Water diffusion in hydrated crystalline and amorphous sugars monitored using H/D exchange

Water interacts with pharmaceutical materials in a number of different ways. The aim of this study was to investigate if exchange experiments with D(2)O can provide useful insights into the structure of hydrated materials. Raffinose pentahydrate, trehalose dihydrate, and sucrose were used as model compounds in conjunction with their amorphous counterparts. Following exposure to D(2)O vapor, the exchange of water of hydration and/or hydroxyl groups was monitored using Raman spectroscopy. For the amorphous materials, all of the sugar hydroxyl groups were found to exchange on exposure to D(2)O, providing evidence that water has no fixed site in amorphous materials, nor is access to different parts of the molecule restricted. For raffinose pentahydrate and trehalose dihydrate, exchange of both hydrate water and hydroxyls was incomplete, suggesting that there are specific pathways for diffusion into and within the crystal structure. The results are rationalized based on the known crystal structures. Using exchange experiments to investigate hydrates thus appears to be a useful probe of structure.