Contribution of the Surface Free Energy Perturbation to Protein-Solvent Interactions

Abstract

Surface tension measurements were carried out at 20 degrees C by a capillary drop-weight method on aqueous solutions of sodium glutamate (NaGlu), lysine hydrochloride (LysHCl), potassium aspartate (KAsp), arginine hydrochloride (ArgHCl), lysylglutamate (LysGlu), argininylglutamate (ArgGlu), guanidinium sulfate, trehalose, trimethylamine N-oxide (TMAO), dimethyl sulfoxide, 2-methyl-2,4-pentanediol (hexylene glycol), and poly(ethylene glycol)s of molecular weights 200, 400, 600, and 1000. All of the salts and the sugar increased the surface tension of water, while the last four compounds decreased it, with 2-methyl-2,4-pentanediol lowering it most effectively and TMAO being the least effective. The preferential hydration of bovine serum albumin (BSA) and lysozyme was measured in KAsp, ArgHCl, LysGlu, and ArgGlu. The high values of preferential hydration found in all cases, except for BSA in ArgHCl, suggest that they should stabilize protein structure, as had been found for lysine hydrochloride and monosodium glutamate [Arakawa, T., & Timasheff, S. N. (1984) J. Biol. Chem. 259, 4979-4986]. A correlation was found for both BSA and lysozyme in KAsp, NaGlu, LysHCl, ArgGlu, and LysGlu between the surface tension effect and the observed preferential interactions, indicating that the change in the surface free energy of the protein-containing cavity due to the surface tension increase for water by these amino acid salts contributes dominantly to the observed increase in the chemical potential of the protein by their addition. The lack of a correlation observed for BSA, but not lysozyme, in ArgHCl at low concentrations where preferential binding is close to zero suggests, however, that the surface tension effect is not the sole factor involved in the protein-solvent interactions in these amino acid salts. Binding of ArgHCl to BSA, probably through hydrogen bonds between the Arg guanidinium group and peptide bonds, was proposed to occur, the affinity of Arg+ being reduced by electrostatic repulsion when proteins carry a net positive charge, such as is the case with lysozyme. Since the four organic solvent additives also lead to protein preferential hydration, no correlation exists between their preferential interactions and the surface free energy perturbation. Therefore, in their case, the preferential hydration must be ascribed to other factors that overcome the preferential binding expected from the Gibbs adsorption isotherm. The surface tension results, however, are consistent with the binding of the organic solvents to proteins through hydrophobic interactions, explaining, at least in part, the observed concentration dependence of the interactions.

On the role of surface tension in the stabilization of globular proteins

“…LysHCl and ArgHCl studies were performed at close to neutral pH to keep them as monovalent salts, i.e., the form for which the surface tension increments had been measured (Kita et al, 1994). lose, LysHC1, and ArgHCl from surface tension increments measured at 20 "C (Kita et al, 1994). The application of these increments to the data at T, was based on the fact that such increments for sucrose and a number of other small organic molecules, e.g., acetic acid, are known not to change with temperature (International Critical Tables, 1928).…”

“…Specifically, it was found that the measured preferential interactions of ArgHCl and LysHCl (Kita et al, 1994) with bovine serum albumin did not give a good correlation with the increase in surface tension. A similar lack of correlation has also been found for MgC12 (Arakawa et al, 1990).…”

“…In other words, the preferential interaction, expressed as preferential binding of co-solvent, will assume positive or negative values in the two respective cases. In dialysis equilibrium experiments, it has been found that sugars, amino acids, and many salts are preferentially excluded from the protein surface, i.e., their preferential interactions with proteins are negative (Lee & Timasheff, 1981;Arakawa & Timasheff, 1982a, 1982b, 1983Kita et al, 1994). All of these co-solvents raise the surface tension of water.…”

“…All of these co-solvents raise the surface tension of water. The good correlation between the measured negative preferential interactions and the positive surface tension increment in their presence (Arakawa & Timasheff, 1982a, 1982b, 1983, 1984bKita et al, 1994) led to the proposal that this is the principal source of the unfavorable free energy of the interactions of these co-solvents with proteins. These observations have suggested that the stabilization of proteins by these co-solvents is due to the increase of the surface tension in their presence.…”

Kosmotropes and chaotropes: modelling preferential exclusion, binding and aggregate stability

“…Because the origin of the kosmotropic and chaotropic effects appears to lie primarily in their influence on the solvent, rather than in direct interactions between cosolvent and solute [22,23,24], a microscopic description must begin with the unique properties of the aqueous medium. Water molecules have the ability to form strong, intermolecular hydrogen bonds, and pure, liquid water may form extended hydrogen-bonded networks, becoming highly ordered (Fig.…”