Spectroscopic and computational investigation of actinium coordination chemistry

Abstract

Actinium-225 is a promising isotope for targeted-α therapy. Unfortunately, progress in developing chelators for medicinal applications has been hindered by a limited understanding of actinium chemistry. This knowledge gap is primarily associated with handling actinium, as it is highly radioactive and in short supply. Hence, AcIII reactivity is often inferred from the lanthanides and minor actinides (that is, Am, Cm), with limited success. Here we overcome these challenges and characterize actinium in HCl solutions using X-ray absorption spectroscopy and molecular dynamics density functional theory. The Ac–Cl and Ac–OH2O distances are measured to be 2.95(3) and 2.59(3) Å, respectively. The X-ray absorption spectroscopy comparisons between AcIII and AmIII in HCl solutions indicate AcIII coordinates more inner-sphere Cl1– ligands (3.2±1.1) than AmIII (0.8±0.3). These results imply diverse reactivity for the +3 actinides and highlight the unexpected and unique AcIII chemical behaviour.

Synthesis and Characterization of the Actinium Aquo Ion

“…Moreover, this procedure was successfully applied in recovering 227 Ac after the XAFS experiment. Although similar elution profiles were previously reported using trace quantities of actinium and lanthanides, 4,17,18,36,38−44 our separation demonstrated that the DGA process accommodated a large mass range. This spanned picogram (pg) to microgram (μg) quantities with successful separations both at the tracer level ( 225 Ac = 1.1 × 10 10 atoms; 173 Lu = 3.6 × 10 11 atoms) and using large masses ( 227 Ac = 7.4 × 10 4 kBq, 7.4 × 10 16 atoms, 2.8 × 10 –5 g; stable Lu = 1.7 × 10 18 atoms, 5.0 × 10 –4 g).…”

“…These data were compared to previous actinium XAFS studies and presented in the context of what is currently understood for the other actinide(III) aquo ions. 4,6−16 Overall, the results highlight the uniqueness of the Ac III ion, most notably in terms of the large coordination numbers and very long Ac–O H2O bond distance.…”

“…45 Figure 2 compares the background subtracted and normalized XANES spectrum from Ac-aquo (obtained in this study) with a spectrum of Ac III dissolved in HCl (Ac III in HCl; 11 M; Ac-HCl) from our previous work. 4 These two spectra are the only XANES measurements reported for actinium to date. The spectra were similar, each having a pronounced edge peak superimposed on an absorption threshold.…”

“…For instance, consider that the first actinium bond distance was not measured until 2016. 4 Perhaps the most well-defined aspect of actinium chelation chemistry is the realization that actinium’s affinities for binding certain donor atoms are difficult to predict. 5 This deficiency, as well as the implications of the large actinium ionic radius on chelation, severely hampers ligand design efforts for stabilizing actinium in targeted α-therapy applications.…”

The Actinium Aqua Ion: A Century in the Making

“…4,9 This difference formed the basis of a recent study by Batista, Kozimor, and co-workers from Los Alamos National Laboratory (LANL), wherein it was found that AcCl 3 retains its chloride ligands in an 11 M HCl solution—a departure from the behavior of the transuranic species AmCl 3 , which forms a dicationic aquo complex. 10 The LANL group’s study simultaneously reported the first bond length to Ac, providing a true landmark in the study of this element. This same team is now reporting for the first time the characterization of the Ac(III) aqua ion, a study that provides much-needed, fundamental data that will be useful in the development of chelating agents tailored to the specific size, shape, and Lewis acidity of Ac(III).…”