Continuum Solvation Model for Studying Protein Hydration Thermodynamics at High Temperatures

A macroscopic solvation model that combines a solvent-accessible surface area term to describe hydration of nonpolar groups with a continuum electrostatics term to describe the hydration of polar groups has previously been shown to provide an excellent description of amino acid hydration free energies at 25 °C (Sitkoff et al. J. Phys. Chem. 1994, 98, pp 1978−1988). This paper describes the extension of this method and its accompanying parameter set (known as PARSE) to handle temperatures in the range from 5 to 100 °C. For the neutral amino acids, hydration free energies were taken from the literature; for the charged amino acids Asp, Glu, and Lys, hydration free energies were obtained by combining results for the neutral analogues with information on the pKa value at the required temperature. An important result of this analysis is that the hydration free energies of the charged residues are much more strongly affected by increasing temperature than their neutral analogues. In extending the PARSE method to reproduce hydration free energies over a range of temperatures, a number of alternative models were investigated: best results were obtained when separate surface area dependent terms were used to represent the hydration of aliphatic and aromatic regions and when the continuum electrostatics term was made strongly temperature dependent. The temperature dependence of the electrostatic component stems partly from changes in the dielectric constant of water but appears to be rather better described when the atomic radii are also made temperature dependent, increasing in size as the temperature rises. This requirement for temperature-dependent radii gains important support from previous studies using the Born model to describe the entropies of hydration of simple ions. The extension of the PARSE method described here permits its use in investigating the effects of hydration on protein stability over a wide range of biologically relevant temperatures.