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 guanidiniurn (GuH(+)) salts on processes t hat 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 interac tions of urea, GuHCl, GuHSCN, and, for comparison, KCI with native bovine s erum albumin (BSA) surface, using vapor pressure osmometry (VPO) to obtain preferential interaction coefficients (Gamma (mu 3)) as functions of nonden aturing concentrations of these solutes (0-1 molal). From analysis of Gamma (mu 3) using the local-bulk domain model, we obtain concentration-independ ent partition coefficients K-P(nat) that characterize the accumulation of t hese solutes near native protein (BSA) surface: K-P,urea(nat) = 1.10 +/- 0. 04, K-P,SCN-(nat) = 2.4 +/- 0,2, K-P,GuH+(nat) = 1.60 +/- 0.08, relative to K-P,K+(nat) equivalent to 1 and K-P,Cl-(nat) = 1.0 +/- 0.08. The relative magnitudes of K-P(nat) are consistent with the relative effectiveness of th ese solutes as perturbants of protein processes. From a comparison of parti tion coefficients for these solutes and native surface (K-P(nat)) with thos e determined by us previously for unfolded protein and alanine-based peptid e surface K-P(unf) we dissect K-P into contributions from polar peptide bac kbone and other types of protein surface. For globular protein-urea interac tions, we find K-P,urea(nat) = K-P,urea .(unf). We propose that this equali ty arises because polar peptide backbone is the same fraction (0. 13) of to tal ASA for both classes of surface. The analysis presented here quantifies and provides a physical basis for understanding Hofmcister effects of salt ions and the effects of uncharged solutes on protein processes in terms of K-P and the change in protein ASA.