Effective Interactions and Colloidal Stability of Bovine γ-Globulin in Solution.

Interactions and phase behavior of γ-globulins are of fundamental interest in biophysical and pharmaceutical research, as these are among the most abundant proteins in blood plasma. In this work, we report the characterization of the oligomeric state of bovine γ-globulin, the effective protein-protein interactions, and the colloidal stability in aqueous solution as a function of protein concentration and ionic strength. Classical biochemical techniques, such as size exclusion chromatography (SEC) and gel electrophoresis, together with small-angle X-ray and neutron scattering (SAXS/SANS), were employed for this study. The results show that bovine γ-globulin solutions are dominated by monomer and idiotype anti-idiotype dimer. Despite the flexibility and highly nonspherical shape of the protein, a simple model with a disk-type form factor and a structure factor of a square-well potential provide a satisfying description of the scattering data. The overall interactions are attractive and the strength decreases with increasing protein concentration, or adding buffer or salts. For higher protein volume fraction (>7%), the model would imply a strong particle-particle correlation which does not appear in the experimental data. This mismatch is most likely due to the smearing effect of the conformation change of proteins in solution. The stability of γ-globulin solutions is highly sensitive to protein concentration, ionic strength, and the type of added salts, such as NaCl, Na2SO4, and NaSCN. For solutions below 50 mg/mL and at low ionic strengths (<0.1 M), protein aggregation is most likely due to subpopulations of IgG molecules with attractive patches of complementary surface charge. This effect is reduced for higher protein concentration due to self-buffering effects. For high ionic strength (>1 M), typical salting-in (with NaSCN) and salting-out effects (with NaCl and Na2SO4) are observed. Results are further discussed in comparison with current studies in the literature on monoclonal antibodies.