Calorimetric studies on viroids.

Thermodynamic studies on highly purified viroid preparations were carried out with the help of a very sensitive adiabatic microcalorimeter. Parallel to the change of UV-absorption at 260 nm as a function of temperature, the additional heat capacity of the dilute viroid solution rises sharply within the melting interval, reaches a maximum at T = Tm and declines to a baseline again when the temperature is increased further. From the peak area the molar transition enthalpy can be calculated. The transition enthalpies of citrus exocortis viroid and cucumber pale fruit viroid are 4200 kJ/mol and 3930 kJ/mol, respectively. The calorimetric results are compared to the results obtained from melting studies using UV-absorption.