Role of the anion in the alkali halides interaction with D-ribose: A ¹H and ¹³C NMR spectroscopy study

The alkali halides interaction with d-ribose in D₂O solutions was studied by ¹H and ¹³C NMR spectroscopy. The observed changes in the NMR spectra are interpreted according to a model in which the hydroxyls rich region, from C₁ to C₄, interacts with the cation while the CH₂ group at C₅ on the opposite side of the sugar interacts with the anion. It seems, during the salt-sugar interaction, cation and anion preserve, at least partially, their ion-pair character. The cooperative interaction of the sugar hydroxyl groups with the cation leads to a polarization within the sugar molecule, which favors the anion interaction with its most positive region. A correlation between the chemical shift of C ₅ atom and the atomic number of the anion was observed, which is discussed as a neighboring paramagnetic effect; as higher is the halogen atom more pronounced is the resulting shift of the C₅ signal. The anion effect is weak but also observed in the ¹³C signals of those carbon atoms bound to hydroxyl groups where the interaction is predominant with the cation. The ¹H signal of the anomeric protons and the relative population of isomers in the alkali halide solution also show an anion dependence.