Precision measurement of ternary diffusion coefficients and implications for protein crystal growth: Lysozyme chloride in aqueous ammonium chloride at 25°C

Isothermal ternary diffusion coefficients for the system lysozyme chloride (1)+NH₄Cl (2)+H₂O at 25°C and pH 4.5 have been measured interferometrically for five mean NH₄Cl concentrations, C₂ = 0.25, 0.5, 0.9, 1.2 and 1.5M, with C₁ = 0.6mM. The main-term diffusion coefficient (D₁₁)_v varies slowly with C₂. The main-term (D₂₂)_v increases with increasing NH₄Cl concentration, as does the binary D_v in aqueous NH₄Cl, but the (D₂₂)_v values are lower in the ternary system. The cross-term (D₂₁)_v, which relates the coupled flow of NH₄Cl to the protein concentration gradient, increases sharply with increasing salt concentration, and is 19 times larger than (D₂₂)_v at the highest concentration. The values of (D₁₂)_v are smaller than the corresponding values previously obtained f or the lysozyme chloride+NaCl+H₂O system over the whole range of salt concentration studied. Using equations based on the Onsager Reciprocal Relations, we have calculated the derivative of the chemical potential of lysozyme chloride with respect to the NH₄Cl concentration, and have estimated the protein cation charge. Integration with respect to the NH₄Cl concentration gives the dependence of the chemical potential of lysozyme chloride on NH₄Cl concentration, providing information about the driving force for nucleation and crystal growth of lysozyme chloride.