Specific Ion–Protein Interactions Dictate Solubility Behavior of a Monoclonal Antibody at Low Salt Concentrations

The perturbation of salt ions on the solubility of a monoclonal antibody was systematically studied at various pHs in Na(2)SO(4), NaNO(3), NaCl, NaF, MgSO(4), Mg(NO(3))(2) and MgCl(2) solutions below 350 mM. At pH 7.1, close to the pI, all of the salts increased the solubility of the antibody, following the order of SO(4)(2-) > NO(3)(-) > Cl(-) > F(-) for anions and Mg(2+) > Na(+) for cations. At pH 5.3 where the antibody had a net positive charge, the anions initially followed the order of SO(4)(2-) > NO(3)(-) > Cl(-) > F(-) for effectiveness in reducing the solubility and then switched to increasing the solubility retaining the same order. Furthermore, the antibody was more soluble in the Mg(2+) salt solutions than in the corresponding Na(+) salt solutions with the same anion. At pH 9.0 where the antibody had a net negative charge, an initial decrease in the protein solubility was observed in the solutions of the Mg(2+) salts and NaF, but not in the rest of the Na(+) salt solutions. Then, the solubility of the antibody was increased by the anions in the order of SO(4)(2-) > NO(3)(-) > Cl(-) > F(-). The above complex behavior is explained based on the ability of both cation and anion from a salt to modulate protein-protein interactions through their specific binding to the protein surface.