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.