Application of the transfer model to understand how naturally occurring osmolytes affect protein stability

A primary thermodynamic goal in protein biochemistry is to attain a predictive understanding of the energetic changes responsible for solvent-induced folding and unfolding. This chapter demonstrates the use of Tanford's transfer model to predict solvent-dependent cooperative protein folding/unfolding free energy changes (m values). This approach provides a thermodynamic description of these free energy changes in terms of individual contributions from the peptide backbone and residue side chains. The quantitative success of the transfer model has been hindered for many years because of unresolved issues involving proper measurement of the group transfer-free energies of amino acid side chains and the peptide backbone unit. This chapter demonstrates what is necessary to design experiments properly so that reliable values of group transfer-free energies are obtainable. It then demonstrates how to derive a prediction of the m value for the description of protein folding/unfolding cooperativity and that the calculated values using the transfer model agree quite well with experimentally measured values.