Preparation, crystallographic and theoretical study on a bifunctional Gd-AAZTA derivative as potential MRI contrast agent precursor

The crystal structure of the complex (αR,6R)-Na[Gd(H ₂O)-AAZTA-C₂H₄COOBn]·3H ₂O·EtOH was determined by single crystal X-ray diffraction; the complex represents a bifunctional derivative of Gd-AAZTA, previously reported as original scaffold for the development of efficient contrast agents for MRI. The Gd(III) ion is nine coordinated with three nitrogens, five carboxylate oxygens and one water oxygen in a monocapped distorted square antiprism coordination polyhedron. Three of the five carboxylic groups are monodentate while a fourth is bidentate bridging an adjacent Gd ion with the formation of a polymeric structure. The influence on the overall structure of the propionate side arm introduced for conjugation purposes was discussed, and a computational approach to predict the structure of its (αR,6S) isomer is presented and compared with the X-ray structure of the (αR,6R) epimer.