Precision Measurements of Binary and Multicomponent Diffusion Coefficients in Protein Solutions Relevant to Crystal Growth:  Lysozyme Chloride in Water and Aqueous NaCl at pH 4.5 and 25 °C⊥

Accurate models of protein diffusion are important in a no. of applications, including liq.-liq. phase sepn. and growth of protein crystals for X-ray diffraction studies. In concd. multicomponent protein systems, significant deviations from pseudobinary behavior can be expected. Rayleigh interferometry is used to measure the four elements (Dij)v of the ternary diffusion coeff. matrix for the extensively investigated protein, hen egg-white lysozyme (component 1) in aq. NaCl (component 2) at pH 4.5 and 25 °C. These are the first multicomponent diffusion coeffs. measured for any protein system at concns. high enough to be relevant to modeling and prediction of crystal growth or other phase transitions, and the first for a system involving lysozyme at any concn. The four ternary diffusion coeffs. for the system lysozyme chloride/NaCl/water are reported for lysozyme chloride at 0.60 mM (8.6 mg/mL) and NaCl at concns. of 0.25, 0.50, 0.65, 0.90, and 1.30 M (1.4, 2.8, 3.7, 5.1, and 7.2 wt %), with the latter two compns. being supersatd. One cross-term, (D21)v, is 80-259 times larger than the main term (D11)v and 7-18 times larger than (D22)v. Std. interferometric diagnostic tests indicate that aggregation is unimportant in our expts. We also present binary diffusion coeffs. Dv for lysozyme chloride/water at concns. from 0.43 to 3.08 mM (6.2-44.1 mg/mL), at the same pH and temp. The precision of the results is about 0.1% for the binary diffusion coeffs. and diagonal ternary diffusion coeffs., and about 1-2% for the cross-terms. For the ternary systems investigated, we show that a single pseudobinary diffusion coeff. does not accurately describe diffusive transport, and predictions by simple models such as the Nernst-Hartley equations are inaccurate at the higher concns. considered here. Finally, dynamic light-scattering diffusion coeffs., differing from both our interferometrically measured (Dij)v and a theor. prediction of light-scattering diffusion coeffs. in multicomponent systems, are reported for the same solns. used for the ternary expts. at 1.30 M