Salt-specific effects in lysozyme solutions

Abstract

The effects of additions of low-molecular-mass salts on the properties of aqueous lysozyme solutions are examined by using the cloud-point temperature, Tcloud- measurements. Mixtures of protein, buffer, and simple salt in water are studied at pH = 6.8 (phosphate buffer) and pH = 4.6 (acetate buffer). We show that an addition of buffer in the amount above Ibuffer = 0.6 mol dm-3 does not affect the Tcloud values. However, by replacing a certain amount of the buffer electrolyte by another salt, keeping the total ionic strength constant, we can significantly change the cloud-point temperature. All the salts de-stabilize the solution and the magnitude of the effect depends on the nature of the salt. Experimental results are analyzed within the framework of the one- component model, which treats the protein-protein interaction as highly directional and of short-range. We use this approach to predictthe second virial coefficients, and liquid-liquid phase diagrams under conditions, where Tcloud is determined experimentally.