Cation exchange equilibria of amino acids

Abstract

Equilibrium ion exchange behavior of typical neutral, acidic, and basic amino acids has been examined. In a pH range from 2 to 7, neutral and acidic amino acids undergo monovalent cation exchange. The amino acid in the form of α-NH 3 + / α-COOH has a high cation exchange affinity, whereas the α-NH 3 + / α-COO − form has a negligible affinity. At a pH near the pK a of α-COOH, both forms coexist; the average affinity changes significantly as pH approaches the pK a of α-COOH. Basic amino acids have two potential cation exchange sites, α-NH 3 + and R(side chain)-NH 3 +. The α-NH 3 + / α-COOH form binds at both sites, whreas the α-NH 3 + / α-COO − form binds only at the R-NH 3 +. As a result, the average valence of a basic amino acid can be a fractional number in the pH range from 2 to 7. As pH increases, the average valence decreases from 2 at low pH (< pK a α) to unity at high pH (> pK a α ). At a given pH, the average valence and average separation factor of a basic amino acid against Na + can change with amino acid concentration, Na + concentration, total concentration, and exchanger capacity. A theory which accounts for the heterogeneous charge structures and multiple equilibria has been developed to explain the fundamental ion exchange behavior. The theory correctly predicts how pH, salt concentration, and composition affect the average valence and the average separation factor. The results on basic amino acids can explain why peptides and proteins exhibit fractional valences and their average affinities change with pH, salt concentration, and solution composition. Our results also indicate that an opposite charge within 5 Å from an ion exchange site of a biochemical can prevent its electrostatic interaction with an ion exchanger.