Thermodynamic and Kinetic Stability of the Gd Complex
COMMUNICATION
namic behavior of the acetate groups and the oxydiethylene
bridge of the OBETA molecules. Comparison with the cal-
GdACTHNGUTERNNUG
(OBETA)ꢀ-based MRI contrast agents (lipophilic deriva-
tives, multimers, nanosized systems, bifunctional agents) will
be undertaken in order to explore the potential of this inter-
esting paramagnetic system.
(H2O)]ꢀ (Figure 4), built on
culated structure of [GdACTHNUTRGENNGU(EGTA)AHCTUNGTRENNGUN
the structural data gained from the molecular dynamics cal-
culation by Yerly et al.[23] shows that these complexes share
the preference for the TTP coordination geometry, with ni-
trogen atoms and a water molecule occupying the three cap-
ping positions. Nevertheless, the more significant difference
is found in the position of the backbone oxyethylenic
bridges, the length and denticity of which dictates the switch
between two different arrangements. Indeed, the oxyethy-
lenic bridge of EGTA is placed close to the water molecule
located in the capping position whereas in the corresponding
OBETA complex the bridge is opposite to the water mole-
Experimental Section
Materials: The chemicals used for the experiments were of the highest
analytical grade. The LnCl3 solutions were prepared by dissolving Ln2O3
(99.9% Fluka) in HCl (6m) and evaporating the excess acid. The concen-
tration of LnCl3, ZnCl2 and CuCl2 stock solutions were determined by
complexometric titration with standard EDTA disodium solution, with
the use of Xylenol Orange (LnCl3, ZnCl2) and murexide (CuCl2) indica-
tors. The concentration of the OBETA and EGTA were determined by
pH-potentiometric titrations in the presence and absence of a 40-fold
excess of Ca2+. The pH-potentiometric titrations were made with stand-
ardized 0.2m KOH.
cule. DFT calculations performed on the [Gd
ACHTUNGTRENN(UNG OBETA)-
A
R
ACHTUNGTRENNUNG
similar bond distances of the metal coordination environ-
ment in both complexes (Table S3 in the Supporting Infor-
mation). This result suggests that the lower stability of
EGTA complexes is related to the important degree of flexi-
bility of the long spacer connecting the two amine nitrogen
atoms of the ligand, whereas OBETA is better suited to effi-
ciently wrap around to the metal ion as a consequence of its
shorter oxydiethylene bridge.
Tetra-tert-butyl 2,2’-oxybis(ethylamine)-N,N,N’,N’-tetraacetate (OBETA-
tBu4): Potassium carbonate (5.12 g, 37 mmol, 5 equiv) was added to a so-
lution of O-(2-aminoethyl)-ethanolamine (770 mg, 7.4 mmol) in acetoni-
trile (10 mL). The solution was cooled to 08C and tert-butyl bromoace-
tate (4.56 mL, 31.1 mmol, 4.2 equiv) was added dropwise. The solution
was then stirred at room temperature for 48 h and the reaction was moni-
tored by TLC. Inorganic salts were filtered off and the solvent was
evaporated. The crude product was purified by gravimetric column chro-
matography (petroleum ether/ethyl acetate 8:2!7:3) to afford OBETA-
tBu4 as clear light-yellow oil (3.01, 73%). 1H NMR (CDCl3, 300 MHz,
298 K): d=3.50 (t, J=6.0 Hz, 4H), 3.41 (s, 8H), 2.85 (t, J=5.8 Hz, 4H),
1.38 ppm (s, 36H); 13C NMR (CDCl3, 75.4 MHz, 298 K): d=172.1 (C),
82.1 (C), 71.5 (CH2), 57.9 (CH2), 54.7 (CH2), 29.5 ppm (CH3); MS
(ESI+) calcd for C28H52N2O9 560.4; found: 561.3 [M+H+].
In conclusion, complexes of Ln3+ ions with the heptaden-
tate ligand OBETA were investigated in terms of solution
equilibria and relaxometric properties. The decrease of the
denticity of the ligand (from 8 to 7) surprisingly results in an
increase of the thermodynamic stability of the Ln3+ com-
plexes as compared with the corresponding Ln
(EGTA)ꢀ
ACHTUNGTRENNUNG
2,2’-Oxybis(ethylamine)-N,N,N’,N’-tetraacetic acid (OBETA): OBETA-
tBu4 (2.99 g) was dissolved in trifluoroacetic acid (15 mL). The solution
was stirred at room temperature, overnight. Volatiles were evaporated
and the product was purified by dissolution in MeOH and precipitation
with diethyl ether. This procedure was repeated three times to afford
1.68 g (94% yield) of OBETA as white powder. 1H NMR (D2O,
300 MHz, 298 K): d=4.06 (s, 8H), 3.76 (bt, J=3.8 Hz, 4H), 3.54 ppm (bt,
J=4.0 Hz, 4H); 13C NMR (D2O, 75.4 MHz, 298 K): d=171.2 (C), 67.2
(CH2), 58.7 (CH2), 58.0 ppm (CH2); IR (KBr): v˜ =2969.9, 1737.1, 1365.6,
1217.0, 1229.8, 1091.2, 885.4 cmꢀ1; MS (ESI+) calcd for C12H20N2O9
336.1; found: 359.1 [M+Na+], 337.1 [M+H+].
complexes. For the Gd3+ complex
a
remarkable
DlogKGdOBETAꢀGdEGTA of 1.71 was observed and indicates the
high affinity of OBETA towards this metal ion. Since the
nature and charge of the donor atoms are not at the origin
of the high stability of GdACHTNUGRTNEUNG
(OBETA)ꢀ, a possible alternative
explanation could be found in the level of preorganization
of the chelator. In EGTA the linker connecting the two ter-
minal amine nitrogen atoms is longer and more flexible,
whereas OBETA presents a better preorganized geometry
for the coordination to the metal ion. In addition, prelimina-
ry results indicate that the kinetic inertness of Gd-
Equilibrium measurements: All the equilibrium measurements were
made at a constant ionic strength maintained with KCl (0.1m) at 258C.
For determining the protonation constants of EGTA and OBETA two
parallel pH-potentiometric titrations were made with KOH (0.2m) in
ligand (0.005m) solutions. The stability constants and protonation con-
ACHTUNGTRENNUNG
stants of complexes LnACTHNUGTRENNUG ACHUTNGTRENNGUN
(EGTA)ꢀ and Ln(OBETA)ꢀ were determined by
A
R
ACHTUNGTRENNUNG
direct pH-potentiometric titration (0.002m Ln3+ and 0.002m ligand solu-
tions). For the calculation of the logKML (KML =[ML]/[M][L]) and
metal chelate has two water molecules in its inner coordina-
tion sphere with a short residence lifetime. These water mol-
ecules are not displaced by dissolved carbonate at high pH
values or by oxoanions at physiological concentration. The
unexpected combination of thermodynamic and kinetic sta-
bility and of relaxometric properties candidates Gd-
logKMLHi (KMLHi =[MLHi]/ACHTNUTRGENUNG
[MLHiꢀ1][H+], i=0) values, the data obtained
in the pH range 1.7–5.5, reporting the volume of the base added versus
pH were used. The pH-potentiometric titrations were carried out in the
pH range 1.7–11.7 with a Metrohm 785 DMP Titrino workstation with
the use of a Metrohm-6.0233.100 combined electrode. The titrated solu-
tion (10 mL) was thermostated at 258C and stirred under N2. For the cali-
bration of the pH meter, potassium hydrogen phthalate (pH 4.005) and
borax (pH 9.177) buffers were used. For the calculation of the H+ con-
centration from the measured pH values, the method proposed by Irving
et al. was used.[25] A 0.01m HCl (0.1m KCl) solution was titrated with
0.2m KOH and the difference between the measured and calculated pH
values was used to calculate the [H+] from the pH values measured in
the titration experiments. For the calculation of the equilibrium constants
the PSEQUAD program was used.[26]
ACHTUNGTRENNUNG
(OBETA)ꢀ as a very promising scaffold for the develop-
ment of novel and efficient MRI contrast agents.
Additional work will be necessary to assess the subtle rea-
sons underlying the higher stabilities and the different selec-
tivity shown by this heptadentate chelating agent towards
Ln3+ ions. This will include necessarily detailed structural
and computational studies. Meanwhile, the preparation of
Chem. Eur. J. 2012, 18, 7680 – 7685
ꢃ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
7683