X-ray crystal structure of a sodium salt of [Gd(DOTP)]5-:: Implications for its second-sphere relaxivity and the 23Na NMR hyperfine shift effects of [Tm(DOTP)]5-

The X-ray structure of the sodium salt of [Gd(DOTP)](5-)shows two different chelates, [Gd-(1)(DOTP)](5-) and [Gd-(2)(DOTP)](5-), bound at either surface of a sheet formed by a cluster of hydrated Na+ ions. Each [Gd-(1)(DOTP)](5-) anion binds directly to four Na' ions of this cluster through the free oxygen atoms of the phosphonate groups of the adjacent ligand, while each [Gd-(2)(DOTP)(5-) unit is connected to the cluster via hydrogen bonds only. The Gd3+ ions in the two moieties do not have any inner-sphere water molecules, and are eight-coordinate. Their coordination polyhedra are twisted square antiprisms, with slightly different twist angles. These m' isomers are found in the crystal structure as racemic mixtures of enantiomers. Only one set of NMR resonances is observed in aqueous solution, corresponding to an averaged m' isomer. In this crystal structure, the Na+ ions bind the phosphonate oxygen atoms of the [Gd-(1)(DOTP)(5-) anion at positions far removed from the main symmetry axis. This is significantly different from the binding mode(s) previously proposed to be occurring in solution between Na+ and [Tm(DOTP)](5-), based on the interpretation of solution paramagnetic Na-23 NMR shifts. This could arise as a result of the effects of the cluster of hydrated Na+ ions that are present, which may hinder axial binding modes and distort lateral binding modes. Further, in the crystal structure, both types of Gd3+ centers have four second-sphere water molecules that are located at distances (4.2-4.5 Angstrom) significantly longer than those previously proposed from the analysis of the NMRD data of [Gd-(1)(DOTP)](5-). This is a result of the coordination of Na' by these water molecules, thus preventing their direct interaction with the phosphonate oxygen atoms. However, in solution such second-sphere water molecules can interact strongly with the phosphonate ligand oxygen atoms, resulting in efficient relaxation if their binding has relatively long lifetimes (> 50 ps). Rotational immobilization will amplify this contribution, thus making it similar to outer-sphere relaxation. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003).