Molecular Theory of Protein Sorption on Weak Polyelectrolyte Gel-Modified Charged Surface

Abstract

We developed a detailed molecular theory to determine protein adsorption on a charged surface as a function of the pH, salt concentration, protein concentration and surface charge density, as well as the modification of the surface by the attachment of a weak polyelectrolyte gel film. The approach includes specific molecular details of the proteins, their translational and rotational degrees of freedom as well as the acid-base equilibrium of the titratable amino acids. We also include the conformational degrees of freedom and acid-base equilibrium of the polymeric gel, the solution entropic contributions, and electrostatic and excluded-volume interactions. The experimental titration curve of lysozyme and the isoelectric point (IP) in solution are reproduced successfully. In low salt conditions, lysozyme adsorption profiles as a function of pH on a non-modified surface are broad and have a maximum value at a pH lower than the isoelectric point of the protein. When the salt concentration is increased, maximum adsorption occurs at evenlower pH values. The level of protein adsorption on a gel-modified surface is two orders of magnitude greater than on a non-modified surface, and has a narrower maximum close to the IP at low salt concentration. The molecular theory may explain the excellent properties of such modified surfaces for chromatographic separation of proteins as well as help to design new chromatographic systems. Moreover, the theory provides with a fundamental understanding of the competition between the different contribution that determine the adsorption behavior.