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 C5 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.