Surface Tension Increment Research Articles

Preferential interactions of proteins with salts in concentrated solutions.

The preferential interactions of proteins with solvent components were studied in concentrated salt by densimetric measurements. Proteins were found to be preferentially hydrated in NaCl, NaCH3COO, and Na2SO4. The resulting unfavorable free-energy change was related to the effects of these salts on solubility and stability of the proteins. This unfavorable free-energy change was correlated with the large, positive surface tension increment of these salts, i.e., their perturbation of surface free energy. On the other hand, KSCN, CaCl2, and MgCl2 showed considerable binding to bovine serum albumin, which could be related to their destabilizing and salting-in effects on macromolecules. Since the last two salts have high surface tension increments, it was concluded that this does not necessarily lead to protein preferential hydration and stabilization.

Stabilization of protein structure by sugars.

The preferential interaction of proteins with solvent components was measured in aqueous lactose and glucose systems by using a high precision densimeter. In all cases, the protein was preferentially hydrated; i.e., addition of these sugars to an aqueous solution of the protein resulted in an unfavorable free-energy change. This effect was shown to increase with an increase in protein surface area, explaining the protein stabilizing action of these sugars and their enhancing effect of protein associations. Correlation of the preferential interaction parameter with the effect of the sugars on the surface tension of water, i.e., their positive surface tension increment, has led to the conclusion that the surface free energy perturbation by sugars plays a predominant role in their preferential interaction with proteins. Other contributing factors are the exclusion volume of the sugars and the chemical nature of the protein surface.

A surface tension approach to the salting-out of nonpolar nonelectrolytes

A simple approach is developed for the salting-out of nonpolar molecules by strongly solvated salts. The standard free energy of transfer of solute from pure solvent to salt solution, and hence the salting coefficient, k, is calculated in terms of the surface tension increment caused by the addition of salt to the solvent. In its simplest form the method gives k in terms of the molar volumes of solvent and solute, and the osmotic coefficient of the salt solution. It is more successful in the prediction of k than the McDevit and Long theory, and it also has advantages over the more complex scaled particle theory of salt effects. In addition to a range of nonpolar solutes, the salt effects on some alcohols are also considered but application to polar solutes generally requires a knowledge of experimentally determined surface tension increments which are not widely available. The correct order of magnitude for the temperature coefficient of k at room temperature for some alkanes and H2 in water is given, although minima in k as a function of temperature, which have sometimes been observed, are not reproduced.

Salt effects on hydrophobic interactions in precipitation and chromatography of proteins: An interpretation of the lyotropic series

Abstract The effect of neutral salts on electrostatic and hydrophobic interactions in salting out and chromatography of proteins is treated by a simple theory. The electrostatic energy change in these processes is expressed according to the classical theory. The corresponding hydrophobic energy change is evaluated from the nonpolar contact area of the interacting species and the corrected surface tension of the medium. The property of a salt that affects hydrophobic interactions is quantified by its molal surface tension increment. This parameter is a measure of the increase in surface tension by the salt and forms the basis of a natural lyotropic series. In agreement with the theory, the analysis of solubility and Chromatographic data shows that the overall salt effect can be described by the two antagonistic effects of salts on electrostatic and hydrophobic interactions. The expression derived for the salting-out constant is employed to calculate the relative surface hydrophobicity of proteins. The results suggest that the salt effect is not only useful in the quantitative treatment of hydrophobic interactions but also in the measurement of hydrophobic properties.

Interfacial tensions at alkane-aqueous electrolyte interfaces

Surface tensions of aqueous electrolytes, and their interfacial tensions against n-dodecane, have been determined at 20°C. The salts studied were LiCl, NaCl, KCl, KBr, NaBr, KI and Na2SO4 at concentrations up to about 1 mol kg–1. For the alkali metal chlorides and Na2SO4 the surface and interfacial tension increments are similar for a given electrolyte. The corresponding increments for KBr, NaBr and KI however are found to differ considerably. The results are discussed in terms of both electrostatic theory and dispersion force theory of interfaces. With respect to the latter, it is found that if due allowance is made for the presence of an ion-free layer of water at the interface, an approximate approach in which only a single dominant interaction frequency is considered, gives results in reasonable accord with experiment.