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

Isothermal ternary diffusion coefficients for the system lysozyme chloride (1) + NH4Cl (2) + H2O at 25 degreesC and pH 4.5 have been measured interfer ometrically for five mean NH4Cl concentrations, C-2 = 0.25, 0.5, 0.9, 1.2 a nd 1.5 M, with C-1 = 0.6 mM. The main-term diffusion coefficient (D-11)(v) varies slowly with C-2. The main-term (D-22)(v) increases with increasing N H4Cl concentration, as does the binary D-v in aqueous NH4Cl, but the (D-22) (v) values are lower in the ternary system. The cross-term (D21)v, which re lates the coupled flow of NH4Cl to the protein concentration gradient, incr eases sharply with increasing salt concentration, and is 19 times larger th an (D-22)(v) at the highest concentration. The values of (D-12)(v) are smal ler than the corresponding values previously obtained for the lysozyme chlo ride + NaCl + H2O system over the whole range of salt concentration studied . Using equations based on the Onsager Reciprocal Relations, we have calcul ated the derivative of the chemical potential of lysozyme chloride with res pect to the NH4Cl concentration, and have estimated the protein cation char ge. Integration with respect to the NH4Cl concentration gives the dependenc e of the chemical potential of lysozyme chloride on NH4Cl concentration, pr oviding information about the driving force for nucleation and crystal grow th of lysozyme chloride. (C) 2001 Elsevier Science B.V. All rights reserved .