Thermodynamic studies of the interactions of alkylureas with glycine, diglycine, and triglycine

Abstract

The solubilities of glycine, diglycine, and triglycine have been measured in methyl‐, N,N '‐dimethyl‐, and ethylurea solutions. Gibbs free energies of transfer of glycine, the two oligoglycines, and the peptide group from water to alkylurea solutions have been calculated. Enthalpies of transfer for the same compounds and the peptide group have been determined from calorimetric measurements. The additivity principle is roughly obeyed for the contribution of the peptide group to the enthalpy of transfer. Comparison of the thermodynamic quantities of transfer to urea and alkylurea solutions reveals significant differences, those of the alkylureas clearly reflecting the hydrophobic interaction. Furthermore, Gibbs free energy of transfer of the peptide group to urea solutions is positive at lower urea concentrations but above 4 M it becomes negative and increases (i.e. becomes more negative) with increasing concentration. The interaction with the peptide group is thus an important factor in the denaturing activity of urea. For the alkylureas, on the other hand, the interaction with the peptide group is generally less favorable for unfolding.