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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

¹ Department of Biochemistry, University of Wisconsin, Madison, Wisconsin, 53706 USA
² Department of Chemistry, University of Wisconsin, Madison, Wisconsin, 53706 USA
³ Department of Bacteriology, University of Wisconsin, Madison, Wisconsin, 53706 USA

Reprint requests to: M. Thomas Record, Dept. of Biochemistry, 433 Babcock Drive, Madison, WI 53706, USA; e-mail: record{at}monte.biochem.wisc.edu; fax: (608) 262-3453.

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 (_µ3) as functions of nondenaturing concentrations of these solutes (0–1 molal). From analysis of _µ3 using the local-bulk domain model, we obtain concentration-independent partition coefficients K^nat_P that characterize the accumulation of these solutes near native protein (BSA) surface: K^nat_P,urea= 1.10 ± 0.04, K^nat_P,SCN^- = 2.4 ± 0.2, K^nat_P,GuH⁺ = 1.60 ± 0.08, relative to K^nat_P,K⁺ 1 and K^nat_P,Cl^- = 1.0 ± 0.08. The relative magnitudes of K^nat_P 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 (K^nat_P) with those determined by us previously for unfolded protein and alanine-based peptide surface K^unf_P, we dissect K_P into contributions from polar peptide backbone and other types of protein surface. For globular protein-urea interactions, we find K^nat_P,urea = K^unf_P,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 K_P and the change in protein ASA.

Keywords: Preferential interactions; protein water-accessible surface area; protein folding; Hofmeister Series; urea; guanidinium chloride; guanidinium thiocyanate; peptide backbone

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