Diffusion of Sucrose and α,α-Trehalose in Aqueous Solutions

Diffusion coefficients for sucrose and trehalose in aqueous solutions were measured using pulsed-gradient-spin-echo NMR and calculated using molecular dynamics simulations. The diffusion coefficients of sucrose and trehalose are comparable at low disaccharide concentrations but differ from each other with increasing concentration and decreasing temperature. At 400 K and the highest disaccharide concentration considered in this work (80 wt %), the simulated trehalose diffusion coefficient is a factor of 3.5 smaller than that of sucrose (2.6 × 10-8 cm2/s and 9.3 × 10-8 cm2/s, respectively). In a 74 wt % disaccharide solution, the experimental diffusion coefficient for trehalose is a factor of 3 smaller than that of sucrose at 358 K. The diffusion coefficients of water are at least 1 order of magnitude larger than those of the disaccharides. This difference increases to 2 orders of magnitude at the highest disaccharide solutions considered here. The decoupling of the water and disaccharide motion is attributed to the different diffusion mechanisms adopted by the two molecules. Simulations reveal that, at high disaccharide concentrations, the diffusion of the disaccharides appears continuous while that of water consists of random jumps followed by rapid vibrations within transient cages created by surrounding molecules.