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

Complexation of d-gluconate (Gluc−) with Ca2+ has been investigated via 1H, 13C and 43Ca NMR spectroscopy in aqueous solutions in the presence of high concentration background electrolytes (1 M ⩽ I ⩽ 4 M (NaCl) ionic strength). From the ionic strength dependence of its formation constant, the stability constant at 6 ⩽ pH ⩽ 11 and at I → 0 M has been derived (logK1,10=1.8±0.1). The protonation constant of Gluc− at I = 1 M (NaCl) ionic strength was also determined and was found to be log Ka = 3.24 ± 0.01 (13C NMR) and log Ka = 3.23 ± 0.01 (1H NMR). It was found that 1H and 13C NMR chemical shifts upon complexation (both with H+ and with Ca2+) do not vary in an unchanging way with the distance from the Ca2+/H+ binding site. From 2D 1H–43Ca NMR spectra, simultaneous binding of Ca2+ to the alcoholic OH on C2 and C3 was deduced. Molecular modelling results modulated this picture by revealing structures in which the Gluc− behaves as a multidentate ligand. The five-membered chelated initial structure was found to be thermodynamically more stable than that derived from a six-membered chelated initial structure.

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