New hydrated crystal form of alpha cyclodextrin found

The group of Francesca Fabbiani (Goettingen, Germany) have found a novel hydrated crystal form of alpha cyclodextrin, which they have crystallised at higher pressures of 0.65 GPa. The surprising behaviour, that the initial hydrated form dissolves with pressure while subsequently the novel crystal form nucleates and grows, is nicely presented by optical microscopy. The high pressure crystal structure shows a 5.5% increase of density in comparison to the structure under ambient conditions. While the host molecules stay almost unchanged with increasing pressure, the novel crystal form shows changes in the positions of the water molecules and their connectivity.

In further studies, the authors plan to investigate the behaviour of alpha cyclodextrin under further increasing pressure. It is possible, that this new hydrate is only an intermediate state in the continuous hydration process induced when pressurising the cyclodextrin.