When are two waters worse than one? Doubling the hydration number of a Gd-DTPA derivative decreases relaxivity.

ne synthesis of a novel ligand, based on N-methyl-diethylene-triaminetetraacetate and containing a diphenylcyclohexyl serum albumin binding group (U) is described and the coordination chemistry and biophysical properties of its Gd-III complex Gd-L1 are reported. The Gd-III complex of the diethylenetriaminepentaacetate analogue of the ligand described here (L2) is the MRI contrast agent MS-325. The effect of converting an acetate to a methyl group on metal-ligand stability, hydration number, water-exchange rate, relaxivity, and binding to the protein human serum albumin (HSA) is explored. The complex Gd-L1 has two coordinated water molecules in solution, that is, [Gd(L1)(H2O)(2)](2-) as shown by D-band proton ENDOR spectroscopy and implied by H-1 and O-17 NMR relaxation rate measurements. The Gd-H-water distance of the coordinated waters was found to be identical to that found for Gd-L2, 3.08 angstrom. Loss of the acetate group destabilizes the Gd-III complex by 1.7 log units (log K-ML= 20.34) relative to the complex with L2. The affinity of Gd-L1 for HSA is essentially the same as that of Gd-L2. The water-exchange rate of the two coordinated waters on Gd-L1 (k(ex)=4.4x10(5) s(-1)) is slowed by an order of magnitude relative to Gd-L2. As a result of this slow water-exchange rate, the observed proton relaxivity of Gd-L1 is much lower in a solution of HSA under physiological conditions (r(1)(obs)=22.0 mM(-1) s(-1) for 0.1 mM Gd-L1 in 0.67mM HSA, HEPES buffer, pH 7.4, 35 degrees C at 20 MHz) than that of Gd-L2 (r(1)(obs)=41.5 mM(-1) s(-1)) measured under the same conditions. Despite having two exchangeable water molecules, slow water exchange limits the potential efficacy of Gd-L1 as an MRI contrast agent.