MODELING OF PROTEIN ADSORPTION ON POLYMER SURFACES - COMPUTATION OF ADSORPTION POTENTIAL

This work is based on a model developed to understand and predict the retention behavior of proteins in ion-exchange chromatography. The adsorption of two types of proteins is studied: a calcium-binding acidic protein, the alpha-lactalbumin (ALC) and a basic protein, the hen egg white lysozyme (HEWL). Their model structures are taken from the protein data bank, and the poly(vinylimidazole) polymer structure is built and used as the model stationary phase. To calculate the interactions between the protein and the polymeric stationary phase, the model explicitly considers all of the atoms. A set of parameters obtained from the AMBER force field is assigned to each of them, and the point charges on the polymer atoms are calculated on a semiempirical basis. The solvent effect is taken into account by introducing in the calculations a Linear dependence of the dielectric permittivity on the separation distance. Protein/polymer interaction energies are evaluated for 2592 configurations scanning the whole space of possible orientations from the bulk down to the contact with the adsorbent. We are able to reproduce the difference in retention between calcium-loaded and -depleted ALC; HEWL is repelled from the surface in agreement with its elution as a nonretained compound. Preferred orientations are found for each of the model proteins that allow the determination of the residues involved in the interaction with the anion-exchanger support. Moreover, we calculate an average effective net charge of the proteins.