RELAXOMETRIC DETERMINATION OF THE EXCHANGE-RATE OF THE COORDINATED WATER PROTONS IN A NEUTRAL GD-III CHELATE

Citation
S. Aime et al., RELAXOMETRIC DETERMINATION OF THE EXCHANGE-RATE OF THE COORDINATED WATER PROTONS IN A NEUTRAL GD-III CHELATE, Chemistry, 3(9), 1997, pp. 1499-1504
Citations number
39
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Chemistry
Journal title
Chemistry → ACNP
ISSN journal
0947-6539
Volume
3
Issue
9
Year of publication
1997
Pages
1499 - 1504
Database
ISI
SICI code
0947-6539(1997)3:9<1499:RDOTEO>2.0.ZU;2-C
Abstract
The exchange rate of the coordinated water molecule in the neutral com plex [Gd(DTPA-BBA)(H2O)] (DTPA-BBA = 1,7-bis[(N-benzylcarbamoyl)methyl ]-1 ,4,7-triazaheptane-1,4,7-triacetate or diethylenetriaminopentaacet ate N,N'-bis(benzylamide)) is slower than in the parent complex [Gd(DT PA)(H2O)](2-). From the analysis of the temperature dependence of the solvent O-17 NMR transverse relaxation time in an aqueous solution of the paramagnetic complex, a value of 4.5 x 10(5) s(-1) (at 298 K) is o btained for the exchange rate of the coordinated water molecule. This rate constant does not vary in the pH range 7-12. Conversely, over the same pH range and at 298 K and 20 MHz, the longitudinal water proton relaxivity increases from 4.8 to 6.5 s(-1) mM(-1). The analysis of the dependence of the longitudinal water proton relaxation rare on magnet ic field and temperature at pH 7 and pH 12 shows that the increase in relaxivity at basic pH has to be assigned to the contribution of the p rototropic exchange at the water molecule in the inner coordination sp here of the metal ion. This exchange process is catalyzed by OH- ions (k(P)=1.7 x 10(9) M-1 s(-1) at 298 K) and causes an increase in the ob served relaxivity when it occurs at a rate larger than the exchange ra te of the entire water molecule. At pH 12 the limiting effect of the s low exchange rate for the coordinated water molecule is removed, and t he longitudinal water proton relaxivity measured at this pH then repre sents the maximum value attainable for this complex.