SOLUTION STUDY, CRYSTAL-STRUCTURE AND RELAXIVITY PROPERTIES OF A GD3+COMPLEX WITH AN UNCHARGED MACROCYCLIC LIGAND BEARING 4 AMIDIC SIDE ARMS

Equilibrium data on the interaction of DTMA [(DTMA = DOTA tetrakis(met hylammide)] with Gd3+ in aqueous solution, properties of the complexes formed in the pH range 0.6-11.8, water proton relaxation rate enhance ment, and the crystal structure analysis of the [Gd(DTMA)H2O](3+) comp lex are reported. In the crystal structure the metal ion is bound to t he nitrogen atoms of the tetraazamacrocyclic moiety, to the amidic oxy gen atoms, and to an oxygen atom of a water molecule. The nine donors are located at the vertices of a distorted square antiprism, which is capped by the coordinated water oxygen atom in the axial position. In solution [Gd(DTMA)](3+) is not very stable [logK(ML) = 12.8(1)] and gi ves rise to the formation of [Gd(DTMA)OH](2+) [pK(a) = 7.9(1)1 and [Gd (HDTMA)](4+) [logK((ML+H)) = 3.4(1)]. The proton solvent relaxivity of aqueous complex solutions assumes a constant value in the pH range 3- 8, increasing at higher and lower pH. For pH > 3 the data are in good agreement with a previous study on the same compound. For pH < 3 a new interpretation is presented, based on the formation of [Gd(HDTMA)](4) and the release of Gd3+.