(8f) The Study of Bovine Serum Albumin Deformation and Adhesion to Self-Assembled Monolayers Using Atomic Force Microscopy

The interactions between proteins and various surfaces created using self-assembled monolayers (SAMs) were studied in order to assess factors affecting biocompatibility. Surfaces of interest include gold surfaces coated with SAMs containing a methyl end group (hexadecane thiol), carboxylic end group (mercaptohexadecanoic acid), as well as a bovine serum albumin (BSA) functionalized surface. Cantilever tips functionalized with BSA were brought into contact with these surfaces using atomic force microscopy (AFM) to resolve the interaction forces for the BSA/SAM pairs. Hamaker constants were determined for each system by modeling the approaching curves in 1M NaCl solution, where affects of electrostatic forces were negligible. The greatest adhesion force and Hamaker constant were found for the BSA interacting with the hydrophobic HDT surface. The adhesion force for the the BSA/MHA system and BSA/BSA were similar in magnitude and much less than that for the BSA/HDT system. The Hamaker constants as well as surface roughness obtain using the AFM were then used to model the adhesion force, believed to be largely due to van der Waals forces. It was found, however, that the modeled adhesion forces were smaller than those experimentally observed. The discrepancy between observed and predicted values of adhesion force was resolved by taking into account deformation of the BSA during application of a load as the tip contacted the substrate. Simulated BSA compression of a few nanometers resulted in the matching of the modeled and the experimental adhesion data. This compression allows for the area of BSA in atomic contact with the substrate to change, generating better alignment of the tip/substrate in the contact adhesion calculations. This model, which considers actual system geometry and roughness, is able to show that accounting for deformation in modeling surface interaction forces provides added accuracy in the prediction of protein interactions with surfaces, which will allow for improved design of biocompatible materials.