The liquid amide transfer model and the unfolding thermodynamics of small globular proteins

In this paper, the solid cyclic dipeptide model developed by Murphy and Gill is analysed in order to point out that, apart from general thermodynamic features shown by well-characterized small globular proteins, only the polar and apolar contributions to the net denaturation heat capacity change are necessary to calculate the so-called protein stability curve, delta dGzero versus temperature. We propose that these specific heat capacity contributions can be determined in a reliable manner by a group additivity analysis of the transfer process of liquid amides from pure liquid phase into water. This suggests that the unfolding process, thought of as the transfer of amino acid residues from the protein 'core' to contact with water molecules, can be modelled based on the transfer process of organic amides. The reliability of the model is tested in comparison with literature data.