Spectroscopic and Quantum Chemical Study of the Curium(III) and Europium(III) Citrate Speciation in Biological Systems

Heavy metals, particularly radionuclides, represent a serious health risk to humans in case of incorpo-ration. For the understanding of their (radio-) toxicity, distribution, deposition and elimination, it is crucial to investigate their aqueous speciation and molecular transport mechanisms in biosystems. Unfortunately, only little is known about the behavior of artificial, radioactive trivalent actinides (An(III)) and naturally occurring, nonradioactive lanthanides (Ln(III)) in the human organism. Nevertheless, quite recently, we showed that citrate complexes are the dominant binding form of An(III) and Ln(III) in human urine at pH < 6. Hence, an accurate prediction of the speciation of these elements in the presence of citrate is crucial for the understanding of the impact on the metabolism of the human or-ganism and the corresponding health risks.
Therefore, we studied the complexation of Cm(III) and Eu(III), as representatives of An(III) and Ln(III), respectively, in aqueous citrate solution over a wide pH range using a combined approach of Time-Resolved Laser-induced Fluorescence Spectroscopy (TRLFS), Fourier-Transform Infrared Spectroscopy with Attenuated Total Reflection (ATR FT IR) and density functional theory (DFT) calculations.
With TRLFS, four citrate complexes were identified for both Cm(III) and Eu(III), which are MHCit0, M(HCitH)HCit 2-, M(HCit)2 3-, and M(Cit)2 5-. Furthermore, the stability constants were determined. Additionally, there were also indications for the formation of MCit- complexes.
With ATR FT IR and DFT, structural details of the EuHCit and EuCit- complexes were obtained. The combination of both methods gave clear evidence for the deprotonation of the hydroxyl group of the citrate ion in the EuCit- complex, what also revealed that the complexation of the Eu 3+ ion takes place not only through the carboxylate groups, like in EuCit0, but additionally via the hydroxylate group. In both EuCit0 and EuCit- the carboxylate binding mode is predominantly mono-dentate.
Under very low metal to citrate ratio that is typical for human body fluids, the Cm(III) and Eu(III) speciation was found to be strongly pH-dependent. The Cm(III) and Eu(III) citrate complexes domi-nant in human urine at pH < 6 were identified to be Cm(HCitH)HCit2- and a mixture of Eu(HCitH)HCit 2- and EuHCit0. The results specify our previous in vitro study using natural human urine samples and point out the importance of low molecular weight ligands for An(III) and Ln(III) complexation in body fluids and other biological systems.