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 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 C NMR spectroscopic measurements. Pronounced pH effects were seen in the presence of Ca indicating strong complex formation. By the evaluation of the experimental data, two highly stable trinuclear species, CaGulH and the CaGulH, are formed in alkaline aqueous solutions. Polarimetric as well as H NMR spectroscopic measurements attested that the increased complex stability was due to the formation of strong metal ion - alcoholate interactions. Moreover, the 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.