A Comparative IRMPD and DFT Study of Fe3+ and UO22+ Complexation with N-Methylacetohydroxamic Acid

Targeted PET/MRI Imaging Super Probes: A Critical Review of Opportunities and Challenges

Recently, the demand for hybrid PET/MRI imaging techniques has increased significantly, which has sparked the investigation into new ways to simultaneously track multiple molecular targets and improve the localization and expression of biochemical markers. Multimodal imaging probes have recently emerged as powerful tools for improving the detection sensitivity and accuracy-both important factors in disease diagnosis and treatment; however, only a limited number of bimodal probes have been investigated in preclinical models. Herein, we briefly describe the strengths and limitations of PET and MRI modalities and highlight the need for the development of multimodal molecularly-targeted agents. We have tried to thoroughly summarize data on bimodal probes available on PubMed. Emphasis was placed on their design, safety profiles, pharmacokinetics, and clearance properties. The challenges in PET/MR probe development using a number of illustrative examples are also discussed, along with future research directions for these novel conjugates.

Assignment of complex species by affinity capillary electrophoresis: The case of Th(IV)-desferrioxamine B

The electrophoretic mobility change of desferrioxamine B (DFO) was monitored by UV absorption spectrophotometry upon increasing the thorium(IV) concentration in the background electrolyte at two acidities ([HClO₄ ]_Tot = 0.0316 and 0.0100 M). These data enabled to assess the speciation model and to determine the equilibrium constant of [Th(DFO)H₂ ]³⁺ at fixed ionic strength (I = 0.1 M (H,Na)ClO₄ ). Affinity capillary electrophoresis (ACE) turned out to be most helpful in identifying the complexed species by ascertaining its charge and protonation state. The assignment of the correct stoichiometry relied on the reliable estimation of the electrophoretic mobility by assuming similar hydrodynamic radii for (DFO)H₄ ⁺ and the chelate. The value of the apparent equilibrium constant (log β₁₁₂ = 38.7 ± 0.4) obtained by ACE compares favorably well with those reported in the literature for thorium and a range of other metal ions, according to a linear free-energy relationship. This method is useful for studying metal-ligand binding equilibria and provides valuable information for further modelling the behavior of tetravalent actinides under environmental conditions. Structural information about the prevalent solution species in acidic conditions was gained by DFT calculations, confirming the bishydroxamato coordination mode of Th⁴⁺ by the diprotonated ligand.

Stability Series for the Complexation of Six Key Siderophore Functional Groups with Uranyl Using Density Functional Theory

Determining stability constants of uranyl complexes with the principal functional groups in siderophores and identifying stability series is of great importance to predict which siderophore classes preferentially bind to U^VI and, hence, impact uranium speciation in the environment. It also helps to develop resins for scavenging U^VI from aqueous solutions. Here, we apply a recently developed computational approach to calculate log β values for a set of geochemically relevant uranium organometallic complexes using Density Functional Theory (DFT). We determined the stability series for catecholate, hydroxamate, α-hydroxycarboxylate, α-aminocarboxylate, hydroxy-phenyloxazolonate, and α-hydroxyimidazole with the uranyl cation. In this work, the stability constants (log β₁₁₀) of α-hydroxyimidazolate and hydroxy-phenyloxazolonate are calculated for the first time. Our approach employed the B3LYP density functional approximation, aug-cc-pVDZ basis set for ligand atoms, MDF60 ECP for U^VI, and the IEFPCM solvation model. DFT calculated log β₁₁₀ were corrected using a previously established fitting equation. We find that the siderophore functional groups stability decreases in the order: α-hydroxycarboxylate bound via the α-hydroxy and carboxylate groups (log β₁₁₀ = 17.08), α-hydroxyimidazolate (log β₁₁₀ = 16.55), catecholate (log β₁₁₀ = 16.43), hydroxamate (log β₁₁₀ = 9.00), hydroxy-phenyloxazolonate (log β₁₁₀ = 8.43), α-hydroxycarboxylate bound via the carboxylate group (log β₁₁₀ = 7.51) and α-aminocarboxylate (log β₁₁₀ = 4.73). We confirm that the stability for the binding mode of the functional groups decrease in the order: bidentate, monodentate via ligand O atoms, and monodentate via ligand N atoms. The stability series strongly suggests that α-hydroxyimidazolate is an important functional group that needs to be included when assessing uranyl mobility and removal from aqueous solutions.

Siderophore-inspired chelator hijacks uranium from aqueous medium

Over millennia, nature has evolved an ability to selectively recognize and sequester specific metal ions by employing a wide variety of supramolecular chelators. Iron-specific molecular carriers—siderophores—are noteworthy for their structural elegance, while exhibiting some of the strongest and most selective binding towards a specific metal ion. Development of simple uranyl (UO₂²⁺) recognition motifs possessing siderophore-like selectivity, however, presents a challenge. Herein we report a comprehensive theoretical, crystallographic and spectroscopic studies on the UO₂²⁺ binding with a non-toxic siderophore-inspired chelator, 2,6-bis[hydroxy(methyl)amino]-4-morpholino-1,3,5-triazine (H₂BHT). The optimal pK_a values and structural preorganization endow H₂BHT with one of the highest uranyl binding affinity and selectivity among molecular chelators. The results of small-molecule standards are validated by a proof-of-principle development of the H₂BHT-functionalized polymeric adsorbent material that affords high uranium uptake capacity even in the presence of competing vanadium (V) ions in aqueous medium.

Development of simple uranyl recognition motifs possessing siderophore-like binding strength and selectivity presents a challenge. Here the authors show a comprehensive theoretical and experimental study on uranyl binding with a polymeric adsorbent material decorated with a non-toxic siderophore inspired small molecule chelator.

Pseudohalide Tectons within the Coordination Sphere of the Uranyl Ion: Experimental and Theoretical Study of C-H···O, C-H···S, and Chalcogenide Noncovalent Interactions

A series of uranyl thiocyanate and selenocyanate of the type [R₄N]₃[UO₂(NCS)₅] (R₄ = ⁿBu₄, Me₃Bz, Et₃Bz), [Ph₄P][UO₂(NCS)₃(NO₃)] and [R₄N]₃[UO₂(NCSe)₅] (R₄ = Me₄, ⁿPr₄, Et₃Bz) have been prepared and structurally characterized. The resulting noncovalent interactions have been examined and compared to other examples in the literature. The nature of these interactions is determined by the cation so that when the alkyl groups are small, chalcogenide···chalcogenide interactions are present, but this "switches off" when R = ⁿPr and charge assisted U═O···H-C and S(e)···H-C hydrogen bonding remain the dominant interaction. Increasing the size of the chain to ⁿBu results in only S···H-C interactions. The spectroscopic implications of these chalcogenide interactions have been explored in the vibrational and photophysical properties of the series [R₄N]₃[UO₂(NCS)₅] (R₄ = Me₄, Et₄, ⁿPr₄, ⁿBu₄, Me₃Bz, Et₃Bz), [R₄N]₃[UO₂(NCSe)₅] (R₄ = Me₄, ⁿPr₄, Et₃Bz) and [Et₄N]₄[UO₂(NCSe)₅][NCSe]. The data suggest that U═O···H-C interactions are weak and do not perturb the uranyl moiety. While the chalcogenide interactions do not influence the photophysical properties, a coupling of the U═O and δ(NCS) or δ(NCSe) vibrational modes is observed in the 77 K solid state emission spectra. A theoretical examination of representative examples of Se···Se, C-H···Se, and C-H···O═U by molecular electrostatic potentials and NBO and AIM methodologies gives a deeper understanding of these weak interactions. C-H···Se are individually weak but C-H···O═U interactions are even weaker, supporting the idea that the -yl oxo's are weak Lewis bases. An Atoms in Molecules study suggests that the chalcogenide interaction is similar to lone pair···π or fluorine···fluorine interactions. An oxidation of the NCS ligands to form [(UO₂)(SO₄)₂(H₂O)₄]·3H₂O was also noted.

Non-covalent interactions of uranyl complexes: a theoretical study

Ab initio and DFT data quantify the ability of model uranyl complexes to engage in hydrogen- and halogen-bonding, quantifying the weakness of U–O_yl as an acceptor but the strength of equatorial OH₂ as a donor.

Effects of preorganization in the chelation of UO22+by hydroxamate ligands: cyclic PIPO−vs.linear NMA−

Thanks to preorganization, 1,2-PIPOH, the six-membered ring cyclic hydroxamic acid, binds uranyl six times more strongly than its linear, methyl-substituted homolog (NMAH).