The solubilities of five cyclic dipeptides in water and in aqueous urea at 298.15 K: a quantitative model for the denaturation of proteins in aqueous urea solutions.

The solubilities of cyclo(L-alanylglycine), cyclo(L-alanyl-L-alanine), cyclo(glycyl-L-leucine), cyclo(L-valyl-L-valine) and cyclo(glycyl-L-phenylalanine) were determined in water and in aqueous urea solutions up to concentrations of 9 molar urea at 298.15 K. The solubilities of all cyclic dipeptides increase with increasing urea concentration. A simple equilibrium model, taking into account the activity of urea and that of water, fits the solubility data yielding apparent equilibrium constants describing the interactions occurring between urea and the peptide groups plus the alkyl groups that are next to these peptide groups. The apparent equilibrium constants were converted to Gibbs energy parameters for each amino acid residue which were then used to make a quantitative estimate of the contribution of urea to the denaturation of proteins.