Relaxometric determination of the exchange rate of the coordinated water protons in a neutral Gd(III) chelate

The exchange rate of the coordinated water molecule in the neutral complex [Gd(DTPA-BBA)(H₂O)] (DTPA-BBA = 1,7-bis[(N-benzylcarbamoyl)methyl]-l,4,7-triazaheptane-l,4,7-triacetate or diethylenetriaminopentaacetate N.N'-bis(benzylamide)) is slower than in the parent complex [Gd(DTPA)(H₂O)]². From the analysis of the temperature dependence of the solvent ¹⁷O NMR transverse relaxation time in an aqueous solution of the paramagnetic complex, a value of 4.5 x 10⁵ s^-1 (at 298 K) is obtained 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^- mM^-1. The analysis of the dependence of the longitudinal water proton relaxation rate on magnetic 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 prototropic exchange at the water molecule in the inner coordination sphere of the metal ion. This exchange process is catalyzed by OH^- ions (k(p)=l.7x 10⁹M^-1S^-1 at 298 K) and causes an increase in the observed relaxivity when it occurs at a rate larger than the exchange rate of the entire water molecule. At pH 12 the limiting effect of the slow exchange rate for the coordinated water molecule is removed, and the longitudinal water proton relaxivity measured at this pH then represents the maximum value attainable for this complex.