Diffusion of water molecules in crystalline beta-cyclodextrin hydrates.
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.
Braesicke
K
K
Department of Biology, Chemistry, and Pharmacy, Freie Universität Berlin, Germany.
Steiner
T
T
Saenger
W
W
Knapp
E W
EW
D000465:Algorithms; D003198:Computer Simulation; D003460:Crystallization; D003505:Cyclodextrins / Q000737:chemistry* / Q000378:metabolism*; D004058:Diffusion; D006813:Humidity; D007700:Kinetics; D008958:Models, Molecular; D011786:Quality Control; D013696:Temperature; D014867:Water / Q000737:chemistry / Q000378:metabolism*; D047392:beta-Cyclodextrins
D003505:Cyclodextrins; D047392:beta-Cyclodextrins; D014867:Water; C031215:betadex