The Ca(ii)-complexation and acid-base properties of l-gulonic acid (HGul), a diastereomer of d-gluconic acid (HGluc) differing only in the configurations of C2 and C5 have been investigated via1H and 13C NMR spectroscopies, Ca-ISE- and pH-potentiometry, polarimetry and freezing point depression. Data obtained for Gul−/HGul have been compared with those of Gluc−/HGluc. It was found that some properties (acid dissociation constant, the stoichiometry and formation constants of the Ca(ii)-complexes) were insensitive to the difference in the configuration. In solutions with pH close to neutral, the presence of the complexes CaGul+ and CaGul2 0 was unambiguously proven, with formation constants of log K1,1 = 0.88 ± 0.02 and log β1,2 = 1.51 ± 0.03 (I = 1 M, T = 25 °C). The formation of Ca(Gluc)2 0 was also observed by others, which implies that the formation of the charge neutral 1 : 2 Ca(ii)-complex of sugar carboxylates is more common than was previously believed. The stability of these species was found not to vary significantly in the ionic strength range of 1-4 M. Polarimetric measurements attested that the structure of Gul− did not change markedly upon complexation. NMR experiments suggest the coordination of C2-OH and C3-OH groups (beside COO−). DFT calculations support the existence of two coordination isomers, in which Ca2+ is attached to the COO−, C2-OH and C3-OH (in agreement with NMR), as well as to the COO−, C3-OH and C4-OH groups.
ASJC Scopus subject areas
- Inorganic Chemistry