Thermal and structural characterization of cyclomaltononaose (delta-cyclodextrin), cyclomaltodecaose (epsilon-cyclodextrin) and cyclomaltotetradecaose (iota-cyclodextrin)

Thermal behavior of cyclomaltononaose (delta-cyclodextrin, delta-CD), cyclomaltodecaose (epsilon-cyclodextrin, epsilon-CD) and cyclomaltotetradecaose (iota-cyclodextrin, iota-CD) and the solid-state phase transitions associated with the respective dehydration and rehydration processes were investigated. delta-CD, epsilon-CD and iota-CD in their amorphous and crystalline states were tested by differential scanning calorimetry (DSC), thermogravimetry (TG) and hot stage microscopy (HSM). X-ray powder diffraction (XRD) patterns both experimental and computer generated from single crystal data were used to support thermal data and to follow the solid-state phase transitions associated with thermal dehydration and rehydration of each cyclodextrin (CD). Thermal events associated with dehydration of delta-CD, epsilon-CD and iota-CD were evaluated and tentatively interpreted on the basis of water arrangements observed in the crystal lattice. Dehydrated delta-CD reverted to the starting 13.75H2O hydrate under high relative humidity (RH) conditions (100% RH) at room temperature (RT), while transformed into a 7H2O hydrate at 30–35% RH. Physicochemical characterization based on thermal behavior, which can be related to water arrangements observed in the respective crystals, permits to distinguish between delta-CD, epsilon-CD and iota-CD, as well as between the amorphous and crystalline states of each CD. The results can be useful for demonstrating the formation of a true inclusion complex of delta-CD, which occurs in two crystalline hydration states, with a guest drug.