Diffusion of water molecules in crystalline beta-cyclodextrin hydrates

Area of Science:

• Physical Chemistry
• Materials Science
• Computational Chemistry

Background:

• Hydrated beta-cyclodextrin (beta-CD) crystals exhibit complex water molecule interactions.
• Understanding water diffusion in crystalline hydrates is crucial for various applications.

Purpose of the Study:

• To elucidate the mechanism of rapid water molecule diffusion within the beta-CD crystal lattice.
• To quantify water transport properties in hydrated beta-CD under varying humidity conditions.

Main Methods:

• Molecular dynamics (MD) simulations were conducted for crystalline beta-CD at hydration levels from beta-CD-9.4H2O to beta-CD-12.3H2O.
• Simulations included crystalline hydrates (4 ns each) and aqueous solution (2 ns).
• Novel algorithms were employed to analyze diffusion pathways and spatially varying diffusion constants.

Main Results:

• A primary diffusion pathway for water molecules was identified, winding through beta-CD cavities and slanted relative to the crystallographic b-axis.
• Water molecules external to the beta-CD cavities can access this main diffusion pathway.
• The calculated diffusion constant along the main pathway is approximately 1/30th of that in bulk water.

Conclusions:

• The study provides a detailed molecular-level understanding of water diffusion in hydrated beta-CD crystals.
• The findings correlate well with experimental data regarding beta-CD crystal behavior under changing relative humidity.
• The identified diffusion pathway and reduced diffusion constant highlight the influence of the crystal lattice structure on water transport.