Barium and Radium Complexation with Ethylenediaminetetraacetic Acid in Aqueous Alkaline Sodium Chloride Media

Abstract

The speciation of Ra 2? and Ba 2? with EDTA was investigated at 25°C in aqueous alkaline NaCl media as a function of ionic strength (0.2-2.5 molÁL -1 ) in two pH regions where the EDTA 4-and HEDTA 3-species dominate. The stability constants for the formation of the [BaEDTA] 2-and [RaEDTA] 2-complexes were determined using an ion exchange method. Barium-133 and radium-226 were used as radiotracers and their concentrations in the aqueous phase were measured using liquid scintillation counting and gamma spectrometry, respectively. The specific ion interaction theory (SIT) was used to account for [NaEDTA] 3-and [NaHEDTA] 2-complex formation, and used to extrapolate the logarithms of the apparent stability constants (log 10 K) to zero ionic strength (BaEDTA 2-: 9.86 ± 0.09; RaEDTA 2-: 9.13 ± 0.07) and obtain the Ba 2? and Ra 2? ion interaction parameters:[e(Na ? , BaEDTA 2-) = -(0.03 ± 0.11); e(Na ? , RaEDTA 2-) = -(0.10 ± 0.11)]. It was found that in the pH region where HEDTA 3-dominates, the reaction of Ba 2? or Ra 2? with the HEDTA 3-ligand also results in the formation of the BaEDTA 2-and RaEDTA 2-complexes (as it does in the region where the EDTA 4-ligand dominates) with the release of a proton. Comparison of the ion interaction parameters of Ba 2? and Ra 2? strongly indicates that both metal ions and their EDTA complexes have similar activity coefficients and undergo similar short-range interactions in aqueous NaCl media.

Chemistry of alkaline earth metals: It is not all ionic and definitely not boring!

“…The coordination geometry relies on a monocapped tetragonal prism and is completed by water molecules [122]. The coordination chemistry of radium is little explored as it is radioactive, yet seems to be similar to the one of barium [123].…”

Towards the stable chelation of radium for biomedical applications with an 18-membered macrocyclic ligand

“…23 Previous efforts to investigate the aqueous coordination chemistry of Ra 2+ have focused on linear polyaminocarboxylate ligands, such as ethylenediamine(tetraacetic) acid (EDTA). [24][25][26][27][28] These studies, which employed either long-lived 226 Ra or tracer 228 Ra, demonstrate that complexes of Ra 2+ with these ligands form, albeit with lower stability constants than the lighter alkaline earth ions. Competition extraction studies have also shown that macrocycles such as DOTA and Kryptox 2.2.2 can bind to 223 Ra, although the biological stability of these complexes has not been evaluated.…”

Elucidating the coordination chemistry of the radium ion for targeted alpha therapy

“…These values can provide valuable insight into the fundamental coordination preferences of Ra 2+ and serve as a useful metric to compare binding strengths and selectivities among chelators. To date, stability constant studies of Ra 2+ complexes have been limited to simple organic acids and acyclic polyaminocarboxylate ligands, 6,10 which are known to form complexes of low stability in vivo with many radiometals. 7 a Here, we expand our knowledge of the stability constants of Ra 2+ complexes to include two chelators that are directly relevant for use in biomedical applications.…”