No abstract

The physical properties of aqueous sucrose near the glass transition temperature have been studied using Monte Carlo and molecular dynamics simulations. The sucrose solutions ranged in concentration from 6 to 100 wt% and in temperature from 300 to 600 K. For concentrated solutions (g 80 wt%), a parallel tempering Monte Carlo algorithm was implemented to circumvent the slow system dynamics and improve sampling of configuration space. Parallel-tempered density calculations agree more closely with experimental data than conventional NPT results. Our simulations indicate that aqueous sucrose retains two intramolecular hydrogen bonds even in dilute solutions. The two hydrogen bonds detected in crystalline sucrose were also observed in the sucrose solutions of 50 wt% sucrose and greater. The hydration number was calculated for sucrose and compared with that for trehalose. Sucrose is less hydrated than trehalose for all concentrations studied. Using parallel-tempered NPT density results, molecular dynamics simulations were utilized to estimate the diffusion of water near the glass transition for concentrated sugar solutions. Diffusion coefficients in aqueous sucrose appear to be a factor of 2 greater than those in trehalose solutions.

The viscosity and electrical conductivity of aqueous solutions of trehalose and other saccharides containing 1:1 electrolytes have been measured. The decrease in the molar conductivity of each of these electrolytes with increasing viscosity is less than that predicted by Walden's rule. Instead, the empirical relation Λη R ) constant, which has already been found for other glass-forming liquids, holds for these mixtures. The microscopic origin of the deviation from the viscous friction model could be related to the presence of local heterogeneities in the distribution of water about the ions. This hypothesis is supported by the results of extensive molecular simulations. The temperature dependence of the Walden product of NaCl in aqueous trehalose mixtures indicates that this salt is fully dissociated close to the glass-transition temperature (T g ).

Transport of D-glucose was studied in Percoll-gradient-purified rat liver lysosomes. D-Glucose uptake had a Km of 22 mM and a t1/2 of approx. 30 s. D-Fucose, 2-deoxyglucose and methyl alpha-glucoside were the most effective competitors for uptake of D-glucose, although D-galactose, D-mannose, D-xylose and L-fucose also appeared to compete for uptake. L-Glucose was a poor competitor for uptake. No competition was observed with N-acetyl-D-glucosamine, N-acetyl-D-galactosamine, D-glucuronic acid, N-acetylneuraminic acid, D-glucosamine or the amino acids L-glycine, L-lysine and L-proline. Uptake was unaffected by N-ethylmaleimide, dithiothreitol, KCl, NaCl, ATP/Mg or alteration of buffer pH. D-Glucose efflux from lysosomes was temperature-dependent, with a Q10 of 2.3, and was inhibited by cytochalasin B. Counter-transport could not be demonstrated. In contrast, L-fucose uptake had a Km of 65 mM and was largely unaffected by 5 M excess of neutral D-sugars. Both uptake and efflux of L-fucose were inhibited by cytochalasin B. It appears that lysosomes possess a facilitated transport system for D-glucose and perhaps other neutral D-sugars that is discrete from transport systems for acetylated and acidic sugars.