The perturbation of salt ions on the solubility of a monoclonal antibody was systematically studied at various pHs in Na
2SO
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 p
I, all of the salts increased the solubility of the antibody, following the order of SO
42鈥?/sup> > NO3鈥?/sup> > Cl鈥?/sup> > F鈥?/sup> for anions and Mg2+ > Na+ for cations. At pH 5.3 where the antibody had a net positive charge, the anions initially followed the order of SO42鈥?/sup> > NO3鈥?/sup> > Cl鈥?/sup> > F鈥?/sup> 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 Mg2+ 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 Mg2+ 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 SO42鈥?/sup> > NO3鈥?/sup> > Cl鈥?/sup> > F鈥?/sup>. The above complex behavior is explained based on the ability of both cation and anion from a salt to modulate protein鈥損rotein interactions through their specific binding to the protein surface.
Keywords:
antibody solubility; Hofmeister salts; ion鈭抪rotein interactions; co-ions and counterions