GD(DOTP)5-OUTER-SPHERE RELAXATION ENHANCEMENT PROMOTED BY NITROGEN BASES

The relaxation properties of Gd(DOTP)5- (1, 4, 7, odecane-N,N',N'',N'' '-tetrakis(methylenephosphonic acid)) have been investigated as a func tion of pH, temperature, concentration, and magnetic field strength, W e have found that the complex has one exchangeable water molecule in i ts inner coordination sphere, at a distance of 3.26 angstrom from the metal ion, and it does not form oligomers in solution in the concentra tion range 0.2 to 10 mM. The possible presence of two species in solut ion with an average fractional hydration number is also taken into acc ounts. The NMRD profiles were recorded at 5-degrees-C, 25-degrees-C, a nd 35-degrees-C and quantitatively analyzed in terms of the paramagnet ic relaxation equations. Interestingly the addition to a solution of t he Gd(III)-complex of nitrogen bases results in a marked relaxation en hancement, which shows a strong pH dependence with a maximum around pH = 9. The relaxivity gain has been shown to depend on outer-sphere eff ects originating from multiple electrostatic interactions between the anionic complex and the organic cations that bring the exchangeable pr otons of the substrate molecules in to close proximity with the parama gnetic center. High resolution NMR relaxation data for N-methyl-D(-)-g lucamine suggest that the hydroxyl group on the beta-carbon plays a ro le in stabilizing the interaction, presumably through a hydrogen bond with an uncoordinated oxygen atom of the complex.