Calcium complexation and acid–base properties ofl-gulonate, a diastereomer ofd-gluconate

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 viaH and C 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 was unambiguously proven, with formation constants of log K = 0.88 ± 0.02 and log β = 1.51 ± 0.03 (I = 1 M, T = 25 °C). The formation of Ca(Gluc) 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 Ca 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.

Self-Correcting Method for the Measurement of Free Calcium and Magnesium Concentrations by 1H NMR

“…The free ion concentrations thus grant valuable insight into the ionresponsive behavior of the systems. 4,5 The free ion concentrations can be measured using electrochemical probes [5][6][7] or fluorescent dyes. [8][9][10] However, such measurements can be expensive and inconvenient to perform in addition to NMR, particularly when only limited quantities of sample are available.…”

“…When complexing species are present, the concentration of free ions can be very different to the total concentrations of Ca or Mg. The free ion concentrations thus grant valuable insight into the ion-responsive behavior of the systems. , The free ion concentrations can be measured using electrochemical probes − or fluorescent dyes. − However, such measurements can be expensive and inconvenient to perform in addition to NMR, particularly when only limited quantities of sample are available. Furthermore, the insertion of electrodes can cause serious damage to soft-solid samples such as gels while electrochemical methods are also wholly unsuited to study the internal ion gradients that can occur in these systems. , Methods for the direct measurement of free Ca 2+ and Mg 2+ concentrations by NMR are thus required.…”

Complex equilibria in strongly alkaline aqueous solutions containing Ca(II), Al(III) and heptagluconate ions

“…As expected, the structure of the titration curves greatly resembled to those of D-gluconate due to the structural similarity of the two ligands: heptagluconate only differs from gluconate in an additional -CH 2 -OH group and thus the same speciation model could be envisaged (Figure 12). Upon fitting the potentiometric data, it was found that assuming the formation of CaHpgl Nevertheless, these results are in line with those reported for heptagluconate [66] and gulonate [142] in I = 1 M (NaCl) medium.…”