Thermal behaviour of anhydrous alpha-, beta- and gamma-cyclodextrin at lowtemperature

Heat capacities of anhydrous alpha- and gamma-cyclodextrin were measured by adiabatic calorimetry between 10 K and 300 K. The thermal behaviour of the two compounds exhibits significant differences. alpha-Cyclodextrin shows a n anomalous excess heat capacity in the entire region between 70 K and 210 K. In the case of gamma-cyclodextrin, an endothermic effect is observed at 240K. This effect is analysed through the study of the corresponding entrop y change and discussed in terms of intramolecular organization. Using the k nown heat capacity values of anhydrous beta-CD, a comparative analysis has been developed. For each cyclodextrin, the average behaviour of a bound alp ha-D-glucopyranose has been calculated and compared. From a thermodynamic p oint of view, the degree of organization of the dehydrated macrocyclic comp ounds could be expressed as beta-CD > alpha-CD > gamma-CD.