Capture and Stabilization of the Hydroxyl Radical in a Uranyl Peroxide Cluster

Radiolysis of water in spent nuclear fuel is known to contribute to the degradation of extractants used in reprocessing schemes; however, much less is known about how free radical species (•OH, HO2-•, e-(aq), •H) formed during water radiolysis interact with actinide cations in aqueous solution. Here we describe the synthesis and characterization of a new uranyl peroxide cluster (UPC), U60Ox30*, that is analogous to the previous characterized U60Ox30, which captures and stabilizes oxygen-based free radicals for more than one week. These radical species were first detected with a nitroblue tetrazolium colorimetric assay and U60Ox30* was characterized by single crystal X-ray diffraction as well as infrared (IR), Raman, UV-Vis-NIR, and electron paramagnetic resonance (EPR) spectroscopies. Identification and further characterization of the free radicals present in U60Ox30* was done via room temperature solid and solution state X-band EPR studies using spin trapping methods. The spin trapping agent 5,5-dimethyl-1-pyrroline N-oxide (DMPO) was definitive for identifying the free radicals in U60Ox30*, which are hydroxyl radicals (•OH) that are stable for up to ten days that also persist upon addition of the metalloenzymes catalase and superoxide dismutase. Addition of the spin trapping agent α-(4-pyridyl N-oxide)-N-tert-butylnitrone (POBN) further confirmed the radicals were oxygen based and not part of the oxalate ligands in the cluster, and deuteration experiments confirmed that the origin of the free radicals was from water radiolysis. The results here demonstrate that highly oxidizing species such as the •OH radical can be stabilized in UPC systems, which alters our understanding of the role of radiolysis products and their potential to interact with actinyl species in spent nuclear fuel.