How do Nanoparticle Size and Protein Charge Affect Gold Nanoparticle-Protein Interactions?

When exposed to a solution containing gold nanoparticles (AuNPs), proteins spontaneously bind to the nanoparticle surface, leading to the formation of a stable surface coating, or biocorona. The composition of this biocorona in biological fluids depends on various factors, and it is currently impossible to predict whether an adsorbed protein will retain its function on the AuNP surface. AuNPs are potentially useful in a variety of diagnostic and therapeutic applications, but it is necessary to understand the physical interaction with host proteins before any of these applications can be realized. Using biophysical NMR spectroscopy, we are investigating the structural consequences and thermodynamic determinants of protein-nanoparticle interactions. Using a dataset of six globular proteins, we have found that all of them appear to remain globular upon binding, forming a single layer on the nanoparticle surface. Protein hydrogen-deuterium exchange rates are not perturbed upon addition of AuNPs, suggesting that at least some native structure is retained on the surface. Our observations hold for a broad range of nanoparticle sizes, suggesting that surface curvature may only play a minor role in protein binding. Most recently, we have investigated protein binding as a function of pH, and, assuming moderate shifts in protein pKa values, we have derived a thermodynamic model for the electrostatic contribution to binding. This model can be used to interpret competition experiments where two proteins are exposed to the same nanoparticle. From our results, we conclude that binding is a complex, electrostatically-driven process with multiple discrete steps. Future investigations will focus on whether the principles derived here can be used to design functional protein-based nanoconjugates.