Interactions between calcium ions and a range of monosaccharides studied by hydroxy-proton resonance spectroscopy

Abstract

At low temperatures in a small pH range, well resolved hydroxy-resonance features have been detected for dilute aqueous solutions of sugars. Shifts in these resonances on the addition of calcium chloride have been followed for the major forms of a range of monosaccharides. These trends take three forms: (i) normal, linear, up-field shifts, comparable to that for the water-proton resonance, indicating no preferential interaction, (ii) reduced up-field or weak down-field shifts, which remain almost linear, assigned to weak binding of two OH groups to Ca2+ ions, either adjacent axial + equatorial groups or two cis axial groups separated by one —CHX— group, and (iii) down-field shifts becoming normal up-field shifts at high salt concentrations, assigned to strong binding of Ca2+ ions to three adjacent OH groups in the axial–equatorial–axial configuration. Only in the case of strong complexing (iii) is there clear evidence for cation-induced conformational changes.These assignments apply to the pyranose forms of the sugars. Our more limited results for the furanose forms strongly support the suggestion that three cis OH groups are required for strong bonding. When the anomeric (O1H) proton in α- or β-D-glucose is replaced by a methyl group the weak binding to Ca2+ ions observed for the O1H and O2H groups is lost, since the down-field shift for the O2H-proton resonance changes to a normal up-field shift. This means that the —OMe group fails to coordinate to the Ca2+ ions. Reasons for this result are proposed.