Gd(III) and Yb(III) Complexes Derived from a New Water-Soluble Dioxopolyazacyclohexane Macrocycle

A new macrocyclic ligand was synthesized by a reaction between diethylenetriaminepentaacetic (DTPA) dianhydride and trans-1,4-diaminocyclohexane, and the Gd(III) and Yb(III) complexes were prepared. The compounds were characterized by spectroscopic methods. Structural calculation by DFT shows that the amide linkages are arranged in such a way that a conformational strain is minimized in the macrocyclic frame. The coordination modes of the ligand and water in the metal complexes were also determined by DFT. The longitudinal relaxation time T₁ was measured for aqueous solutions of the Gd(III) complex. The T₁ relaxivity arises from the structural feature that a water molecule coordinated to the paramagnetic metal is surrounded by a large open space, through which the exchange of water occurs readily to shorten the relaxation time of water in the entire region, as a result of the chelate conformation defined strictly by the amide groups and the cyclohexane ring.