Multinuclear NMR and molecular modelling investigations on the structure and equilibria of complexes that form in aqueous solutions of Ca2+ and gluconate

Complex formation and lactonization reactions in aqueous solutions containing Ca2+ or Nd3+ ions and sugar-type ligands

“…The observed changes were, however, the result of simultaneous deprotonation and complexation reactions, thus, the proposed coordination motif is questionable. In a recent publication, a 1 H- 43 Ca HMQC NMR measurement ( Figure 8) attested the coordination of the C2-OH and C3-OH groups [37]. The authors suggested the simultaneous formation of linkage isomers in which either the C2-OH or both the C2-OH and C3-OH functional groups act as binding sites.…”

“…For the most frequently studied Gluc - (Figure 7), the thermodynamic association constant, log β11 0 , was found to be 1.7-1.9 [37,[84][85][86]. At varying ionic strength, log β11 varies in the range of 0.37-1.36 obtained from Ca-ISE potentiometric [22,62,75,[87][88][89][90], solubility [82], ion-exchange [91] and 13 C NMR spectroscopic [37] measurements. The value of 0.37 [62] is probably underestimated, since it was obtained indirectly from pH-potentiometric titrations performed in alkaline solutions.…”

“…The NMR peaks of gluconate were identified previously [36,37]. The 1 H peaks of gulonate were assigned by calculating the vicinal or geminal coupling constants (| 3 JH,H| or | 2 JH,H|, presented in Table 3), both in neutral and alkaline media.…”

“…The 1 H peaks of gulonate were assigned by calculating the vicinal or geminal coupling constants (| 3 JH,H| or | 2 JH,H|, presented in Table 3), both in neutral and alkaline media. Performing measurements at high pH was chosen because more resolved spectra with smaller line widths were obtained for gluconate [37] and heptagluconate [41], respectively. The present |JH,H| values (corresponding to pH ≈ 7) were compared to those measured in D2O earlier [151].…”

“…Depending on the ionic strength and experimental technique used, the log Kp of Glucranges from 3.23 to 3.86 determined via potentiometry [21,[22][23][24][25][26][27][28][29][30][31][32][33][34], potentiometry combined with polarimetry [35] or with NMR spectroscopy [36,37]. These data (mostly correspond to T = 25 °C) are tabulated in Table 1 for the sake of perspicuity.…”