Volumetric properties of amphionic molecules in water. Part 2. Thermal expansibility and compressibility related to the formation of zwitterionic structures

The dependence of the partial molar volumes of a number of α-amino acids, ω-amino acids, dipeptides and diketopiperazines on temperature has been determined at various concentrations and for the temperature range 0-55°C using an automatic dilatometer; the limiting partial molar expansibilities, Φ°E, have been calculated. Measurements of the sound velocity at 15 MHz and at various concentrations allowed us to determine the compressibility of the solutions at 25°C and to calculate the limiting partial compressibilities, Φ°K,S, for some of the above mentioned compounds. Finally, by using the known values Φ°E and Φ°K,S together with the partial molar heat capacity of the solute and some physical properties of pure water, the limiting isothermal partial molar compressibilities, Φ°K,T, have been obtained. The values of Φ°E and Φ°K,S and the trends they show when the structure of the amphionic molecule is changed have been analysed by taking into account the features exhibited by Φ°E and Φ°K,S of related charged and non-charged compounds. The dependence on temperature and pressure of the internal and external proton-exchange reaction between a carboxyl and an amino group have been evaluated and compared.