Multinuclear NMR and molecular modelling investigations on the structure and equilibria of complexes that form in aqueous solutions of Ca(2+) and gluconate.

Complexation of d-gluconate (Gluc(-)) with Ca(2+) has been investigated via (1)H, (13)C and (43)Ca NMR spectroscopy in aqueous solutions in the presence of high concentration background electrolytes (1MI4M (NaCl) ionic strength). From the ionic strength dependence of its formation constant, the stability constant at 6pH11 and at I-->0M has been derived (logK(1,1)(0)=1.8+/-0.1). The protonation constant of Gluc(-) at I=1M (NaCl) ionic strength was also determined and was found to be logK(a)=3.24+/-0.01 ((13)C NMR) and logK(a)=3.23+/-0.01 ((1)H NMR). It was found that (1)H and (13)C NMR chemical shifts upon complexation (both with H(+) and with Ca(2+)) do not vary in an unchanging way with the distance from the Ca(2+)/H(+) binding site. From 2D (1)H-(43)Ca NMR spectra, simultaneous binding of Ca(2+) 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.