ML and ML2 complex formation between Ca(ii) and d-glucose derivatives in aqueous solutions

Abstract

The complex formation equilibria between Ca²⁺ ions and six carbohydrate derivatives related to D-glucose was quantitatively characterized by potentiometry, freezing point depression and polarimetry. Complexation could not be observed for D-glucose, while weak association was deduced for D-sorbitol and D-mannitol. Stronger complexes are formed with D-gluconate and D-heptagluconate due to the presence of the carboxylate group. In addition to the plausible 1 : 1 species, the 1 : 2 species can also be detected at higher ligand to metal ratios. The ML type complex is also formed with D-glucuronate and D-glucarate. The strong association for D-gluconate and D-heptagluconate was attested by freezing point depression measurements. Polarimetric results show that for these two ligands the specific rotation of the complexed and free anions is only slightly different. The stability of the 1 : 1 complexes follows the order: mannitol < sorbitol < glucuronate < heptagluconate ≈ gluconate < glucarate. The formation of the ML₂ type species has been established for polyhydroxy ligands having at least one carboxylate group in addition to the conformational flexibility.