Characterization of Thorium-Pyrazinoic acid complexation and its decorporation efficacy in human cells and blood

Thorium Complexation with Aliphatic and Aromatic Hydroxycarboxylates: A Combined Experimental and Theoretical Study

Hydroxycarboxylic acids, viz., α-hydroxyisobutyric acid (HIBA) and mandelic acid (MA), have been widely employed as eluents for inner transition metal separation studies. Both extractants have identical functional groups (OH and COOH) with different side-chains. Despite their similarities in binding motifs, they show different retention behaviors for thorium and uranium in liquid chromatography. To understand the mechanism behind the trend, a detailed study on the aqueous phase interaction of thorium with both extractants is carried out by speciation, spectroscopy, and density functional theory-based calculations. Potentiometric titration experiments are carried out to reveal the stability and species formed. Electrospray ionization mass spectrometry is performed to identify the formation of different species by Th with both HIBA and MA. It is seen that for Th-HIBA and Th-MA, the dominating species are ML3 and ML4, respectively. A similar pattern observed in potentiometric speciation analysis supports the tendency of Th to form higher stoichiometric species with MA than with HIBA. The difference in the dominating species thus helps in explaining the reversal in the retention behavior of uranium and thorium in the reverse-phase liquid chromatographic separation. The results obtained are corroborated with extended X-ray absorption fine structure spectroscopic measurements and density functional theory (DFT) calculations.

Combined Thermodynamic, Theoretical, and Biological Study for Investigating N-(2-Acetamido)iminodiacetic Acid as a Potential Thorium Decorporation Agent

The most effective approach to mitigate the toxic effects of internal exposure of radiometals to humans is metal-ligand (ML) chelation therapy. Thorium (Th)-induced carcinogenesis as well as other health hazards to humans as a result of chronic internal exposure necessitates the development of efficient Th-decorporating agents. In this regard, chemical and biological studies were carried out to evaluate N-(2-Acetamido)iminodiacetic acid (ADA), a comparatively cost-effective, readily available, and biologically safe complexing agent for Th decorporation. In the present work, detailed thermodynamic studies for complexation of ADA with Th(IV) have been carried out to understand Th-ADA interaction, using potentiometry, calorimetry, electrospray ionization mass spectrometry, and theoretical studies, followed by its biological assessment for Th decorporation. Thermodynamic studies revealed the formation of strong Th-ADA complexes, which are enthalpically as well as entropically favored. Interestingly, density functional theory calculations, to obtain a thermodynamically favored mode of coordination, showed the uncommon trend of lower denticity of ADA in ML than in ML2, which has been explained on the basis of stabilization of ML by hydrogen bonding. The same was also reflected in the unusual trend of enthalpy for Th-ADA complexes. Biological experiments using human erythrocytes, whole human blood, and lung cells showed good cytocompatibility and ability of ADA to significantly prevent Th-induced hemolysis. Th removal of ADA from erythrocytes, human blood, and normal lung cells was found to be comparable with that of diethylenetriamine pentaacetate (DTPA), an FDA approved decorporating agent. The present study contributed significant data about Th complexation chemistry of ADA and its Th decorporation efficacy from human erythrocytes, blood, and lung cells.

Experimental and theoretical approach to probe the aquatic speciation of transuranic (neptunyl) ion in presence of two omnipresent organic moieties

Pyrazines are omnipresent in nature and have their occurrence in plants, microbes, food supplies, marine arenas. The present studies aimed at aquatic speciation of the neptunyl ion (NpO₂⁺) with two pyrazine compounds namely pyrazine monocarboxylic acid (PMC) and pyrazine dicarboxylic acid (PDC). Absorption spectrophotometry was used to probe the stability, speciation and spectral properties for the complexation process. NpO₂⁺ forms a more stable complex with PMC than PDC for 1:1 (ML), while for 1:2 (ML₂) the opposite trend is observed. The extent of shift in λ_max, which is also an indicator for the strength of complexation, reflected similar trends for the complexation process. Isothermal titration calorimetry was employed to determine the enthalpies of complex formation, which is found to be endothermic. The complexation process is entropy driven. Linear free energy correlations were established to retrieve the coordination modes of the complexes. The variation in peak potentials (the cyclic voltammograms) with change in pH and metal to ligand ratio were explored to understand redox speciation, electron transfer kinetics and Eh-pH characteristics for the interaction of NpO₂⁺ with pyrazine carboxylate ligands. Density functional theory calculations were employed to optimize the geometries and to calculate the bond distances and partial charges on key atoms of the optimized geometries. The theoretical calculations helped to reveal the contributions from two different configurations of the same geometry towards the optical absorption. The bond distances and partial charges estimated theoretically helped to understand the aqueous interactions at the molecular level.

Stabilization of uranyl(v) by dipicolinic acid in aqueous medium

Preparation of a stable U(v) complex in an aqueous medium is a challenging task owing to its disproportionation nature (conversion into more stable U(vi) and U(iv) species) and sensitivity to atmospheric oxygen. The stable uranyl (UO₂²⁺)/dipicolinic acid (DPA) complex ([U^(VI)O₂(DPA)(OH)(H₂O)]^-) was formed at pH 10.5-12.0, which was confirmed by potentiometric and spectrophotometric titrations, and NMR, ESI-MS and EXAFS spectroscopy. The complex [U^(VI)O₂(DPA)(OH)(H₂O)]^- can be electrochemically reduced on the Pt electrode at -0.9 eV (vs. Ag/AgCl) to [U^(V)O₂(DPA)(OH)(H₂O)]^2- in aqueous medium under an anaerobic environment. According to cyclic voltammetric analysis, a pair of oxidation and reduction waves at E'₀ = -0.592 V corresponds to the [U^(VI)O₂(DPA)(OH)(H₂O)]^-/[U^(V)O₂(DPA)(OH)(H₂O)]^2- redox couple and the formation of [U^(V)O₂(DPA)(OH)(H₂O)]^2- was confirmed by the electron stoichiometry (n = 0.97 ± 0.05) of the reduction reaction of [U^(VI)O₂(DPA)(OH)(H₂O)]^-. The pentavalent uranyl complex [U^(V)O₂(DPA)(OH)(H₂O)]^2- was further characterized via UV-vis-NIR absorption spectrophotometry and X-ray absorption (XANES and EXAFS) spectroscopy. The [U^(V)O₂(DPA)(OH)(H₂O)]^2- complex is stable at pH 10.5-12.0 in anaerobic water for a few days. DFT calculation shows the strong complexing ability of DPA stabilizing the unstable oxidation state U(v) in aqueous medium.