Interactions between Amino Acids and Zirconia Modified with Ethylenediaminetetra(methylenephosphonic acid): Mechanistic Insights into the Selective Binding of Antibodies.

Zirconia modified with ethylenediaminetetra(methylenephosphonic acid) (EDTMP) has an affinity for antibodies, including immunoglobulin G (IgG) and immunoglobulin M (IgM). However, little is known about the mechanism underlying antibody selectivity. In this study, we examined the interactions of EDTMP-modified zirconia with proteinogenic amino acids using chromatographic and batch methods to gain mechanistic insights into antibody selectivity at the amino acid level. We demonstrated that EDTMP-modified zirconia has an affinity for amino acids with a positively charged side chain, especially lysine. Similar trends were observed for oligopeptides. This affinity was reduced by the addition of sodium phosphate or sodium polyphosphates. Thus, the antibody selectivity of EDTMP-modified zirconia is primarily ascribable to electrostatic attractions between the EDTMP moieties of the zirconia surfaces and the constant region of antibodies that are rich in lysine residues. Consistent with this, the human IgG antibody has a higher adsorption ability onto EDTMP-modified zirconia than the rabbit IgG antibody, which has fewer lysine residues in the constant region. These findings are useful not only for improving antibody purification but also for developing new applications, including purification of proteins tagged with positively charged amino acid residues.