Production and Characterization of Desmalonichrome Relative Binding Affinity for Uranyl Ions in Relation to Other Siderophores

Siderophores are iron (Fe)-binding secondary metabolites that have been investigated for their uranium-binding properties. Previous work has focused on characterizing hydroxamate types of siderophores, such as desferrioxamine B, for their uranyl (UO2)-binding affinity. Carboxylate forms of these metabolites hold potential to be more efficient chelators of UO2, yet they have not been widely studied. Desmalonichrome is a carboxylate siderophore that is not commercially available and so was obtained from the fungus Fusarium oxysporum cultivated under Fe-depleted conditions. The relative affinity for UO2 binding of desmalonichrome was investigated using a competitive analysis of binding affinities between UO2 acetate and different concentrations of Fe(III) chloride using electrospray ionization mass spectrometry. In addition to desmalonichrome, three other siderophores, including two hydroxamates (desferrioxamine B and desferrichrome) and one carboxylate (desferrichrome A), were studied to understand their relative affinities for the UO2(2+) ion at two pH values. The binding affinities of hydroxamate siderophores to UO2(2+) ions were observed to decrease with increasing Fe(III)Cl3 concentration at the lower pH. On the other hand, decreasing the pH has a smaller impact on the binding affinities between carboxylate siderophores and the UO2(2+) ion. Desmalonichrome in particular was shown to have the greatest relative affinity for UO2 at all pH and Fe(III) concentrations examined. These results suggest that acidic functional groups in the ligands are important for strong chelation with UO2 at lower pH.