Adsorption of serum albumins at the air/water interface

The adsorption of bovine serum albumin and human serum albumin (HSA) at the air/water interface has been studied by neutron specular reflection. All the neutron measurements were performed in null reflecting water (D2O:H2O similar or equal to 1:11), and the specular reflectivity at this water contrast is entirely from the adsorbed protein layer at the air/water interface. Accurate measurement of surface excesses and layer thicknesses has allowed us to infer the possible structural conformation of the two protein molecules on the surface of water. The effect of bulk protein concentration on the adsorbed amount and the total thickness of the adsorbed layer was examined at pH 5, close to the isoelectric point of 4.8 for the two albumins. The surface excess (Gamma) of both proteins increased sharply over the concentration range of 5 x 10(-4) to 5 x 10(-2) g dm(-3), beyond which Gamma tended to the respective saturation limits. The saturation value of surface excess of BSA at 1 g dm(-3) was found to be 2.8 +/- 0.3 mg m(-2) as compared with 2.1 +/- 0.3 mg m(-2) for HSA. The thicknesses of the protein layers at concentrations below 0.1 g dm(-3) were found to be close to the short axial length of 40 Angstrom of the globular protein solution structure, suggesting that albumin molecules adsorb with their long axes parallel to the surface of water. The effect of solution pH on the adsorption of the two proteins was examined over the pH range between 3 and 7 and at the two fixed protein concentrations of 5 x 10(-3) and 1 g dm(-3). At the low protein concentration the thicknesses of the protein layers were found to be constant at 30 +/- 3 Angstrom and the corresponding surface excesses to be peaked at pH 5. At the high protein concentration the reduction in surface excess at the pH values away from the IP was more substantiated for both proteins, as evident from the variation of both surface excess and layer thickness with solution pH.