Configuration-dependent complex formation between Ca(II) and sugar carboxylate ligands in alkaline medium: Comparison of L-gulonate with D-gluconate and D-heptaguconate.

The calcium sugar carboxylate interactions in hyperalkaline solutions are of relevance in radioactive waste repositories and in certain industrial processes. The complex formation between L-gulonate and Ca ²⁺ ions was studied in strongly alkaline medium at 25 °C and 1 M ionic strength and was compared with previous results reported for D-gluconate and D-heptagluconate. The deprotonation of the ligand was confirmed by potentiometric and ¹³ C NMR spectroscopic measurements. Pronounced pH effects were seen in the presence of Ca ²⁺ indicating strong complex formation. By the evaluation of the experimental data, two highly stable trinuclear species, Ca ₃ Gul ₂ H _-3 ⁺ and the Ca ₃ Gul ₂ H _-4 ⁰ , are formed in alkaline aqueous solutions. Polarimetric as well as ¹ H NMR spectroscopic measurements attested that the increased complex stability was due to the formation of strong metal ion - alcoholate interactions. Moreover, the ¹ H NMR spectra of the three anions refer to the role of configuration in metal ion-binding. That is, the participation of the C3-OH or C4-OH group is governed by the relative position (i.e., threo or erythro) of the C2-OH and C3-OH groups.
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