The stability of native conformations of proteins is strongly affected by the addition of solutes to solutions, but due to the structural complexities of proteins, direct studies of this phenomenon are extremely difficult [1]. Thermodynamic investigations of aqueous solutions containing amino acids, building stones of peptides, will be helpful in the fundamental understanding of the nature of interactions between amino acids and other solutes existing in solution. On the other hand, our studies [2] have been shown that poly(propylene glycol) (PPG) polymers and amino acids can induce phase separation in aqueous media over part of the composition space. From this point of view, the investigation of various thermodynamic properties of PPG-amino acid aqueous mixtures is necessary for elucidating of phase forming behavior of these systems and also for optimum design of industrial purification and separation process dealing with this type of aqueous biphasic systems [3]. In this work a comprehensive study of volumetric and compressibility properties have been made for PPG-amino acid aqueous systems.
