Molecular motion and interactions in aqueous carbohydrate solutions. III. A combined nuclear magnetic and dielectric-relaxation strategy

Abstract

A strategy is developed for the complementary use of dielectric and nuclear magnetic relaxation methods to elucidate the molecular dynamics in aqueous solutions of small hydrophilic molecules, and hence determine extents of hydration. The nuclear magnetic relaxation data, as well as characterizing the motional properties of various carbohydrate solutes, is used here to test alternative models for the resolution of the dielectric spectra into their component relaxation processes. This approach results in a much more confident analysis of solvent relaxation properties than has in the past been usual, to yield information relating to the extents of hydration of small sugars and the possible orientation-specific nature of this hydration. It is demonstrated that the dielectric relaxation of the sugar molecules themselves is unequivocally not due to the reorientation of the whole molecule and most likely is dominated by the rotation of hydrate side chain groups (hydroxyls and hydroxymethyl). In proton magnetic relaxation studies of glucose in D₂O it is observed that one particular proton (H-1 in the α-form only) is extremely susceptible to inter-molecular proton-proton interactions while the remaining protons are very effectively shielded. This observation is shown to be fully consistent with the conformational and hydration properties of glucose.