Excess enthalpies of ternary aqueous solutions of amides and ureas at 298.15 K.

Aqueous solutions of amides have been studied for their importance as uncharged model molecules of structural units of polypeptide chains and proteins. Urea and its alkyl derivatives are known as denaturing agents against proteins. In this paper some new experimental calorimetric data from our laboratory concerning ternary aqueous solutions, each containing one amide and one urea species, are presented, discussed and compared with literature data. The relevance of the weak interactions occurring between these types of cosolutes in water is discussed from the point of view of the biochemical consequences on the stability of proteins. It is confirmed that both polar and hydrophobic intermolecular interactions perturb the native conformation of polypeptides and proteins, as they compete with intramolecular interactions. Urea is either ineffective or it enhances the hydrophobic interactions, contrary to what apparently is suggested by the increase of solubility of lighter hydrocarbons in urea-water mixtures.