Application of the Steric Mass Action formalism for modeling under high loading conditions: Part 1. Investigation of the influence of pH on the steric shielding factor.

In ion exchange chromatography, the Steric Mass Action (SMA) formalism is frequently used to simulate sorption processes at low and high column load conditions. To apply the SMA model for describing protein elution over wide ranges of pH, it is necessary to use pH-dependent model parameters. In the past, some publications have already described the pH-dependence of the characteristic protein charge and the equilibrium constant, while the influence of pH on the steric shielding factor has been mostly neglected. In this work, the pH-dependences of all relevant model parameters, including the shielding factor, were investigated, described, and implemented into the SMA model. Therefore, the elution behavior of a bispecific monoclonal antibody on the strong cation exchange resin POROS™ XS was modeled over broad ranges of pH, salt concentrations, and protein concentrations. Linear gradient elution experiments were performed to generate an extensive data set by using increasing column loadings from 0.5 up to 75.0 mg/mL. By using an inverse peak fitting method, shielding factors were estimated at various pH values ranging from 4.5 to 8.9. The results showed that an increasing buffer pH resulted in strongly increasing shielding factors. A semi-empirical correlation describing the shielding factor as a function of pH was established and implemented into the SMA formalism. This approach led to precise prediction of protein elution behavior using a single-component simulation. This was demonstrated by accurate simulation of linear salt, pH and dual gradient elution experiments conducted under high loading conditions.