Complex Formation of Curium(III) with the Amino Acids L2-Aminobutyric Acid and L-Threonine and the Corresponding Phosphate Ester O-Phospho-L-Threonine

The complex formation of curium(III) with L2-Aminobutyric acid, L-Threonine and O-Phospho-L-Threonine was determined by time-resolved laser-induced fluorescence spectroscopy (TRLFS) at trace Cm(III) concentrations (3x10-7 M). The concentrations of L2-Aminobutyric acid and L-Threonine were varied between 0.001 and 0.25 M in the pH range of 2 to 9.6 using 0.5 M NaCl as background electrolyte. In the case of O-Phospho-L-Threonine, lower concentrations were applied varying from 1x10-4 to 0.025 M in the pH range 2.8 to 7.6 using 0.154 M NaCl as background electrolyte.
Curium species of the type MpHqLr were identified from the fluorescence emission spectra. Three Cm–L2-Aminobutyrate complexes could be determined: (i) CmL2+ with a peak maximum at 596.0 nm, (ii) CmL2+ with a peak maximum at 600.8 nm, and (iii) CmL3 with a peak maximum at 603.8 nm. The corresponding formation constants were calculated from TRLFS measurements to be log β101 = 5.17 ± 0.07, log β102 = 9.00 ± 0.07, and log β103 = 11.30 ± 0.09. In the Cm–Threonine system, also three different species could be characterized: (i) CmThr2+ with a peak maximum at 599.6 nm, (ii) CmThr2+ with a peak maximum at 603.2 nm, and (iii) CmThr2- with a peak maximum at 605.7 nm. These species show the following formation constants: log β101 = 6.72 ± 0.07, log β102 = 10.22 ± 0.09, and log β1-22 = -(7.22 ± 0.19). Two species could be identified in the Cm–Phosphothreonine system: (i) CmH2PThr2+ with a peak maximum at 597.0 nm and (ii) CmHPThr+ with a peak maximum at 601.0 nm. The formation constants of the Cm-Phosphothreonine complexes were calculated to be log β121 = 18.03 ± 0.13 and log β111 = 14.17 ± 0.09. Possible structures of the identified curium species will be discussed on the basis of the fluorescence lifetime measurements.
For a better understanding of the actinide interaction processes in biological systems such as microbes and plants on a molecular level, it is essential to explore their complexation with selected bioligands of relevant functionalities as model compounds.

This work was funded by the BMWi under contract number: 02E9985.