Evaluation of the Langmuir Formalism for Modeling the Adsorption Isotherms of Proteins and Polyelectrolytes in Simulations of Ion Exchange Chromatography

Abstract

AbstractThe adsorption behavior of two cationic proteins (cytochrome c and α‐chymotrypsinogen) and one synthetic polycation (poly(diallylmethylammonium chloride), polyDADMAC) was investigated on a porous particle based and a monolithic cation exchanger column. All isotherms could be fitted to the Langmuir equation. Subsequently, the derived Langmuir equation parameters were combined with two kinetic models, the transport dispersive (TD) and the lumped pore diffusion (POR) models, to simulate breakthrough curves of the substances on the ion exchanger columns. In the case of the proteins, simulations based on the experimentally derived Langmuir parameters showed poor correlation with the experimental breakthrough curves. When the Langmuir parameters were instead fitted in the simulations, adequate agreement could be achieved. In the case of the TD model, the overall mass transfer parameter had to also be fitted as a function of the protein concentration to optimize the fit. In the case of the POR model, one set of transport parameters could be used to simulate the breakthrough curves independent of the mobile phase composition (buffer concentration 75 or 120 mM, NaCl concentration 0–5 %) or the protein concentration (0.001–0.3 mg/mL). As in the TD model, it was not possible to use the experimentally derived Langmuir parameters to simulate the breakthrough curves with the POR model. Instead, the Langmuir parameters had to be fitted for each individual case in order to achieve agreement. Neither the TD nor the POR model could satisfactorily simulate the behavior of the synthetic polyelectrolyte. The Langmuir equation can thus be used to provide standard transport models with an adsorption formalism to simulate breakthrough curves in ion exchange chromatography. However, the existence of a close correlation of the actually observed physico‐chemical adsorption behavior of the involved proteins could not be supported.