Electrostatic and van der Waals Contributions to Protein Adsorption: Computation of Equilibrium Constants

Although protein adsorption has been much described and exploited, little effort has been directed toward the development of models for its a priori prediction. Here we describe a methodology that allows for the computation of protein-surface equilibrium constants K_eq (i.e., equilibria at low surface coverages) based on protein molecular structure and surface properties. The crux of the model is the computation of the electrostatic and van der Waals energies of interaction between a colloidal protein molecule and a planar, charged surface at a fixed distance from and orientation with respect to it. Results for the protein lysozyme are presented; however, these calculations are computationally very time-intensive. Consequently, we utilize a simplified description of the protein as a low dielectric sphere with its net charge placed at the center. It is used in particular to compute the relationship, relevant to ion-exchange chromatography, between ionic strength and K_eq, due to the strong effect which salt exerts on electrostatic interactions. The physical properties that affect the value of the equilibrium constant are protein and surface net charges, Hamaker constant, and protein size; the first two influence electrostatic interactions, the third characterizes dispersion forces, and the last affects both types of interactions. In addition to allowing a priori prediction of adsorption equilibria, the construct presented in this paper can allow for improved understanding and interpretation of electrostatic and dispersive mechanisms for protein adsorption.