Translational Diffusion Coefficients of Bovine Serum Albumin in Aqueous Solution at High Ionic Strength

We report static and dynamic light scattering measurements on bovine serum albumin (BSA) solutions at high ionic strength (I) where potential and hydrodynamic interactions between BSA molecules are of comparable strengths. Measurements of the concentration dependence of the osmotic compressibility, (dpi/dc), and the translational diffusion coefficient, D(m), are presented for several solvent systems: (a) at the isoelectric pH = 4.7 and I = 0.1, where long-range electrostatic repulsions are absent; (b) at pH = 7.4 and I = 0.15, 1.5, and 3.3, where a well-screened electrostatic repulsion is present. The results are compared with theoretical predictions which involve a microscopic hard-sphere treatment of the potential and hydrodynamic interactions. At pH = 7.4 and I = 1.5, our experimental results for dpi/dc are in good agreement with the hard-sphere prediction, and our values for D(m) are, likewise, consistent with a hard-sphere hydrodynamic analysis in which contributions from the divergence terms in the velocity field are neglected. At the isoelectric pH, similar agreement with theory is obtained, provided the contribution of an attractive potential is included; at pH 7.4 and I = 0.15, the contribution from a long-range repulsion must be included; at pH 7.4 and I = 3.3, onset of protein aggregation is observed. Copyright 1999 Academic Press.