The structure and dynamics of the lanthanide(III) complexes
of DTPA-BGLUCA3- (DTPA-bis(glucamide)),
DTPA-BENGALAA3-
(DTPA-bis(ethylenegalactamine−amide)),
DTPA-BEA3- (DTPA-bis(ethanolamide)), and
DTPA-BPDA3- (DTPA-bis(propanediolamide)) in
water have been investigated. These complexes are of relevance
as
potential MRI contrast agents. 13C relaxation times of
the Nd(III) complexes show octadentate binding of the
organic ligand via the three amines, the three carboxylates, and the
two amide oxygens. 17O NMR measurements
indicate that the coordination sphere is completed by one water ligand.
Eight diastereomeric pairs of isomers are
observed for the DTPA-bis(sugaramides). Data sets obtained
from variable-temperature and -pressure 17O
NMR
at 9.4 T and variable-temperature 1H nuclear magnetic
relaxation dispersion (NMRD) on the Gd(III) complexes
were fitted simultaneously to give insight into the parameters
governing the water 1H relaxivity. The
water
exchange rates, k
ex
298, on
[Gd(DTPA-BPDA)(H2O)],
[Gd(DTPA-BGLUCA)(H2O)] and
[Gd(DTPA-BENGALAA)(H2O)] are 3.6 ± 0.3, 3.8 ± 0.2, and 2.2 ± 0.1 ×
105 s-1, and the activation
volumes are +6.7, +6.8, and +5.6
cm3 mol-1 (±0.2 cm3
mol-1), respectively, indicating a strongly
dissociatively activated mechanism. The sugar
moieties have no significant influence on the coordination of the
Gd(III) ion and on the parameters governing the
relaxivity, apart from the expected increase in the rotational
correlation time. The relaxivity under the usual
MRI conditions is limited by the water exchange rate and the electronic
relaxation. The data obtained are used
to explain the relaxivity of conjugates of polysaccharides and
Gd(DTPA).
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