Structure and Thermodynamics of Eu(III) and Cm(III) Complexes with Glucuronic Acid

Complexation by small organic ligands controls the bioavailability of contaminants and influences their mobility in the geosphere. We have studied the interactions of Cm3+, as a representative of the trivalent actinides, and Eu3+, as an inactive homologue, with glucuronic acid (GlcA) a simple sugar acid. Time-resolved laser-induced luminescence spectroscopy (TRLFS) shows that complexation at pH 5.0 occurs only at high ligand to metal ratios in the form of 1:1 complexes with standard formation constants log beta(0) = 1.84 +/- 0.22 for Eu3+ and log beta(0) = 2.39 +/- 0.19 for Cm3+. A combination of NMR, QMMM, and TRLFS reveals the structure of the complex to be a half-sandwich structure wherein the ligand binds through its carboxylic group, the ring oxygen, and a hydroxyl group in addition to five to six water molecules. Surprisingly, Y3+, which was used as a diamagnetic reference in NMR, prefers a different coordination geometry with bonding through at least two hydroxyl groups on the opposite side of a distorted GlcA molecule. QMMM simulations indicate that the differences in stability among Cm, Eu, and Y are related to ring strain induced by smaller cations. At higher pH a stronger complex was detected, most likely due to deprotonation of a coordinating OH group.

Automated summary

The complexation of small organic ligands with metal ions under different pH conditions results in varied structures and stability. Cm3+ and Eu3+ form 1:1 complexes with glucuronic acid, while Y3+ prefers a different coordination geometry with the same ligand. The differences in stability among the metal ions are related to the ring strain induced by smaller cations, with stronger complexes detected at higher pH due to deprotonation of a coordinating group.