Effect of gold nanoparticle morphology on adsorbed protein structure and function

Many biomedical applications of gold nanoparticles (NPs) rely on proteins that are covalently attached or adsorbed on the NP surface. The biological functionality of the protein-NP conjugate depends on the protein's ability to interact with target molecules, which is affected by NP characteristics such as size, curvature, aspect ratio, morphology, crystal structure, and surface chemistry. In the present study, the effect of gold nanoparticle morphology on the structure and function of adsorbed enzymes, lysozyme (Lyz) and α-chymotrypsin (ChT), has been investigated. Gold nanospheres (AuNS) were synthesized with diameters 10.6 ± 1 nm, and gold nanorods (AuNR) were synthesized with dimensions of (10.3 ± 2) × (36.4 ± 9) nm. Under saturating conditions, proteins adsorb with a higher surface density on AuNR when compared to AuNS. In the case of Lyz, adsorption on AuNS and AuNR resulted in a 10% and 15% loss of secondary structure, respectively, leading to conjugate aggregation and greatly reduced enzymatic activity. ChT retained most of its secondary structure and activity on AuNS and AuNR at low surface coverages; however, as protein loading approached monolayer conditions on AuNR, a 40% loss in secondary structure and 86% loss of activity was observed. Subsequent adsorption of ChT in multilayers on the AuNR surface allowed the conjugates to recover activity and remain stable. It is clear that AuNP morphology does affect adsorbed protein structure; a better understanding of these differences will be essential to engineer fully functional nanobioconjugates.