The Role of Electrostatics in Protein–Protein Interactions of a Monoclonal Antibody

Understanding how protein–protein interactions depend on the choice of buffer, salt, ionic strength, and pH is needed to have better control over protein solution behavior. Here, we have characterized the pH and ionic strength dependence of protein–protein interactions in terms of an interaction parameter <i>k</i>_D obtained from dynamic light scattering and the osmotic second virial coefficient <i>B</i>₂₂ measured by static light scattering. A simplified protein–protein interaction model based on a Baxter adhesive potential and an electric double layer force is used to separate out the contributions of longer-ranged electrostatic interactions from short-ranged attractive forces. The ionic strength dependence of protein–protein interactions for solutions at pH 6.5 and below can be accurately captured using a Deryaguin–Landau–Verwey–Overbeek (DLVO) potential to describe the double layer forces. In solutions at pH 9, attractive electrostatics occur over the ionic strength range of 5–275 mM. At intermediate pH values (7.25 to 8.5), there is a crossover effect characterized by a nonmonotonic ionic strength dependence of protein–protein interactions, which can be rationalized by the competing effects of long-ranged repulsive double layer forces at low ionic strength and a shorter ranged electrostatic attraction, which dominates above a critical ionic strength. The change of interactions from repulsive to attractive indicates a concomitant change in the angular dependence of protein–protein interaction from isotropic to anisotropic. In the second part of the paper, we show how the Baxter adhesive potential can be used to predict values of <i>k</i>_D from fitting to <i>B</i>₂₂ measurements, thus providing a molecular basis for the linear correlation between the two protein–protein interaction parameters