Salt-specific effects in lysozyme solutions

The effects of additions of low-molecular-mass salts on the properties of aqueous lysozyme solutions are examined by using the cloud-point temperature, T-cloud, 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 I-buffer = 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 predict the second virial coefficients, and liquid-liquid phase diagrams under conditions, where T-cloud is determined experimentally.