Eu(III) and Cm(III) Complexation by the Aminocarboxylates NTA, EDTA, and EGTA Studied with NMR, TRLFS, and ITC-An Improved Approach to More Robust Thermodynamics

Sebastian Friedrich; Claudia Sieber; Björn Drobot; Satoru Tsushima; Astrid Barkleit; Katja Schmeide; Thorsten Stumpf; Jerome Kretzschmar

doi:10.3390/molecules28124881

Eu(III) and Cm(III) Complexation by the Aminocarboxylates NTA, EDTA, and EGTA Studied with NMR, TRLFS, and ITC-An Improved Approach to More Robust Thermodynamics

Molecules. 2023 Jun 20;28(12):4881. doi: 10.3390/molecules28124881.

Authors

Sebastian Friedrich¹, Claudia Sieber¹, Björn Drobot¹, Satoru Tsushima^{1

2}, Astrid Barkleit¹, Katja Schmeide¹, Thorsten Stumpf¹, Jerome Kretzschmar¹

Affiliations

¹ Helmholtz-Zentrum Dresden-Rossendorf, Institute of Resource Ecology, 01328 Dresden, Germany.
² International Research Frontiers Initiative, Institute of Innovative Research, Tokyo Institute of Technology, Meguro, Tokyo 152-8550, Japan.

Abstract

The complex formation of Eu(III) and Cm(III) was studied via tetradentate, hexadentate, and octadentate coordinating ligands of the aminopolycarboxylate family, viz., nitrilotriacetate (NTA^3-), ethylenediaminetetraacetate (EDTA^4-), and ethylene glycol-bis(2-aminoethyl ether)-N,N,N',N'-tetraacetate (EGTA^4-), respectively. Based on the complexones' pK_a values obtained from ¹H nuclear magnetic resonance (NMR) spectroscopic pH titration, complex formation constants were determined by means of the parallel-factor-analysis-assisted evaluation of Eu(III) and Cm(III) time-resolved laser-induced fluorescence spectroscopy (TRLFS). This was complemented by isothermal titration calorimetry (ITC), providing the enthalpy and entropy of the complex formation. This allowed us to obtain genuine species along with their molecular structures and corresponding reliable thermodynamic data. The three investigated complexones formed 1:1 complexes with both Eu(III) and Cm(III). Besides the established Eu(III)-NTA 1:1 and 1:2 complexes, we observed, for the first time, the existence of a Eu(III)-NTA 2:2 complex of millimolar metal and ligand concentrations. Demonstrated for thermodynamic studies on Eu(III) and Cm(III) interaction with complexones, the utilized approach is commonly applicable to many other metal-ligand systems, even to high-affinity ligands.

Keywords: calorimetry; complex formation constant; high-affinity ligand; molecular structure; spectroscopy.

Abstract

Grants and funding