Protein Adsorption to Charged Gold Nanospheres as a Function of Protein Deformability

The corona that forms as protein adsorbs to gold nanospheres (AuNSs) is directly influenced by the surface chemistry of the AuNS. Tools to predict adsorption outcomes are needed for intelligent design of nanomaterials for biological applications. We hypothesized that the denaturation behavior of a protein might be a useful predictor of adsorption behavior to AuNSs, and used this idea to study protein adsorption to anionic citrate-capped AuNSs and to cationic poly(allylamine hydrochloride) (PAH) wrapped AuNSs. Three proteins (alpha-amylase (A-Amy), beta-lactoglobulin (BLG), and bovine serum albumin (BSA)), representing three different classes of acid denaturation behavior, were selected with BLG being the least deformable and BSA being the most deformable. Protein adsorption to AuNSs was monitored via UV-vis spectrophotometry and dynamic light scattering. Changes to the protein structure upon AuNS interaction were monitored via circular dichroism spectroscopy. Binding constants were determined using the Langmuir adsorption isotherm, resulting in BSA > BLG A-Amy affinities for citrate-capped gold nanospheres. PAH-coated AuNSs displayed little affinity for these proteins at similar concentrations as citrate-coated AuNSs and became agglomerated at high protein concentrations. The enzymatic activity of A-Amy/citrate AuNS conjugates was measured via colorimetric assay, and found to be 11% of free A-Amy, suggesting that binding restricts access to the active site. Across both citrate AuNSs and PAH AuNSs, the changes in secondary structure were greatest for BSA > A-Amy > BLG, which does follow the trends predicted by acid denaturation characteristics.