Acidic pH impact on BSA domains I & II adsorption to a silica dioxide surface

Bovine Serum Albumin (BSA) is a protein used as a passive blocker, adsorbing to various surfaces to reduce nonspecific interactions. BSA contains three structurally homologous domains but comprised of a different number of basic/acidic amino acids, causing each domain to respond to pH changes differently. At neutral pH, domain I contains many negatively charged residues producing a total net charge of −17. At pH 5.2, these residues become protonated, shifting the net charge to +1. The change in pH alters the charge of BSA and therefore may affect protein-surface interactions. Silica is a common surface used in bead-based immunoassays. Experimental observations suggest that adsorption occurs when both BSA and silica are negatively charged but the mechanism is not fully understood. By using theoretical techniques like molecular dynamics (MD) simulations we can analyze this protein-surface interaction and begin to understand it on a molecular level. In this study, we investigated how the change in pH can impact the adsorption of BSA to silica using MD simulations. BSA binding to silica at pH 5.2 and pH 7.0 were compared by performing simulations using NAMD. We observed that at pH 5.2, domains I and II orient closer to the surface to create many intermolecular interactions, while at pH 7.0 only domain II interacted with the silica surface. This interaction could be due to the change in surface charge of domain I at pH 7.0 and pH 5.2. At pH 7.0 domain I is highly negatively charged impacting the electrostatic interactions that are observed at pH 5.2, where domain I is less negatively charged. With MD simulations, we found pH impacts protein-surface interaction and can affect the mechanism of protein adsorption.