ELECTROSTATIC CONTRIBUTION TO THE ENERGY AND ENTROPY OF PROTEIN ADSORPTION

Protein adsorption is complicated by the many contributions to the ove rall thermodynamics, and as a result there is a need for mechanistic m odels in interpreting experimental data. We focus on the electrostatic contribution to the adsorption thermodynamics and, specifically, on c alculating the relative contributions of enthalpic and entropic drivin g forces to the electrostatic component of the free energy of adsorpti on. The model used is a colloidal one in which the protein, modeled as a sphere carrying a point charge at its center, interacts with a plan ar adsorbent surface of opposite charge as well as with neighboring ad sorbent molecules. It is found that the entropic contribution dominate s the protein-surface interaction, which is generally but not uniforml y attractive; this contribution reflects the liberation of counterions from the double layers adjacent to the protein and adsorbent surfaces , Protein-protein interactions are found to be generally repulsive but weak, with energetic and entropic contributions similar in magnitude. The dominance of entropic effects is consistent with several reported calorimetric measurements, but additional effects, such as the releas e of water molecules from the interacting surfaces, confound attempts to compare theory and experiment directly. (C) 1998 Academic Press.