Understanding adsorption behavior of α-chymotrypsin onto cation exchanger using all-atom molecular dynamics...

The interest for a better understanding of ion-exchange mechanisms at the atomic level has strongly increased over the past decades. Indeed, molecular-level information about physico-chemical mechanisms could help optimizing chromatographic processes for protein purification, which are sub-optimized due to the lack of predictive models. A promising approach is based on the use of Molecular Dynamics (MD) simulations to study local phenomena inside adsorbents which can then be challenged against experimental results. In this work, macroscopic experimental data, consisting in the ion-exchange uptake of α-chymotrypsin onto SP Sepharose FF, have been compared to the adsorption behavior predicted by MD simulations. The chromatographic surface, represented as a uniform distribution of ligands with a counterion layer, in the presence of the protein was modeled using all-atom representation. The SMA formalism was used to describe single adsorption isotherms and to relate macroscopic observations with molecular simulations. Two SMA parameters based on physical principles, the characteristic charge n and the steric factor σ, have been estimated by both experiments and MD simulations. At pH 5 and NaCl concentration of 100 mM, our study shows a fairly good agreement between both results, especially for the characteristic charge. It is shown that the steric factor calculation is strongly dependent on the ligand density on the adsorbent surface, whose value must be carefully determined in order to obtain reliable predictions. In addition, four binding patches were identified as being involved in the adsorption, which have been confirmed through binding free energy calculations.