Experimental evidence for a second coordination sphere water molecule in the hydration structure of YbDTPA - Insights for a re-assessment of the relaxivity data of GdDTPA

The low temperature (-100 degrees C) X-ray structure of the complex K-2[Yb( DTPA)(H2O)] has been determined. The metal ion is at the center of a tricap ped trigonal prism and is nine-coordinate, binding to the three nitrogens a nd five oxygens of the ligand and one water molecule. From the structure ob tained, three well-defined hydration shells can be observed consisting of: i) one coordinated water molecule; ii) several water molecules in the outer coordination sphere of the Yb-III ion and iii) one water molecule surprisi ngly close to the metal center and hydrogen-bonded to proximate carboxylate groups. A variable-field and temperature NMRD study of the corresponding G d complex has been performed and the data interpreted by taking into accoun t the presence of second-sphere water molecule(s). The results of the analy sis are in excellent agreement with the X-ray structural data.