Salting Up and Salting Down of Bovine Serum Albumin Layers at the Air-Water Interface
Guangcui Yuan, Paul A. Kienzle, Sushil K. Satija
The surface adsorption of BSA in pure water and salted aqueous solutions was studied by neutron reflection. With contract match technique, the surface excess in null reflect water as a function of protein concentration was revealed. It is found that, in a concentration range of 0.2 ppm to 1000 ppm, without salts, the surface excess shows a profound peak at around 20 ppm; with salts, the surface excess increases steadily with the protein concentration. When the surface excess at a specific protein concentration is viewed, the introduce of sodium chloride causes either a salting-down effect --- surface adsorption decline or a salting-up effect --- surface adsorption enhance, depending on the protein concentration. Salting-up effect is observed at the low ( 1 ppm) and high ( 1000 ppm) concentration ends , and salting-down effect dominates the intermediate concentration range. The salting-up or salting-down behaviors is found to be directly related to the solution pH, which is not a constant but varies with protein - protein interaction (here, bulk protein concentration).When the solution pH is shifted toward to the isoelectric point by adding salts, surface adsorption enhances; When the solution pH is shifted away from the isoelectric point by adding salts, surface adsorption declines.
Bovine Serum Albumin, surface adsorption, neutron reflection, salt effects