Gd(DOTP)⁵‐ outer‐sphere relaxation enhancement promoted by nitrogen bases

The relaxation properties of Gd(DOTP)⁵⁻ (1, 4, 7, 10‐tetra‐azacyclododecane‐N,N',N “,N”' ‐tetrakis(methylenephosphon‐ic acid)) have been investigated as a function of pH, temperature, concentration, and magnetic field strength. We have found that the complex has one exchangeable water molecule in1 its inner coordination sphere, at a distance of 3.26 Å from the metal ion, and it does not form oligomers in solution in the Concentration range 0.2 to 10 mM. The possible presence of two species in solution with an average fractional hydration number is also taken into accounts. The NMRD profiles were recorded at 5°C, 25°C, and 35°C and quantitatively analyzed in terms of the paramagnetic relaxation equations. Interestingly the addition to a solution of the Gd(lll)‐complex of nitrogen biases results in a marked relaxation enhancement, which shows a strong pH dependence with a maximum around pH = 9. The relaxivity gain has been shown to depend on outer‐sphere effects originating from multiple electrostatic interactions between the anionic complex and the organic cations that bring the exchangeable protons of the substrate molecules in tal close proximity with the paramagnetic center. High resolution NMR relaxation data for N‐methy1‐D(‐)‐glucamine suggest that the hydroxyl group on the β‐carbon plays a role in stabilizing the interaction, presumably through a hydrogen bond with an uncoordinated oxygen atom of the complex.