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 $T_{cloud}$ 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.