Calcium complexation and acid-base properties of l-gulonate, a diastereomer of d-gluconate.

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 via H-1 and C-13 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 CaGul(2)(0) was unambiguously proven, with formation constants of log K-1,K-1 = 0.88 +/- 0.02 and log beta(1,2) = 1.51 +/- 0.03 (I = 1 M, T = 25 degrees 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.