Temperature and pH Dependence XAFS Study of Gd(DOTA)^- and Gd(DTPA)^2- Complexes: Solid State and Solution Structures

Abstract

We present an X-ray absorption spectroscopy study of the local structures of Gd(DTPA)(2)(-) and Gd(DOTA)(-) complexes in the crystalline state (at room and low temperatures) and in aqueous solutions exhibiting various pH values (0.15-7) at different temperatures (25-90 degrees C). Using X-ray absorption fine structure (XAFS) analysis procedures and ab initio multiple scattering calculations of XAFS spectra at the Gd L(3) edge, we reconstructed the Gd(3+) local environment, and compared it with existing structural models. From neutral pH to a value of 1.5, we found that the local environment and complex dynamics around the gadolinium ions were conserved up to 4.5 Å, and the structure agreed well with the known crystallographic data. In these solutions, the gadolinium ions in the complex Gd(DOTA)(-) are bonded to the four carboxylate oxygen atoms [R(Gd-O(av)) 2.38 Å, Debye-Waller (DW) factor 0.006 Å(2)], to the four nitrogen atoms [R(Gd-N(av)) 2.65 Å, DW factor 0.006 Å(2)] and to one water molecule [R(Gd-O(w)) 2.46 Å, DW factor 0.012 Å(2)]. Concerning the complex Gd(DTPA)(2)(-), the gadolinium ions are bonded to the five carbonyl oxygen atoms [R(Gd-O(av)) 2.39 Å, DW factor 0.007 Å(2)], to the three nitrogen atoms [R(Gd-N(av)) 2.64 Å, DW factor 0.006 Å(2)], and to one water molecule [R(Gd-O(w)) 2.47 Å, DW factor 0.018 Å(2)]. In the range of pH (0.15-1.5) for the Gd(DTPA)(2)(-) complexes, thanks to the pH strong dependence of the XAFS signals, we observed a progressive complex dissociation. On the other hand, the XAFS signals of Gd(DOTA)(-) complexes exhibited only a slight pH (1-1.5) dependence. Concerning both complexes, we noted just a slight temperature dependence.

Complexing Mechanism of the Lanthanide Cations Eu3+, Gd3+, and Tb3+ with 1,4,7,10‐Tetrakis(carboxymethyl)‐1,4,7,10‐tetraazacyclododecane (dota)—Characterization of Three Successive Complexing Phases: Study of the Thermodynamic and Structural Properties of the Complexes by Potentiometry, Luminescence Spectroscopy, and EXAFS

“…Moreover, our results in solution for the [Gd(dota)] À compound are in good agreement with EXAFS studies. [52][53][54] This EXAFS study confirms unambiguously the existence of intermediate species, and provides reliable information about the lanthanide ion local structure. A first intermediate compound, [LnH 2 (dota)] + **, was underlined by the coordination of four carboxylate moieties.…”

“…The Fourier transforms (Figure 9b) exhibit well-defined features up to about 5.5-6 (apparent distances, uncorrected phase shift). As previously shown, [38,52] the first intense peak at around 1.9 is related to the carboxylic and water oxygen nearest-neighbors. The exact position of the peak depends on the nature of the central atom.…”

“…A water molecule caps the plane of the oxygen atoms to assure the coordination number of nine for gadolinium and europium. BØnazeth et al [52] showed by EXAFS the great similarity of the structures of the [Gd(dota)]…”

Structural Investigation of the Aqueous Eu2+ Ion: Comparison with Sr2+ Using the XAFS Technique

“…In the case of the L 3 -edge of rare earth elements, E 0 is located at energies higher than the white line. 34 For Eu 3+ and Eu 2+ the energies E 0 were located at ca. 2 eV above the white line maxima.…”

Multimetal - multiligand complexes of Hg (II), Cd (II) and Pb (II) ions involving 2-aminosuccinic acid and 5 methyluracil

“…Metal chelates include substances such as hemoglobin, haemocyanin, myoglobin, vitamin B 12 , chlorophyll, nucleic acid and various enzymes. 4 Mixed chelation occurs commonly in biological fluids, i.e. as million of potential ligands are likely to compete for metal ions found in vivo.…”