THERMODYNAMICS OF PROTEIN ADSORPTION - THEORY WITH SPECIAL REFERENCE TO THE ADSORPTION OF HUMAN-PLASMA ALBUMIN AND BOVINE PANCREAS RIBONUCLEASE AT POLYSTYRENE SURFACES

A thermodynamic analysis is given to unravel the various subprocesses that contribute to protein adsorption, taking the adsorption of human plasma albumin (HPA) and bovine pancreas ribonuclease (RNase) on negatively charged polystyrene latices as the examples. A simple three-layer model of the adsorbed protein is put forward. Given this model, there are only two main adjustable parameters, viz. the standard enthalpy of transfer of ions from bulk to adsorbate and the relative dielectric permittivity of the Stern layer of the adsorbent before adsorption. The following five subprocesses are considered: Uptake or exclusion of protons, uptake or exclusion of other ions, overlap of electrical fields, van der Waals attraction, including dehydration of the surface, and structural alterations of the protein. The last mentioned process is the most elusive one. Its enthalpy was estimated by subracting the sum of the enthalpy changes due to the first four subprocesses from the total, measured enthalpy of adsorption. A similar procedure was adopted for the Gibbs energy. The main conclusions for the two systems investigated are: (1) The adsorption process is entropically driven; (2) the gain in entropy stems from dehydration of the absorbent surface, from structural rearrangements inside the protein molecule including changes in the state of hydration and from field overlap; (3) the enthalpy of reconformation is endothermic; (4) the uptake of ions is an important concomitant feature. Arguments are put forward to show that the analysis has a more general applicability. © 1979.