r1p at 20 MHz and 298 K versus pH for
GdEGT4P (filled circles), GdEGT2P (open diamonds) and GdEGT1P (down
triangles).
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L. Tei et al.
Figure 3. Temperature dependence of the paramagnetic contribution
to the water 17O NMR transverse relaxation rate R2p for GdEGT4P.
Experimental conditions: 18 mM, 2.1 T.
Figure 2. Plot of the paramagnetic contribution to the 17O transverse
relaxation rate R2p as a function of pH at 2.1 T (12.2 MHz) and 298 K for a
8.3 mM aqueous solution of GdEGT4P.
the phosphonic groups, and therefore at a distance sufficiently
short from the gadolinium to contribute to the relaxivity. The
increase of r1p by lowering pH could be attributed to an increase
of the hydration state of the complex resulting from the stepwise
protonation of the phosphonic groups and to the formation of
oligomers. Both the tendency to oligomerize and to increase
the q value by passing from basic to neutral/acidic conditions
have been previously observed and reported in the case of
phosphonate complexes.[14–16] These two hypotheses can be
verified by analysing 17O NMR data and 1H nuclear magnetic
relaxation dispersion (NMRD) profiles.
The 17O water transverse relaxation rates, R2, are determined
by the contact (through-bond) Gd–Ow interaction and provide
direct access to information on the hydration state of the complex.
This interaction is not influenced by the molecular reorientation
of the complex, but it depends on the hydration number q, the
electronic relaxation parameters, the rate of water exchange, and
the scalar coupling constant A/ꢀ.[17,18] The pH dependency of the
17O R2 values measured at 2.1 T and 298 K for a 31 mM solution
of GdEGT4P is reported in Fig. 2. The behavior reproduces quite
closely that observed for the 1H R1 data to indicate that the
relaxivity changes involve the whole water molecule and do not
simply depend on proton exchange processes. Strictly speaking,
the 17O R2 pH dependency could arise either from hydration
equilibria (q variations) and/or large variation of kex. In order to
discriminate between these alternatives, it is useful to measure the
temperature dependence of the 17O R2 data, a well-established
procedure to obtain accurate estimates of q and kex. The data,
recordedatbothpH5and9,arereportedinFig. 3.Itisquiteevident
that at basic pH GdEGT4P does not possess any coordinated water
molecule (q = 0). The Gd–OW coupling involves only the dipolar
interaction with the water molecules in the outer coordination
shell, which is quite small owing to the low γ value of the 17O
nuclei. This result reproduces closely what is found in the case of
GdDOTP. On the other hand, the profile at pH = 5.5 is typical of a
system with q > 0 in the fast exchange regime.[18] Then, whereas
the high steric encumbrance of the phosphonic groups does not
allow access of a water molecule to the inner coordination sphere
of the metal ion at basic pH (8/12), their protonation occurring at
lower pH values favors an increase of the hydration state and the
formation of aggregated species.
Figure 4. 1H 1/T1 NMRD profiles of GdEGT4P at pH = 9.5 (filled diamonds)
and 5.5 (open circles) at 298 K. The solid curve through the data points is
calculated with the parameters reported in Table 1.
featuring a single dispersion around 3–5 MHz.[8] We know from
the 17O NMR data that q = 0 and then we have to model the
relaxation behavior in terms of the outer and second coordination
sphere relaxation contributions only. By adopting standard values
for the distance of closest approach of the water molecules
(a = 4.0 Å) to the metal ion and for the relative diffusion
coefficient (D = 2.4 × 10−5 cm2 s−1), and by assuming that each
phosphonic group is H-bonded to a single water molecule, a
satisfactory fit is obtained with the parameters listed in Table 1.
It is worth underlining the fact that this analysis represents only
a crude approximation that enables the estimation of the order
of magnitude of the contribution of the solvent molecules of the
second coordination shell of the complex. On the other hand,
the NMRD profile measured at pH = 5 shows the characteristic
features of a high-molecular-weight system: a minimum of the
relaxivity at ca 10 MHz followed by a broad peak centred
around 40 MHz.[8] This profile results from the occurrence of
two conditions: q > 0 and slow molecular tumbling rate, as
expected for oligomeric species. Clearly, the complexity of the
species present in solution prevents us from any attempts to fit
the data.
It has been shown that GdDOTP is able to interact with the
positively charged domains of bovine serum albumin (BSA) and
to form adducts endowed with high relaxivity (r1p ∼ 22 mM−1 s−1
at 20 MHz and 298 K), whose enhancement is attributed to
exchangeable protons on the protein close to the interaction
site of the complex and to a number of hydrogen-bonded water
molecules in the second coordination sphere of the metal ion.[11]
The behavior of GdEGT4P is expected to be rather similar owing
This latter hypothesis finds a clear support in the 1/T1 NMRD
profiles measured at pH 9 and 5 (Fig. 4). The NMRD profile
recorded at pH = 9 is characterized by the typical shape
and amplitude of monomeric, low-molecular-weight complexes,
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Copyright ꢀ 2008 John Wiley & Sons, Ltd.
Magn. Reson. Chem. 2008, 46, S86–S93

1H and 17O relaxometric study of Gd(III) complexes of EGTA derivatives
Table 1. Selected best-fit parameters obtained from the analysis of
the 1/T1 NMRD profile (298 K, pH = 7.2) and 17O NMR data (9.4 T) of
the Gd complexes
Parameter
GdEGT4P GdEGT2P GdEGT1P Gd–EGTAa
ꢀ2 (s−2 · 1019
)
6.2 0.4 4.2 0.2 4.3 0.1
3.4 0.2
24
1.00
32.00
298τV (ps)
23 0.3 22
1
23
1
1
EV (kJ mol−1
298τM (ns)
)
–
–
–
–
–
–
0
–
–
–
–
–
–
0
1.5 0.2
2.4 0.03
ꢀH#M (kJ mol−1
)
28.9 1.4 42.7 3.1
ꢀS#M (J mol−1 K−1
)
+17.2
3
+42
3
298τR (ps)
r (Å)b
qb
70
2
58
6
3.00
3.00
Figure 5. 1H 1/T1 NMRD profiles of GdEGT2P at pH = 7.2 and 298 K. The
solid curve through the data points and the second sphere contribution
are calculated with the parameters reported in Table 1.
1.00
4.00
2.24
1.00
3.80
1.00
3.80
a (Å)b
4.00
2.24
4.00
3.82
4.00
2.24
2.00
3.70
D (cm2 s−1 × 10−5 b
)
2.0
2
qꢁ c
–
–
rꢁ (Å)d
steric constraint in the first coordination shell of the Gd(III) ion
to destabilize the nonacoordinate (q = 1) state in favor of an
octacoordinate (q = 0) ground state in solution.
a From Ref. [9].
b Fixed during the least-squares procedure.
c Number of second sphere water molecules.
d Mean Gd–H (water) distance of the second sphere waters.
Characterization of GdEGT1P
The relaxivity of the gadolinium complex with the monophospho-
nate ligand EGT1P, r1p = 5.4 mM−1 s−1, is sensibly higher than
that of GdEGT4P (+17% in the case of the monomeric form)
and GdEGT2P (+46%) and even higher than that measured for
GdEGTA. Therefore, we may safely assume that this complex is
monohydrated and stable over a large range of pH values (Fig. 1).
So, only the replacement of a single carboxylate with a phospho-
nic group maintains the hydration state of the parent GdEGTA
complex. The steric compression of the oxoethylenic bridge on
the adjacent bound water molecule, which is responsible of the
fast rate of exchange, is reinforced by the introduction of the
bulky phosphonic groups. When the substitution involves more
than a single carboxylic group, the steric constraint is such as
to invert the relative stabilities of the nona- and octacoordinate
states, and only q = 0 complexes are found in solution. The high
relaxivity value of GdEGT1P, larger than for other complexes of
similar molecular weight, suggests also in this case a contribu-
tion from water molecule(s) present in the second coordination
shell. The NMRD profile is reported in Fig. 6 and shows char-
acteristic features of monohydrated Gd(III) complexes: the low
frequency/high relaxivity plateau (r1p > 9 mM−1 s−1), the disper-
sion at ca 6–8 MHz, and then the high frequency/low relaxivity
plateau (r1p ≈ 5.0 mM−1 s−1). Again, for the data-fitting it is nec-
essary to make some reasonable estimate of the contribution of
the second solvation sphere. In analogy with the previous cases,
we associate a single water molecule to the phosphonic group of
the ligand and then fit the data to a model that includes inner-,
second-, and OS relaxation mechanisms. For the IS relaxivity we
made the further assumption of q = 1, r = 3.0 Å (r is the Gd–Hw
distance), and used for τM the value calculated from 17O NMR
data (see below). The different contributions to the relaxivity cal-
culated with the parameters listed in Table 1 are shown in Fig. 6.
At 20 MHz the inner-, second-, and OS relaxation mechanisms
make a contribution to the global relaxivity of ca 50, 10, and 40%,
respectively.
to the identical overall electric charge and number of phosphonic
donor groups. We evaluated the binding interaction by titrating
a dilute solution of the complex at pH 7 with the protein and
measuring the changes of the longitudinal relaxation rate. The
resulting binding isotherm was then analysed according to the
proton relaxation enhancement (PRE)[19] equations to obtain the
values of the affinity constant (KA = 1440 M−1) and of the relaxivity
ofthemacromolecularadduct(r1p = 19.5 mM−1 s−1).Asexpected,
these values reproduce quite closely the non-specific binding
properties found for GdDOTP.
Characterization of GdEGT2P
The relaxivity of the bis-(methylenephosphonate) derivative at
20 MHz and 298 K is 3.7 mM−1 s−1 and is independent of pH
in the range 4.5–11. Then, unlike the previous case, GdEGT2P
follows the behavior of the parent compound GdEGTA and
appears to be present in aqueous solution as a monomeric
complex over a wide range of pH values. The value of the
relaxivity is intermediate between the corresponding values of
a pure OS complex (r1p ≈ 2.0–2.5 mM−1 s−1) and of a q = 1
complex (r1p ≈ 4.5–5.5 mM−1 s−1) of similar molecular size. This
represents a good indication of the absence of water molecules
in the inner coordination sphere of the metal ion and of the
occurrence of a sizeable contribution of the second hydration
sphere relaxation mechanism. The octacoordinate ground state
of gadolinium in this complex is confirmed by the VT 17O NMR
R2 data, which show a profile quite similar to that of GdEGT4P
at basic pH (Fig. 3). The 1H 1/T1 NMRD profile, recorded at 298 K
and pH = 7.2 (Fig. 5), has a shape rather similar to that of the
monomeric form of GdEGT4P. A good fit is indeed obtained by
assuming the presence of two water molecules hydrogen-bonded
to the two phosphonic groups of the ligand and by adopting
standard values for the parameters a and D in the evaluation
of the OS relaxivity. The best-fit parameters are listed in Table 1,
which outline the similarity between the monomeric forms of
GdEGT4P and GdEGT2P. Clearly, even the replacement of only two
carboxylates with two phosphonic moieties introduces enough
A rather crucial parameter that in general cannot be determined
with accuracy by the analysis of the NMRD profiles is the mean
residence lifetime of the coordinated water molecule, τM. As it is
well known, the kinetic parameters of the water exchange can be
c
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L. Tei et al.
Figure 7. Temperature dependence of the paramagnetic contribution to
the water 17O NMR (9.4 T) transverse relaxation rate for a 25 mM aqueous
solution of GdEGT1P. Solid curve fitted with the values of Table 1 for ꢀ2,
τV, EV, and A/ꢀ = −3.80 × 106 rad s−1. The dashed curve is the calculated
profile of GdDTPA, according to the data in Ref. [17].
Figure 6. 1H 1/T1 NMRD profiles of GdEGT1P at pH = 7.2 and 298 K. The
solid curve through the data points and the different contributions to
the relaxivity are calculated with the parameters reported in Table 1. The
dashed curve is the calculated NMRD profile of GdEGTA, according to the
data in Ref. [9].
to be ca 30 ns at 20–40 MHz, and ca 2–4 ns at 80–100 MHz.[20]
So, GdEGT1P is endowed with an optimal rate of water exchange
for the attainment of enhanced relaxivity for its multimeric or
macromolecular derivatives.
evaluated by measuring the temperature dependence of the 17
O
transverse relaxation rate, R2, of the water molecules in a solution
of the paramagnetic complex.[17,18] This experiment was carried
out at 9.4 T on a 25 mM solution of GdEGT1P at pH = 7.2. The
results are reported in Fig. 7 and compared with the calculated
curve of the corresponding complex [GdDTPA(H2O)]2− for which
a τM value of 300 ns (at 298 K) was calculated. The exponential
increase of R2 with decreasing temperature is indicative of a
rate of water exchange much faster than for GdDTPA. For this
latter, the maximum of the curve is at about 315 K, whereas for
GdEGT1P it is well below 273 K. The fit of the experimental data to
the Swift–Connick equations confirmed this qualitative analysis
and provided the following values for the kinetic pa−ra1meters:
298τM = 2.4 ns (kex = 4.1 × 108 s−1),ꢀH# = 28.9 kJ mol ,ꢀS# =
+17.2 J mol−1. This is one of the fastest water exchange rates
measured for a monohydrated Gd complex, about an order of
magnitude faster than that of GdEGTA. The value of τM should
not be too short so as to influence the overall correlation time
and not too long to limit the relaxivity. Its optimal value decreases
with increasing magnetic field strength and it can be estimated
Synthesis
While the synthesis of EGT4P was accomplished following a
reportedprocedure,thatofEGT1PandEGT2Pwascarriedoutusing
similar synthetic procedures except for the first step (Scheme 2). In
thisfirststep,thereductiveaminationof1,8-diaza-3,6-dioxaoctane
with 2.5 or 1 M equivalents of benzaldehyde and NaBH4 led
to the formation of di- and monobenzyl amines, respectively.
Whereas EGT2P was obtained starting from the dibenzyl derivative
(N,Nꢁ-dibenzyl-1,8-diaza-3,6-dioxaoctane, 1) in four steps, EGT1P
was synthesized starting from the mono-benzyl derivative (N-
benzyl-1,8-diaza-3,6-dioxaoctane, 5) in the same number of
steps. The alkylation with tert-butyl bromoacetate followed by
hydrogenolysis of the N-benzyl groups with ammonium formate
andPd/Cledtotheformationof thederivatives 3and7having two
and one secondary amino groups, respectively. The phosphonic
Scheme 2. i: ClCH2PO3H2, CH2O, HCl; ii: PhCHO, CH2Cl2; iii: NaBH4, CH3CN; iv: BrCH2COOtBu, K2CO3, NMP; v: HCOONH4, Pd/C, MeOH; vi: HPO3Et2, CH2O,
toluene; vii: HBr.
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1H and 17O relaxometric study of Gd(III) complexes of EGTA derivatives
groups were inserted by a Mannich reaction of 3 and 7 with
diethyl phosphite and paraformaldehyde. Derivatives 4 and 8
were purified by column chromatography, and then the tert-butyl
and ethylphosphonic esters were hydrolysed with a 33% solution
of HBr in acetic acid. Ligands EGT2P and EGT1P were obtained as
analytically pure white solids by precipitation with diethyl ether
in overall 18 and 10% yield, respectively. All intermediates and
products were characterized by 1H and 13C NMR spectroscopy and
electrospray ionization mass spectra (ESI-MS).
temperature overnight. The mixture was then filtered and dried
underreducedpressuretoobtainayellowoilthatwasre-dissolved
in acetonitrile (10 ml). NaBH4 (1.5 g, 40.5 mmol) was added in
portions at 0 ◦C and then the reaction was refluxed for 2 h. HCl
concentrate was added slowly to the cooled solution until pH 1 is
reached and then the reaction was refluxed for other 30 min. The
product precipitated as a white solid, which was filtered, washed
with cold diethyl ether (2 × 5 ml), and re-crystallized from H2O.
Yield: 5.5 g, 14 mmol (83%). 1H NMR, CD3OD, 400 MHz, 25 ◦C, δ
7.46 (m, 4H, Har), 7.45 (m, 6H, Har), 4.24 (s, 4H, CH2Ph), 3.78 (t,
J = 5.2 Hz, 4H, NCH2CH2O), 3.71 (s, 4H, CH2O), 3.21 (t, J = 5.2 Hz,
4H, NCH2CH2O); 13C NMR, CD3OD, 100 MHz, 25 ◦C, δ 140.0, 128.4,
127.03 (Car), 70.6 (CH2O), 70.4 (NCH2CH2O), 54.0 (CH2Ph), 48.7
(NCH2CH2O). ESI-MS (m/z): 329.21 (M + H+) (calc. 329.46).
Conclusions
ThecoordinationpolyhedronofGdEGTAisconvenientlydescribed
as a tricapped trigonal prism in which the capping positions are
occupied by the two nitrogen atoms and the IS water oxygen,
and the trigonal faces are composed of the oxygen atoms of the
ether and the carboxylate groups.[9] The substitution of the four
carboxylate groups with four bulky and sterically demanding
phophonate groups introduces significant differences in the
properties of the corresponding Gd(III) complex. The coordinated
water molecule is destabilized and, unlike GdEGTA, GdEGT4P has
an octacoordinate ground state with q = 0. The steric interaction
and electrostatic repulsion between phosphonate groups on the
same trigonal face or on adjacent positions on opposite faces
plays a predominant role in the formation of polymeric species.
The tendency to release these electrostatic and steric interactions
may induce de-coordination of one or more phosphonate groups
and formation of oligomeric systems with concomitant access of
water molecule(s) to the metal centre. This process is favored
1,2-bis(N-Benzyl-Nꢀ-tert-butoxycarbonylmethyl-2-
aminoethoxy)etane (2)
1 (1.7 g, 4.2 mmol) and K2CO3 (4.1 g, 29.6 mmol) are suspended
in 25 ml of anhydrous N-methylpyrrolidone (NMP) and stirred at
room temperature for 30 min. Then, 2.5 M equivalent of tert-butyl
bromoacetate (1.5 ml, 10.6 mmol) in 5 ml of NMP was added drop-
wise in 45 min. The reaction was stirred at room temperature
overnight and then 10 ml of H2O was added. After extraction
with diethyl ether (3 × 20 ml), the organic phase was dried over
Na2SO4, filtered, and evaporated. The yellow oil obtained was
purified by column chromatography (petrol ether/ethyl acetate
8.5/1.5; Rf = 0.86) to obtain a final white solid. Yield: 1.05 g,
1.9 mmol (45%). 1H NMR, CDCl3, 400 MHz, 25 ◦C, δ 7.32–7.28 (m,
10H, Har), 3.82 (s, 4H, CH2Ph), 3.56 (t, J = 6.1 Hz, 4H, NCH2CH2O),
3.54 (s, 4H, CH2O), 3.28 (s, 4H, NCH2CO), 2.86 (t, J = 6.1 Hz, 4H,
NCH2CH2O), 1.45 (s, 18H, C(CH3)); 13C NMR, CDCl3, 100 MHz, 25 ◦C,
δ 171.0 (COO-tBu), 139.4, 129.0, 128.3, 127.1 (Car), 80.8 (C(CH3)),
70.4 (CH2O), 70.2 (NCH2CH2O), 58.7 (CH2Ph), 55.8 (NCH2CO), 53.1
(NCH2CH2O), 28.3 (C(CH3)). ESI-MS (m/z): 557.43 (M + H+) (calc.
557.75).
2−
by the stepwise protonation of the –PO3 groups occuring at
pH < 8.
We may suppose that in GdEGT2P the two methylenphospho-
nate groups are located in opposite positions on the two trigonal
faces in order to minimize the repulsive interactions. In fact, no
oligomerization processes were observed to indicate a higher sta-
bility of the system. On the other hand, the capping coordination
site of the water molecule is still destabilized and the octacoordi-
nate, q = 0, state is characterized by lower energy than the ninth
coordinate, q = 1, state.
1,2-bis(N-tert-Butoxycarbonylmethyl-2-aminoethoxy)etane
(3)
2 (1.05 g, 1.9 mmol) was dissolved in MeOH (10 ml) and Pd/C (10%,
100 mg) was added. A solution of ammonium formate (0.5 g,
7.6 mmol) in MeOH (5 ml) was added to the stirring suspension
at room temperature, and then the reaction was refluxed for 2 h.
After cooling, the heterogeneous catalyst was filtered over celite
and the solvent was evaporated. The oil obtained was re-dissolved
in H2O, basified with 1 M solution of NaOH, and extracted with
CH2Cl2 (3×30 ml). The organic phase was then dried over Na2SO4,
filtered, and evaporated to obtain a pale yellow oil. Yield: 0.57 g,
1.5 mmol (81%). 1H NMR, CDCl3, 400 MHz, 25 ◦C, δ 3.60 (s, 4H,
CH2O), 3.57 (t, J = 5.2 Hz, 4H, NCH2CH2O), 3.30 (s, 4H, NCH2CO),
2.77 (t, J = 5.2 Hz, 4H, NCH2CH2O), 1.44 (t, 18H, C(CH3)); 13C
NMR, CDCl3, 100 MHz, 25 ◦C, δ 171.6 (COO-tBu), 81.1 (C(CH3)), 70.8
(CH2O), 70.4 (NCH2CH2O), 51.8 (NCH2CO), 48.8 (NCH2CH2O), 28.2
(C(CH3)). ESI-MS (m/z): 377.31 (M + H+) (calc. 377.50).
Finally, the presence of a single methylenphosphonate group
allows the Gd(III) to maintain a ninth coordination site occupied
by a water molecule while increasing its steric compression, which
results in a extremely short residence lifetime. This finding, along
with the presence of a sizeable contribution to relaxivity of
second sphere water molecules, is relevant for the preparation
of multimeric systems of high relaxivity optimized for use at high
magnetic field strengths.[21]
Experimental
All chemicals were purchased from Sigma–Aldrich Co. and were
used without purification, unless otherwise stated. NMR spectra
were recorded on a JEOL ECP-400 (operating at 9.4 T). ESI mass
spectra were recorded on a Waters Micromass ZQ.
Ethylene glycol-bis-(2-aminoethylether)-N,Nꢀ-methyl-
phosphonate-N,Nꢀ-tert-butylacetate (4)
Synthesis of 1,2-bis(N-benzyl-2-aminoethoxy)etane (1)
(1,2-bis(2-Aminoethoxy)etane) (3 ml, 20.2 mmol) in CH2Cl2 (5 ml)
was added drop-wise (30 min) to a suspension of benzaldehyde
(5.1 ml, 50.6 mmol) and Na2SO4 (7.2 g, 50.6 mmol) in CH2Cl2
(20 ml). After the addition, the reaction was stirred at room
Diethylphosphite (0.58 ml, 4.5 mmol) is added in 30 min to a
solution of 3 (0.57 g, 1.5 mmol) in anhydrous toluene (12 ml)
and under N2. Then, a suspension of paraformaldehyde (0.2 g,
6 mmol) in 4 ml of toluene is added in 10 min and the solution
c
Magn. Reson. Chem. 2008, 46, S86–S93
Copyright ꢀ 2008 John Wiley & Sons, Ltd.
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L. Tei et al.
obtained is refluxed for 6 h. After cooling, the solvent is removed
under reduced pressure and the oily product is purified by
column chromatography (ethyl acetate/MeOH/NH3 9.5/0.5/0.05,
Rf = 0.33).Yield:0.69 g,1.04 mmol(68%).1HNMR,CDCl3,400 MHz,
25 ◦C,δ 4.11(m,8H,POCH2CH3),3.56(t,J = 5.7 Hz,4H,NCH2CH2O),
3.54 (s, 4H, CH2O) 3.51 (s, 4H, NCH2CO), 3.19 (d, J = 9.9 Hz,
4H, NCH2P), 2.96 (t, J = 5.7 Hz, 4H, NCH2CH2O), 1.42 (s, 18H,
C(CH3)), 1.31 (t, J = 7.1 Hz, 12H, POCH2CH3); 13C NMR, CDCl3,
100 MHz, 25 ◦C, δ 170.5 (COO-tBu), 80.9 (C(CH3)), 70.4 (CH2O), 70.1
(NCH2CH2O), 62.0 (POCH2CH3), 56.8 (NCH2CO), 55.0 (NCH2CH2O),
drop-wise in 45 min. The reaction was stirred at room temperature
overnight and then 10 ml of H2O was added. After extraction with
CH2Cl2 (3 × 30 ml), the organic phase was dried over Na2SO4,
filtered, and evaporated. The crude obtained was purified by col-
umn chromatography (CH2Cl2/MeOH/NH3 9.5/0.5/0.05; Rf = 0.4)
to obtain a final white solid. Yield: 3.66 g, 6.3 mmol (91%). 1H
NMR, CDCl3, 400 MHz, 25 ◦C, δ 7.32–7.28 (m, 5H, Har), 3.85 (b s,
2H, CH2Ph), 3.59 (t, J = 5.9 Hz, 4H, NCH2CH2O), 3.55 (s, 4H, CH2O),
3.46 (s, 6H, NCH2CO), 2.91 (t, J = 5.9 Hz, 4H, NCH2CH2O), 1.47 (s,
27H, C(CH3)); 13C NMR, CDCl3, 100 MHz, 25 ◦C, δ 170.9 (COO-tBu),
139.4, 129.0, 128.4, 127.2 (Car), 80.9 (C(CH3)), 70.5 (CH2O), 70.3
(NCH2CH2O), 58.6 (CH2Ph), 56.8 (NCH2CO), 53.5, 53.1 (NCH2CH2O),
28.1 (C(CH3)). ESI-MS (m/z): 571.56 (M + H+) (calc. 571.78).
50.2 (d, J = 162.1 Hz, NCH2P), 28.2 (C(CH3)), 16.5 (POCH2CH3); 31
P
NMR, CDCEGTP1, 25 ◦C, δ 24.3. ESI-MS (m/z): 677.46 (M + H+) (calc.
677.73).
N,N,Nꢀ-tert-Butoxycarbonylmethyl-1,8-diaza-3,6-
dioxaoctane (7)
Ethylene glycol-bis-(2-aminoethylether)-N,Nꢀ-diphosphonic-
N,Nꢀ-diacetic acid (EGT2P)
6 (3.6 g, 6.2 mmol) was dissolved in MeOH (15 ml), and Pd/C
(10%, 360 mg) was added. The mixture was introduced into a
hydrogenation bottle, purged with nitrogen, and then stirred
under hydrogen (1 atm.) for 4 h. The catalyst was removed by
filtration on celite, the solvent was evaporated, and a pale yellow
oil was obtained. Yield: 2.8 g, 5.7 mmol (92%). 1H NMR, CDCl3,
400 MHz, 25 ◦C, δ 3.58 (t, J = 5.8 Hz, 4H, NCH2CH2O), 3.54 (s, 4H,
CH2O), 3.42 (s, 6H, NCH2CO), 2.89 (t, J = 5.8 Hz, 4H, NCH2CH2O),
1.46(s, 27H, C(CH3));13CNMR, CDCl3, 100 MHz, 25 ◦C, δ 171.0(COO-
tBu), 81.0 (C(CH3)), 70.6 (CH2O), 70.4 (NCH2CH2O), 56.7 (NCH2CO),
53.6, 53.2(NCH2CH2O), 28.1(C(CH3)). ESI-MS (m/z):481.49(M+H+)
(calc. 481.66).
4 (0.68 g, 1 mmol) was dissolved in 5 ml of a 33% solution of
HBr in acetic acid and stirred overnight at room temperature.
The solvent was evaporated and the crude product was re-
dissolved in acetonitrile; slow addition of excess diethyl ether
led to precipitation of EGT2P as a white amorphous solid, and
was isolated by centrifugation. This precipitation procedure was
repeated twice obtaining analytically pure EGT2P. Yield: 0.400 g,
0.9 mmol (89%). 1H NMR, D2O, 400 MHz, 25 ◦C, δ 3.98 (s, 4H,
CH2O), 3.88 (t, J = 5.2 Hz, 4H, NCH2CH2O), 3.71 (s, 4H, NCH2CO),
3.60 (t, J = 5.2 Hz, 4H, NCH2CH2O), 3.19 (d, J = 11.3 Hz, 4H,
NCH2P); 13C NMR, D2O, 100 MHz, 25 ◦C, δ 170.9 (COOH), 69.9
(CH2O), 64.9 (NCH2CH2O), 58.1 (NCH2CO), 55.5 (NCH2CH2O), 53.1
(d, J = 124.6 Hz, NCH2P);31P NMR, D2O, 25 ◦C, δ 6.75. ESI-MS (m/z):
403.13 (M + H+) (calc. 403.29).
Ethylene glycol-bis-(2-aminoethylether)-N,-methyl-
phosphonate-N,Nꢀ,Nꢀ-tert-butylacetate (8)
Synthesis of N-benzyl-1,8 diaza-3,6-dioxaoctane (5)
Diethylphosphite (1.2 ml, 9.7 mmol) was added in 30 min to a
solution of 7 (2.3 g, 4.8 mmol) in anhydrous toluene (20 ml)
and under N2. Then, a suspension of paraformaldehyde (0.3 g,
9.7 mmol) in 2 ml of toluene was added in 10 min and the
solution obtained was refluxed for 4 h. After cooling, the solvent
was removed under reduced pressure and the oily product was
purifiedbycolumnchromatography(CH2Cl2/MeOH/NH3 9/1/0.01;
Rf = 0.48) Yield: 1.0 g, 1.56 mmol (42%). 1H NMR, CDCl3, 400 MHz,
25 ◦C, δ 4.14 (m, 4H, POCH2CH3), 3.77 (s, 4H, CH2O), 3.61 (m,
4H, NCH2CH2O), 3.58 (s, 6H, NCH2CO), 3.23 (d, J = 9.9 Hz, 2H,
NCH2P), 3.01 (m, 4H, NCH2CH2O), 1.46, 1.44 (s, 27H, C(CH3)), 1.31
(t, J = 7.0 Hz, 6H, POCH2CH3); 13C NMR, CDCl3, 100 MHz, 25 ◦C, δ
170.5 (COO-tBu), 81.4 (C(CH3)), 70.3 (CH2O), 70.1 (NCH2CH2O), 62.3
(POCH2CH3), 56.7, 55.3 (NCH2CO), 55.2, 53.9 (NCH2CH2O), 50.1 (d,
J = 160.0 Hz, NCH2P), 28.2 (C(CH3)), 16.6 (POCH2CH3); 31P NMR,
CDCl3, 25 ◦C, δ 25.0. ESI-MS (m/z): 632.51 (M + H+) (calc. 632.78).
1,2-bis(2-Aminoetoxy)etane (10.0 g, 67.5 mmol) in CH2Cl2 (15 ml)
was added dropwise (30 min) to a suspension of benzaldehyde
(7.0 g, 67.5 mmol) and Na2SO4 (9.6 g, 67.5 mmol) in CH2Cl2
(20 ml). After the addition, the reaction was stirred at room
temperature overnight. The mixture was then filtered and the
solvent evaporated obtaining a yellow oil that was re-dissolved
in MeOH (30 ml). NaBH4 (2.5 g, 67.5 mmol) was added in small
portions at 0 ◦C and then the reaction was refluxed for 2 h. HCl
concentratewasaddedslowlytothecooledsolutionuntilpH1was
reached and then the reaction was refluxed for another 30 min.
The crude product precipitated as a pale yellow solid which was
filtered, washed with cold diethyl ether (2 × 10 ml), dissolved in
10% solution of NaHCO3, and extracted with CH2Cl2 (3 × 15 ml).
The product was obtained as a white solid after purification by
columnchromatography(CH2Cl2/CH3OH/NH3 9 : 1:0.1, Rf = 0.12).
Yield 4.0 g, 16.8 mmol (56%). 1H NMR, CDCl3, 400 MHz, 25 ◦C, δ
7.31 (m, 5H, Har), 7.22 (m, 1H, NH), 3.79 (s, 4H, CH2Ph), 3.59 (s, 6H,
Ethylene
glycol-bis-(2-aminoethylether)-N-phosphonic-
CH2O), 3.48 (t, J = 5.2 Hz, 2H, CH2O), 2.81 (m, 4H, NCH2CH2O); 13
C
N,Nꢀ,Nꢀ-triacetic acid (EGT1P)
NMR, CDCl3, 100 MHz, 25 ◦C, δ 140.0, 128.4, 127.0 (Car), 73.3, 70.6,
70.4 (CH2O), 54.0 (CH2Ph), 48.7 (Bz-NHCH2), 41.7 (CH2NH2). ESI-MS
(m/z): 239.12 (M + H+) (calc. 239.34).
8 (0.65 g, 1 mmol) was dissolved in 3 ml of a 33% solution of HBr
in acetic acid and stirred overnight at room temperature. The
solvent was evaporated and the crude product was re-dissolved
in acetonitrile; slow addition of excess diethyl ether led to the
precipitation of EGT1P as a white amorphous solid, isolated by
centrifugation. This precipitation procedure was repeated twice
obtaining analytically pure EGT1P. Yield: 0.200 g, 0.5 mmol (50%).
1H NMR, D2O, 400 MHz, 25 ◦C, δ 4.32, 4.27 (s, 4H, NCH2CO), 3.87
(s, 4H, CH2O), 3.70–3.63 (m, 8H, NCH2CH2O), 3.53 (d, J = 11.3 Hz,
2H, NCH2P); 13C NMR, D2O, 100 MHz, 25 ◦C, δ 168.4 (COOH),
N-Benzyl-N,Nꢀ,Nꢀ-tert-butoxycarbonylmethyl-1,8-diaza-3,6-
dioxaoctane (6)
5 (4.0 g, 16.8 mmol) and K2CO3 (11.6 g, 83.9 mmol) were sus-
pended in 30 ml of anhydrous acetonitrile and stirred at room
temperature for 30 min. Then, 3 M equivalents of tert-butyl bro-
moacetate (7.4 ml, 50.3 mmol) in 5 ml of acetonitrile were added
c
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Copyright ꢀ 2008 John Wiley & Sons, Ltd.
Magn. Reson. Chem. 2008, 46, S86–S93

1H and 17O relaxometric study of Gd(III) complexes of EGTA derivatives
69.9 (CH2O), 65.1, 64.7 (NCH2CH2O), 56.1 (NCH2CH2O), 55.7, 55.4
(NCH2CO), 51.4 (d, J = 136.1 Hz, NCH2P);31P NMR, D2O, 25 ◦C, δ
7.82. ESI-MS (m/z): 367.16 (M + H+) (calc. 367.32).
equipped with a 5-mm probe and standard temperature control
units. Aqueous solutions of the complexes (8–25 mM) containing
2.0% of the 17O isotope (Cambridge Isotope) were used. The
observed transverse relaxation rates were calculated from the
signal width at half-height.
Synthesis of the Gd(III) complexes
The three complexes were synthesized as follows: the ligand
(0.2 mmol) was dissolved in H2O (5 ml) and the pH of the solution
adjusted to 7.5 with NaOH 1N. To this solution, 3 ml of an aqueous
solution of GdCl3 (0.2 mmol) was added drop-wise by maintaining
the pH at 7.5, again with NaOH 1N. At room temperature the
complex formation was instantaneous. The pH of the solution was
then increased to 8–9 by adding NaOH 1N in order to precipitate
the possible excess of uncomplexed Gd(III) ions. The solution was
then evaporated under reduced pressure and the residue dried at
70 ◦C overnight.
Acknowledgements
Financial support from PRIN 2005 (Grant no. 2005039914) is
gratefully acknowledged. This work was carried out under the
auspices of CIRCMSB and COST Action D38.
References
[1] J.-C. Bu¨nzli, G. R. Choppin, Lanthanide Probes in Life, Chemical and
Earth Sciences, Elsevier: Amsterdam, 1989.
[2] J. Rocha, L. D. Carlos, Curr. Opin. Solid State Mater. Sci. 2003, 7, 199.
[3] D. Parker, Chem. Soc. Rev. 2004, 33, 156.
Proton relaxometric studies
[4] J.-C. Bu¨nzli, C. Piguet, Chem. Soc. Rev. 2005, 34, 1048.
[5] S. Pandya, J. Yu, D. Parker, Dalton Trans. 2006, 2757.
[6] See the dedicated issue of Chem. Soc. Rev. 2006, 35, 499.
[7] P. Caravan, J. J. Ellison, T. J. McMurry, R. B. Lauffer, Chem. Rev. 1999,
99, 2293.
[8] S. Aime, M. Botta, M. Fasano, E. Terreno, Acc. Chem. Res. 1999, 32,
941.
[9] S. Aime, A. Barge, A. Borel, M. Botta, S. Chemerisov, A. E. Merbach,
U. Mu¨ller, D. Pubanz, Inorg. Chem. 1997, 36, 5104.
[10] L. A. Bromley, D. Cottier, R. J. Davey, B. Dobbs, S. Smith, Langmuir
1993, 9, 3594.
Thewaterproton1/T1 longitudinalrelaxationrates(20 MHz, 25 ◦C)
were measured on a Stelar Spinmaster Spectrometer (Mede, Pv,
Italy) on 0.5–2 mM aqueous solutions of the complexes. For the
T1 determinations the standard inversion–recovery method was
used with typical 90◦-pulse width of 3.5 µs, 16 experiments of 4
scans. The reproducibility of the T1 data was estimated to be 1%.
ThetemperaturewascontrolledwithaStelarVTC-91airflowheater
equipped with a copper–constantan thermocouple (uncertainty
0.1 ◦C). In a typical titration experiment, several (five to eight)
aqueous solutions at pH = 7.2 of the paramagnetic complex
were prepared containing different concentrations of the anionic
species (0–0.04 M) and the water proton relaxation rate of each
solution was measured at 25 ◦C. The starting pH was adjusted by
eitherHClorKOH. Moreover, thepHofthesolutionswascontrolled
before and after the measurement. The proton 1/T1 NMRD profiles
were measured on a fast field-cycling Stelar Spinmaster FFC
relaxometer over a continuum of magnetic field strengths from
0.00024 to 0.5 T (corresponding to 0.01–20 MHz proton Larmor
frequencies). The relaxometer operates under computer control
with an absolute uncertainty in 1/T1 of 1%. Additional data
points at 20–70 MHz were obtained on a Stelar Relaxometer.
[11] M. Botta, Eur. J. Inorg. Chem. 2000, 399.
[12] J. Ren, A. D. Sherry, J. Magn. Reson. 1996, B111, 178.
[13] F. Avecilla, J. A. Peters, C. F. G. C. Geraldes, Eur. J. Inorg. Chem. 2003,
4179.
[14] C. F. G. C. Geraldes, R. D. Brown 3rd, W. P. Cacheris, S. H. Koenig,
A. D. Sherry, M. Spiller, Magn. Reson. Med. 1989;, 9, 94.
[15] C. F. G. C. Geraldes, A. D. Sherry, W. P. Cacheris, Inorg. Chem. 1989,
28, 3336.
[16] J. Gałezowska, R. Janicki, A. Mondry, R. Burgada, T. Bailly,
M. Lecouvey, H. Kozłowski, Dalton Trans. 2006, 4384.
[17] D. H. Powell,
O. M. Ni
Dhubhghaill,
D. Pubanz,
L. Helm,
Y. S. Lebedev, W. Schlaepfer, A. E. Merbach, J. Am. Chem. Soc.
1996, 118, 9333.
[18] L. Helm, A. E. Merbach, Chem. Rev. 2005, 105, 1923.
[19] S. Aime, M. Botta, M. Fasano, E. Terreno, in TheChemistry of Contrast
Agents in Medical Magnetic Resonance Imaging, (Eds: A. E. Merbach,
E. To´th, John Wiley and Sons: Chichester, 2001, pp 193.
[20] D. M. J. Doble, M. Botta, J. Wang, S. Aime, A. Barge, K. N. Raymond,
J. Am. Chem. Soc. 2001, 123, 10758.
17O NMR
Variable temperature 17O NMR measurements were recorded
on JEOL ECP-400 (9.4 T) and JEOL EX-90 (2.1 T) spectrometers
[21] J. B. Livramento, L. Helm, A. Sour, C. O’Neil, A. E. Merbach, E. To´th,
Dalton Trans. 2008, 1195.
c
Magn. Reson. Chem. 2008, 46, S86–S93
Copyright ꢀ 2008 John Wiley & Sons, Ltd.
www.interscience.wiley.com/journal/mrc