Thermodynamics of interactions of urea and guanidinium salts with protein surface: Relationship between solute effects on protein processes and changes in water‐accessible surface area

To interpret effects of urea and guanidinium (GuH+) salts on processes that involve large changes in protein water‐accessible surface area (ASA), and to predict these effects from structural information, a thermodynamic characterization of the interactions of these solutes with different types of protein surface is required. In the present work we quantify the interactions of urea, GuHCl, GuHSCN, and, for comparison, KCl with native bovine serum albumin (BSA) surface, using vapor pressure osmometry (VPO) to obtain preferential interaction coefficients (Γμ3) as functions of nondenaturing concentrations of these solutes (0–1 molal). From analysis of Γμ3 using the local‐bulk domain model, we obtain concentration‐independent partition coefficients KnatP that characterize the accumulation of these solutes near native protein (BSA) surface: KnatP,urea= 1.10 ± 0.04, Knat P,SCN − = 2.4 ± 0.2, Knat P,GuH + = 1.60 ± 0.08, relative to Knat P,K + ≡ 1 and Knat P,Cl − = 1.0 ± 0.08. The relative magnitudes of KnatP are consistent with the relative effectiveness of these solutes as perturbants of protein processes. From a comparison of partition coefficients for these solutes and native surface (KnatP) with those determined by us previously for unfolded protein and alanine‐based peptide surface KunfP, we dissect KP into contributions from polar peptide backbone and other types of protein surface. For globular protein‐urea interactions, we find KnatP,urea = KunfP,urea. We propose that this equality arises because polar peptide backbone is the same fraction (0.13) of total ASA for both classes of surface. The analysis presented here quantifies and provides a physical basis for understanding Hofmeister effects of salt ions and the effects of uncharged solutes on protein processes in terms of KP and the change in protein ASA.