Excess enthalpies of dilute aqueous solutions of model peptides and urea at 25°C.

Excess enthalpies of four dilute aqueous solutions each containing urea and a model peptide (N-acetyl-N′-methylamides of either glycine or L-alanine, N-acetylamides of either L-valine or L-proline) have been determined by calorimetry. These results are compared with the excess enthalpies of aqueous solutions of amides and other uncharged peptides. The second enthalpic virial coefficients are discussed on the basis of McMillan-Mayer theory and additivity of group interactions. The results confirm the preliminary conclusions of previous works in this series, i.e., that peptideurea interactions are water assisted, and an extra contribution appears when more than one peptidic or amidic group is present on the same molecule.