The Effect of Salt on Protein Chemical Potential Determined by Ternary Diffusion in Aqueous Solutions.

We use accurate thermodn. derivs. extd. from high-precision measurements of the four vol.-fixed diffusion coeffs. in ternary solns. of lysozyme chloride in aq. NaCl, NH4Cl, and KCl at pH 4.5 and 25 to (a) assess the relative contributions of the common-ion and nonideality effects to the protein chem. potential as a function of salt concn., (b) compare the behavior of the protein chem. potential for the three salts, which we found to be consistent with the Hofmeister series, and (c) discuss our thermodn. data in relation to the dependence of the protein soly. on salt concn. The four diffusion coeffs. are reported at 0.6 mM lysozyme chloride and 0.25, 0.5, 0.9, 1.2, and 1.5 M KCl and extend into the protein-supersatd. region. The chem. potential cross-derivs. are extd. from diffusion data using the Onsager reciprocal relation and the equality of molal cross-derivs. of solute chem. potentials. They are compared to those calcd. previously from diffusion data for lysozyme in aq. NaCl and NH4Cl. We est. the effective charge on the diffusing lysozyme cation at the exptl. concns. Our diffusion measurements on the three salts allowed us to analyze and interpret the four diffusion coeffs. for charged proteins in the presence of 1:1 electrolytes. Our results may provide guidance to the understanding of protein crystn.