Structure and stability of columnar cyclomaltohexaose (alpha-cyclodextrin) hydrate

Area of Science:

• Supramolecular Chemistry
• Materials Science
• Crystallography

Background:

• Cyclodextrins are cyclic oligosaccharides with unique inclusion properties.
• Alpha-cyclodextrin (alpha-CD) can form various crystalline structures with guest molecules.
• Understanding the structural stability of alpha-CD complexes is crucial for their applications.

Purpose of the Study:

• To investigate the crystal structure of alpha-cyclodextrin (alpha-CD) formed from aqueous solutions.
• To elucidate the phase behavior of the alpha-CD columnar structure under varying water vapor sorption levels.
• To determine the stability limits of the alpha-CD columnar structure in the presence of water guests.

Main Methods:

• Rapid recrystallization of alpha-cyclodextrin from aqueous solution.
• Water vapor sorption experiments at 40°C.
• Wide-angle X-ray diffractometry (WAXD) to analyze crystal structure changes over time.

Main Results:

• Alpha-cyclodextrin rapidly forms a columnar crystal structure with water as the sole guest molecule.
• The columnar structure is unstable above a water activity of approximately 0.67 at 40°C.
• WAXD confirmed a phase transformation from the columnar to the cage structure within 0.25 hours at 40°C and a water activity of 1.0.

Conclusions:

• The columnar crystal structure of alpha-cyclodextrin is sensitive to water activity.
• High humidity induces a phase transition to a more stable cage structure.
• These findings provide insights into the structural dynamics and stability of cyclodextrin-water complexes.