Peptide-urea interaction. Heats of mixing of cyclic dipeptides with urea

The excess enthalpies of the ternary aqueous solutions containing urea and the glycyl-glycine, glycyl-L-alanine, L-alanyl-L-alanine and sarcosyl-sarcosine diketopiperazines respectively have been determined. A weak but favourable enthalpic contribution to the interaction between these solutes is found. The difference between "strong" and "weak" interactions in aqueous solutions of non-electrolytes is stressed and the role of water in the weak, non-specific interactions, is discussed. The consequence of the weakness of the urea-peptide interactions on the binding of urea to the proteins is also briefly discussed.