Diffusion of water molecules in crystalline beta-cyclodextrin hydrates

• Published in: Journal of molecular graphics & modelling Vol. 18; no. 2; pp. 143 - 152
• Main Authors: Braesicke, K, Steiner, T, Saenger, W, Knapp, E W
• Format: Journal Article
• Language: English
• Published: United States 01-04-2000
• Subjects: Algorithms; beta-Cyclodextrins; Computer Simulation; Crystallization; Cyclodextrins - chemistry; Cyclodextrins - metabolism; Diffusion; Humidity; Kinetics; Models, Molecular; Quality Control; Temperature; Water - chemistry; Water - metabolism
• ISSN: 1093-3263
• DOI: 10.1016/S1093-3263(00)00046-2

To understand the rapid diffusion mechanism of water molecules in the crystal lattice of hydrated beta-cyclodextrin (beta-CD), molecular dynamics (MD) simulations of crystalline beta-CD were performed at five different relative humidities corresponding to hydration states ranging from beta-CD-9.4H2O to beta-CD-12.3H2O, and in aqueous solution. The trajectories for the crystalline beta-CD hydrates had lengths of 4 ns each, whereas the simulation in aqueous solution extended to 2 ns. Transport of water molecules in the crystal was characterized in terms of a spatially varying diffusion constant and the main direction of diffusion, which were evaluated using newly developed algorithms. The main diffusion pathway winds through the cavities of adjacent doughnut shaped beta-CD molecules and is slightly slanted with respect to the crystallographic b-axis. Water molecules outside the beta-CD cavities have access to the main diffusion pathway. The diffusion constant for transport of water molecules along the main pathway calculated from the MD simulation data adopts 1/30 of the value in bulk water at room temperature. This is in agreement with estimates that can be made from experimental data on the adjustment of a beta-CD crystal to changes in relative humidity.