Protein-Protein Interactions, Clustering, and Rheology for Bovine IgG up to High Concentrations Characterized by...

A systematic understanding of intermolecular interactions is necessary for designing concentrated monoclonal and polyclonal antibody solutions with reduced viscosity and enhanced stability. Here, we determine the effects of pH and cosolute on the strength and geometry of short-range anisotropic protein-protein attractions for a polyclonal bovine IgG by comparing intensities [I(q)] obtained from small-angle X-ray scattering to those computed in molecular dynamics simulations with 12-bead models. As our model embodies key features of the protein shape, it can describe the experimental I(q) for solutions of 10-200 mg/mL protein with only a small (<1 k_BT) variation in the model's well depth. At high concentration, small changes in the interaction potential produce large increases in clustering given the close interprotein spacing. Reducing the pH below the pI or adding NaCl weakens short-range anisotropic attractions but not enough to remove large reversible oligomers that raise viscosity. In contrast, for arginine added at pH 5.5, a uniform attraction model is sufficient to describe the I(q) that plateaus at low q. With primarily monomers and dimers, the viscosity is reduced relative to the other systems that have larger clusters as described with a model that includes the cluster size distribution.