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

Bence Kutus; Ákos Buckó; Gábor Peintler; István Pálinkó; Pál Sipos

doi:10.1039/c6dt03907c

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

Dalton Trans. 2016 Nov 15;45(45):18281-18291. doi: 10.1039/c6dt03907c.

Authors

Bence Kutus¹, Ákos Buckó¹, Gábor Peintler², István Pálinkó³, Pál Sipos¹

Affiliations

¹ Department of Inorganic and Analytical Chemistry, University of Szeged, H-6720 Dóm tér 7, Szeged, Hungary. sipos@chem.u-szeged.hu and Materials and Solution Structure Research Group, Institute of Chemistry, University of Szeged H-6720 Aradi vértanúk tere 1, Szeged, Hungary.
² Department of Organic Chemistry, University of Szeged, H-6720 Dóm tér 8, Szeged, Hungary and Materials and Solution Structure Research Group, Institute of Chemistry, University of Szeged H-6720 Aradi vértanúk tere 1, Szeged, Hungary.
³ Department of Physical Chemistry and Materials Science, University of Szeged, H-6720 Aradi vértanúk tere 1, Szeged, Hungary and Materials and Solution Structure Research Group, Institute of Chemistry, University of Szeged H-6720 Aradi vértanúk tere 1, Szeged, Hungary.

PMID: 27801462
DOI: 10.1039/c6dt03907c

Abstract

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 CaGul₂⁰ was unambiguously proven, with formation constants of log K_1,1 = 0.88 ± 0.02 and log β_1,2 = 1.51 ± 0.03 (I = 1 M, T = 25 °C). The formation of Ca(Gluc)₂⁰ 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.