Gibbs free energy of adsorption for biomolecules in ion-exchange systems

The Gibbs free energy of adsorption (delta G0ads) was estimated for several amino acids, peptides and proteins in a cation-exchange system. Using the steric mass action formalism that describes biomolecular adsorption in ion-exchange systems, the delta G0ads was chromatographically determined under infinitely dilute conditions. The delta G0ads measured for seven globular proteins ranged from -5.7 to -13.9 kcal/mol. The average bond energy (defined as delta G0ads divided by the number of bonds formed between the protein and the surface) for these proteins varied from -1.1 to -1.7 kcal/mol. These bond energies were found to be comparable to the bond energies for lysine and arginine (-1.1 and -1.5 kcal/mol, respectively), the amino acids which primarily contribute to the cation-exchange of proteins. In contrast, an elevated average bond energy of -2.6 kcal/mol was observed for two peptides and protamine (a polypeptide) suggesting that synergistic binding may play a role for unstructured macromolecules, but not for globular proteins.