A Three-Step Model for Protein–Gold Nanoparticle Adsorption

Gold nanoparticles (AuNPs) are an attractive delivery vector in biomedicine because of their low toxicity and unique electronic and chemical properties. AuNP bioconjugates can be used in many applications, including nanomaterials, biosensing, and drug delivery. While the phenomenon of spontaneous protein–AuNP adsorption is well-known, the structural and mechanistic details of this interaction remain poorly understood. As a result, predicting the orientation and structure of proteins on the nanoparticle surface remains a challenge. New techniques are therefore needed to characterize the structural properties of proteins as they bind to AuNPs. We have developed a straightforward and rapid NMR-based approach to quantitatively characterize the protein–AuNP interaction. This approach is immune to the inner filter effect, which complicates fluorescence measurements, and it can be performed without prior centrifugation of samples. Using a data set of six proteins, ranging in size from 3 to 583 residues, we measured the stoichiometry of binding to AuNPs with a diameter of 15 nm. The stoichiometry of binding can be predicted based on simple geometric considerations assuming that proteins remain globular on the AuNP surface. Using our approach, we find that a protein lacking cysteine residues can be displaced from AuNPs using a small organothiol compound, but proteins with surface cysteines are resistant to displacement. From this data we develop a model for adsorption consisting of three steps: an initial reversible association step, a rearrangement/reorientation step on the AuNP surface, and a final cysteine-dependent “hardening” step, after which binding becomes irreversible