Dalton Transactions Dynamic Article Links Cite this: Dalton Trans., 2012, 41, 13969 www.rsc.org/dalton PAPER Curium(III) citrate speciation in biological systems: a europium(III) assisted spectroscopic and quantum chemical study† Anne Heller,* Astrid Barkleit, Harald Foerstendorf, Satoru Tsushima, Karsten Heim and Gert Bernhard Received 6th July 2012, Accepted 3rd September 2012 DOI: 10.1039/c2dt31480k Citrate complexes are the dominant binding form of trivalent actinides and lanthanides 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 their impact on the metabolism of the human organism and the corresponding health risks. We studied the complexation of Cm(III) and Eu(III), as representatives of trivalent actinides and lanthanides, respectively, in aqueous citrate solution over a wide pH range using time-resolved laser-induced fluorescence spectroscopy. Four distinct citrate complexes were identified and their stability constants were determined, which are MHCit 0 , M(HCitH)HCit 2- , M(HCit) 2 3- , and M(Cit) 2 5- (M = Cm, Eu). Additionally, there were also indications for the formation of MCit - complexes. Structural details on the EuHCit 0 and EuCit - complexes were obtained with FT-IR spectroscopy in combination with density functional theory calculations. IR spectroscopic evidence for the deprotonation of the hydroxyl group of the citrate ion in the EuCit - complex is presented, which also revealed that the complexation of the Eu 3+ ion takes place not only through the carboxylate groups, like in EuHCit 0 , but additionally via the hydroxylate group. In both EuHCit 0 and EuCit - the carboxylate binding mode is mono-dentate. Under a very low metal : 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 dominant in human urine at pH < 6 were identified to be Cm(HCitH)HCit 2- and a mixture of Eu(HCitH)HCit 2- and EuHCit 0 . The results specify our previous in vitro study using natural human urine samples (Heller et al., Chem. Res. Toxicol., 2011, 24, 193–203). Introduction Heavy metals, particularly radionuclides, represent a serious health risk to humans in the case of incorporation. For the under- standing of their (radio-) toxicity, distribution, deposition and elimination, it is crucial to investigate their aqueous speciation and molecular transport mechanisms in biosystems. Unfortu- nately, only little is known about the behavior of trivalent acti- nides (An(III)) and lanthanides (Ln(III)) in the human organism. An(III) are artificial, highly radioactive elements that are mainly produced within the nuclear fuel cycle in nuclear power plants and, therefore, are contained in nuclear waste. Due to nuclear incidents, natural disasters, or non-professional storage of radio- active waste, An(III) can be released into the environment and the biosphere. In contrast to this, Ln(III) are naturally occurring, non- radioactive elements with a variety of applications in technology and medicine. 1 Especially the use of Ln(III) chelate complexes as contrast enhancing agents in magnetic resonance imaging and as molecular sensors and probes has grown enormously during the last few decades. 2 Consequently, there are several possibilities how humans potentially may get in contact with both An(III) and Ln(III). Actinide and lanthanide elements are hazardous to health due to their radiological and/or chemical toxicity. Irrespective of the uptake pathway, i.e., inhalation, ingestion or cutaneous absorp- tion, the heavy metal ions are resorbed and transported by the bloodstream prior to deposition in target organs or tissue. For all An(III) and Ln(III) these are, in particular, the bones and liver, respectively. 3–5 Excretion of these elements is very low and occurs mainly via the kidneys and, therefore, with urine. 3–5 In general, trivalent actinides and lanthanides exhibit analog chemi- cal properties due to their similar ionic radii and oxidation states. This, in turn, results also in a very similar behavior of An(III) and Ln(III) in the human organism. Apart from the well-studied effects of distribution, accumu- lation and elimination, only a few investigations on the molecu- lar chemical binding form of these elements in body fluids and tissues have been performed. The speciation of An(III) in human blood was studied by some authors, who showed that these metal ions mainly bind to the plasma proteins transferrin and serum albumin. 5,6 Additionally, a significant part of the heavy metal ions was found to bind to low molecular weight ligands, such as phosphate and citrate. 5,6 Preliminary investigations on An(III) in urine suggested citrate complexation of the investigated † Electronic supplementary information (ESI) available. See DOI: 10.1039/c2dt31480k Helmholtz-Zentrum Dresden-Rossendorf, Institute of Resource Ecology, P.O. Box 510119, 01314 Dresden, Germany. E-mail: a.heller@hzdr.de; Fax: +49 351 260 3553; Tel: +49 351 260 2251 This journal is © The Royal Society of Chemistry 2012 Dalton Trans., 2012, 41, 13969–13983 | 13969