Hydration of Monosaccharides: A Study by Dielectric and Nuclear Magnetic Relaxation

NMR studies indicate that the relaxation rate of (17)O-enriched water is enhanced in monosaccharide solutions, and it is greater in hexose solutions than with a pentose, ribose. Three dielectric relaxations have been isolated and assigned to bulk water < solute reorientation < hydration water in order of increasing relaxation time. The extent of hydration evaluated from dielectric and NMR measurements agree. If, for example, we define "hydration number" as the average number of water molecules which are hydrogen-bonded to the monosaccharides such that the lifetime of the bond is longer than that between molecules in pure water, then at 50 degrees C the hydration number of glucose is 2.3, and that of ribose is 1.2. The hydration number of glucose decreases with increasing temperature, and it is suggested that at 80 degrees C both hexoses and the pentose only form stable monohydrates. At lower temperature the hydration depends on the compatibility of the monosaccharide conformation with the "lattice" structural component of water. This model also explains (1) the relaxation mechanisms, (2) the thermodynamic properties of the solutions and, in particular, the similarity between the hydration numbers calculated from relaxation data and from free-energy measurements, (3) why the concentration of tetrahedrally bonded water molecules, as detected by Raman measurements, in sugar solutions is higher than that implied by thermodynamic and relaxation measurements, and (4) the temperature dependence of the conformational equilibria of ribose.