Adsorption of Proteins onto Ion-Exchange Chromatographic Media: A Molecular Dynamics Study

Molecular dynamics (MD) simulations with coarse-grained models are used to investigate the adsorption process of proteins onto an ion-exchange chromatographic medium. The adsorption of human serum albumin (HSA) and bovine hemoglobin (bHb) on the anion exchanger Q Sepharose FF is studied. These two proteins have similar molecule sizes with different isoelectric points. To obtain a reliable set of data, simulations with different initial conditions are carried out for each protein. Convincingly, the results of the MD simulations are qualitatively consistent with those of previous experiments. HSA reaches a stable adsorption in all simulations, while the binding sites can differ. In contrast, bHb reaches a stable adsorption only in one simulation. Due to the stronger protein-ligand interaction of HSA, it adsorbs faster, stronger, and with more binding sites onto Q Sepharose FF than bHb. The spatial movements during the adsorption process and the adsorbed states are investigated in detail. Moreover, we studied the underlying physical relevance of two parameters (characteristic charge and steric factor) of the steric mass action (SMA) model for HSA and for bHb with MD simulations and compared them with the results of experiments. The results reveal that MD simulations are useful to interpret the physical consistence of the SMA parameters.