Thermal Behaviour of Anhydrous α-, β- and γ-Cyclodextrin at Low Temperature

Heat capacities of anhydrous α-and γ-cyclodextrin were measured by adiabaticcalorimetry between 10 K and 300 K. The thermalbehaviour of the two compounds exhibits significantdifferences. α-Cyclodextrin shows an anomalousexcess heat capacity in the entire region between 70 Kand 210 K. In the case of γ-cyclodextrin, anendothermic effect is observed at 240K. This effect isanalysed through the study of the correspondingentropy change and discussed in terms ofintramolecular organization.Using the known heat capacity values of anhydrousβ-CD, a comparative analysis has been developed.For each cyclodextrin, the average behaviour of abound α-D-glucopyranose has been calculated andcompared. From a thermodynamic point of view, thedegree of organization of the dehydrated macrocycliccompounds could be expressed asβ-CD α-CD γ-CD.