Protein adsorption on hematite (α-Fe 2O 3) surfaces

Abstract

Adsorption isotherms for human plasma albumin (HPA) and bovine pancreas ribonuclease (RNase) on hematite surfaces have been determined at different ionic strength and pH. Calorimetric data point to different modes of adsorption of the HPA molecule at and away from isoelectric conditions, respectively. Under most conditions the adsorption enthalpy is positive, even if the HPA molecule and the hematite surface are oppositely charged, so that the adsorption must be driven by an increase of entropy. At high surface coverage θ lateral interactions between the adsorbed HPA molecules are shown to play an important role in the adsorption mechanism, the enthalpy of those interactions becoming more endothermic with increasing distance from the isoelectric point of the protein. For HPA, additional information is obtained from infrared spectroscopy and electrophoresis, both as a function of pH and θ. At the isoelectric point of the protein the extent of reconformation of the HPA molecule is a minimum; this extent increases with increasing distance from the i.e.p. The variation of reconformation with θ is most pronounced at low θ. Adsorption is accompanied by the uptake of low-molecular-weight electrolyte. The general conclusion is that the adsorption of RNase is to a large extent determined by coulombic interactions, whereas with HPA other factors are dominant. For the latter protein, the electrical contributions to the interaction with the substrate can to a large extent be unraveled.