Transfer free energies and average static accessibilities for ribonuclease A in guanidinium hydrochloride and urea solutions

Abstract

Isopiestic vapor pressure measurements have been used to obtain free energies of transfer of ribonuclease A from dilute buffer to solutions of either urea or guanidine hydrochloride (GdnHCl) over a wide cosolute concentration range. The free energies of transfer vary monotonically from 0 to -8 kcal/mol in 8 M urea and to -18 kcal/mol in 6 M GdnHCl. These values are not large in relation to free energies of transfer of constituent groups of the protein from water to cosolute solutions of the same concentration. The assumption is made that the magnitude of the free energy of transfer of the protein is governed by the average static accessibility of the constituent groups to the solution. The free energies of transfer to different cosolute concentrations of a hypothetical 100% accessible ribonuclease A were determined using literature values of the free energies of transfer of constituent groups and the amino acid composition. The ratio of the experimentally determined free energy of transfer to the free energy of transfer of the 100% accessible protein gave 11% accessible surface area for the native protein in 1 M GdnHCl or 2 M urea. Additional considerations led to a value of 36% for the accessible surface area of the denatured protein in 6 M GdnHCl or 8 M urea.