The peptide–urea interaction. Excess enthalpies of aqueous solutions of N-acetylamides of amino acids and urea at 298.15 K

Abstract

The peptide–urea interactions in water are relevant for the understanding of the conformational stability of polypeptides and the denaturation of proteins. Interesting calorimetric results on ternary and quaternary aqueous solutions containing urea and some N-acetylamides of simple amino acids (glycine, D- and L-alanine and L-leucine) are reported, discussed and compared with the properties of similar amide–urea–water and cyclic dipeptide–urea–water systems. The results were used to calculate the excess enthalpies and their virial coefficients. The usual treatment of the data was enlarged to cover solution containing racemates. The second virial coefficients of the excess enthalpies are discussed on the basis of the McMillan–Mayer theory and additivity of groups. However, the increasing amount of data at our disposal, allows us to refine the conclusions of preceding works from both our own and other laboratories. The additivity of the water-mediated interactions between urea and alkylic groups is confirmed, but evidence is given that extra contributions appear for the peptide–urea interaction, when more than one peptidic or amidic group is present on the same molecule.