| Abstract
| - For ternary systems, we present a method for using measured values of the four ternary diffusioncoefficients and the Onsager reciprocal relations to extract derivatives of solute chemical potentials with respectto solute molar concentrations. The method is applicable to systems in which the molar concentration of onesolute is very small compared to that of the other, and also small enough that an inverse concentration dependencedominates certain activity coefficient derivatives. These conditions apply to a large number of aqueous systemsinvolving macromolecules of biological interest. Unlike other techniques, the present method can be used tostudy undersaturated and supersaturated solutions. The approach is illustrated for the lysozyme chloride−NaCl−H2O system at 25 °C, using data reported here for pH 6.0 at 0.60 mM (8.6 mg/mL) lysozyme chlorideand 0.25, 0.50, 0.65, 0.90, and 1.30 M (1.4, 2.8, 3.7, 5.1, and 7.2 wt %) NaCl concentrations, and our earlierdata for pH 4.5 at the same concentrations. We use these solute chemical potential derivatives to compute theprotein cation charge approximately, and to construct a function approximating the derivative of the lysozymechloride chemical potential with respect to NaCl concentration, which we integrate over a range of NaClconcentrations. This provides the change of the lysozyme chloride chemical potential with NaCl concentrationwell into the supersaturated region, and hence provides the driving force for nucleation and crystal growth oflysozyme chloride as a function of the extent of supersaturation. We also compute the diffusion Onsagercoefficients (Lij)0 for each composition at pH 4.5 and 6.0. Binary diffusion coefficients of aqueous lysozymechloride at 0.89 mM (12.7 mg/mL) for pH values from 4.0 to 6.0, and at pH 6.0 for concentrations from 0.25to 1.95 mM (3.6−27.9 mg/mL) are also reported.
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