The first reported single-crystal X-ray structure of an actinium compound shows that Ac(III) does not coordinate exactly like its lanthanum counterpart in the same molecular setting. In a 2024 study, researchers captured an actinium–HOPO complex inside the protein siderocalin: the actinium complex had an approximate square-pyramidal arrangement and longer average metal–oxygen distances than the lanthanum comparison. The result is a direct chemical measurement—not a crystal of elemental actinium or a test of an actinium cancer treatment.
What did the first actinium crystal structure show?
Jennifer N. Wacker and colleagues reported the structure in Nature Communications on 15 July 2024. It is the first reported single-crystal X-ray structure for an actinium compound. The object crystallized was a macromolecular assembly containing Ac(III) bound by the synthetic chelator 3,4,3-LI(1,2-HOPO), called HOPO in the paper, and held inside the protein siderocalin (Scn). It was not a crystal of pure actinium metal.
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The structure crystallized in space group P4₁2₁2 and was determined at 2.08 Å resolution. HOPO surrounded Ac(III) in an approximate square-pyramidal geometry. The paper reports an average Ac–O(HOPO) distance of 3.2(7) Å, with different averages for the two kinds of oxygen: 2.9(5) Å for N-oxide oxygens and 3.5(8) Å for ketone oxygens. These are structural measurements in this protein-scaffold crystal, not universal bond lengths for every actinium complex. Read the study in Nature Communications.
How did researchers make actinium crystallography possible?
Actinium is scarce and radioactive, so researchers have little material to work with. The team used actinium-227, the longest-lived actinium isotope, to study binding in solution and determine the solid-state structure. HOPO first bound Ac(III); siderocalin then recognized the complex and held it in a binding pocket. That protein scaffold made it possible to grow crystals suitable for X-ray analysis.
According to Lawrence Berkeley National Laboratory, the researchers used 5 micrograms of purified actinium-227, grew crystals over a week, cooled them in liquid nitrogen, and collected X-ray data at the Advanced Light Source, beamline 5.0.2. The tiny actinium-containing complex was therefore studied within a protein crystal rather than as a bulk sample. Berkeley Lab describes the experiment.
How did actinium differ from lanthanum?
The researchers compared the Ac–HOPO–siderocalin structure with a La(III)–HOPO–siderocalin structure crystallized in the same space group. The lanthanum structure was determined at 2.0 Å resolution. The comparison shows that the two metals did not produce identical coordination in this particular HOPO–protein system.
| Feature | Ac(III)–HOPO–siderocalin | La(III)–HOPO–siderocalin |
|---|---|---|
| Crystal space group | P4₁2₁2 (Wacker et al., 2024) | P4₁2₁2 (Wacker et al., 2024) |
| Structure resolution | 2.08 Å (Wacker et al., 2024) | 2.0 Å (Wacker et al., 2024) |
| Average metal–O(HOPO) distance | 3.2(7) Å (Wacker et al., 2024) | 2.5(2) Å (Wacker et al., 2024) |
| Coordination arrangement | Approximate square-pyramidal geometry (Wacker et al., 2024) | Not described as the same approximate square-pyramidal arrangement in the paper (Wacker et al., 2024) |
| Ligand arrangement | Two HOPO aryl groups are displaced relative to the lanthanum structure (Wacker et al., 2024) | Reference arrangement for the reported aryl-group displacement (Wacker et al., 2024) |
The authors suggest that movement of two HOPO aryl groups helps accommodate the larger Ac(III) ion. They also caution that protein crystallography generally does not reach the atomic resolution of small-molecule crystallography; in this experiment, actinium’s high atomic number helped distinguish it from the low-atomic-number protein scaffold. The comparison supports a specific conclusion: lanthanum did not fully reproduce actinium’s coordination behavior in this system. It does not show that lanthanum or other non-radioactive surrogates are useless in all actinium research. C&EN also reports on the comparison.
What do the solution measurements add?
The crystal structure is one part of the study. Solution measurements characterize how strongly the components associate under specified conditions; their values should not be read as treatment outcomes or compared without their pH and measurement context.
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- At pH 7.36, the conditional formation constant for [Ac(III)(HOPO)]⁻ was log β′ = 17.0(1), as reported by Wacker et al. in 2024.
- At pH 7.4, the dissociation constant KD for Ac–HOPO binding to siderocalin was 6(1) nM. Under the same reported pH, the corresponding values were 20(5) nM for the lanthanum complex and 43(17) nM for free HOPO (Wacker et al., 2024).
The paper also cites earlier measurements for context, rather than reporting them as results of the 2024 crystal experiment: an Ac–O(H₂O) distance of 2.63(1) Å from X-ray absorption spectroscopy, and six-coordinate ionic radii of 1.065 Å for Ac(III) and 1.032 Å for La(III). Different experiments and coordination environments cannot be collapsed into a single direct measurement, but the context helps explain why the actinium complex’s fit and ligand arrangement merit direct study.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why does this matter for actinium-225 therapy research?
Actinium-225 is of interest for targeted alpha therapy, but the structural study used actinium-227, not actinium-225. The paper gives half-lives of 9.920(3) days for actinium-225 and 21.772(3) years for actinium-227. Because the isotopes are chemically the same element, studying Ac(III) coordination can inform the chemical foundations of future ligand and delivery-system design. It does not by itself establish that a particular ligand will deliver a therapeutic dose to a tumor or remain there.
The study is fundamental chemical characterization. It did not test an actinium-225 radiopharmaceutical in patients, demonstrate clinical benefit, or establish the HOPO–siderocalin construct as a therapy. Its practical contribution is narrower and important: a platform for measuring actinium chemistry directly, combining solution binding data with a crystal structure, despite the element’s scarcity and radioactivity.
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