Schrier M Y, Schrier E E
Biochemistry. 1976 Jun 15;15(12):2607-12. doi: 10.1021/bi00657a020.
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.
等压蒸气压测量已被用于在较宽的共溶质浓度范围内，获得核糖核酸酶A从稀缓冲液转移至尿素或盐酸胍（GdnHCl）溶液中的转移自由能。在8M尿素中，转移自由能从0单调变化至-8千卡/摩尔，在6M GdnHCl中则变化至-18千卡/摩尔。相对于蛋白质组成基团从水转移至相同浓度共溶质溶液的转移自由能而言，这些值并不大。假定蛋白质转移自由能的大小由组成基团对溶液的平均静态可及性决定。利用组成基团转移自由能的文献值和氨基酸组成，确定了假设100%可及的核糖核酸酶A转移至不同共溶质浓度时的转移自由能。实验测定的转移自由能与100%可及蛋白质的转移自由能之比，得出天然蛋白质在1M GdnHCl或2M尿素中的可及表面积为11%。进一步考虑得出，变性蛋白质在6M GdnHCl或8M尿素中的可及表面积值为36%。
