In vitro simulation of solid-solid dehydration, rehydration, and solidification of trehalose dihydrate using thermal and vibrational spectroscopic techniques

The processes of dehydration, rehydration, and solidification of trehalose dihydrate were examined to simulate it in the drying and wetting states.

Techniques included differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and Fourier transform infrared (FT-IR) microspectroscopy combined with thermal analysis. Trehalose dihydrate was pressed on one KBr pellet (IKBr method) or sealed within two KBr pellets (2KBr method) for FT-IR measurement.

On the DSC thermogram, the shoulder between 60 degrees C and 80 degrees C represented a transitional change because no weight loss occurred in this area of the TGA curve. The endothermic peak at 100 degrees C represented dehydration of trehalose dihydrate to anhydrous trehalose; a 9.5% weight loss in the TGA curve occurred from 81 degrees C. The thermal-dependent FT-IR spectra for trehalose dihydrate prepared by the IKBr method changed markedly with temperature in the 1800-1500 cm(-1) region during dehydration. IR peak intensity at 1687 cm(-1) assigned to the bending vibrational mode of solid-like water declined with temperature and decreased sharply at 67 degrees C. Another IR peak at 1640 cm(-1) associated with the bending of liquid water increased at 67 degrees C but disappeared at 79 degrees C as a result of water evaporation. Both peaks for the sample prepared by the 2KBr method changed dramatically at 64 degrees C; peak intensity at 1640 cm(-1) remained constant above 64 degrees C.

A new polymorphic transition of trehalose dihydrate was first evidenced at 64-67 degrees C from both DSC curves and thermal-related FT-IR spectra. This transitional temperature reflected the thermal-dependent transformation from solid-like water to liquid water in the trehalose dihydrate structure during dehydration. During rehydration, trehalose anhydrate was rehydrated to the dihydrate, and liquid water in the dihydrate structure was again transformed to solid-like water. The polymorphic transition within this temperature range seems to correlate with the bioprotective effect of trehalose dihydrate in preserving protein stability.