Investigating The Reversibility Of Protein Adsorption On Gold Nanoparticles And The Role Of Free Thiol

Protein-gold nanoparticle (AuNP) bioconjugates have many potential applications in nanomedicine. A thorough understanding of the interaction between the protein and AuNP is critical to engineering these functional bioconjugates with desirable properties. In this work, we investigate the role of free thiols presented by the protein on the stability of the protein-AuNP conjugate. Human serum albumin (HSA) was modified with 2-iminothiolane (Traut’s reagent) to introduce additional thiols on the protein surface, and three variants of HSA were synthesized to present 1, 5, and 20 free thiols by controlling the molar excess of the chemical modifier. Protein exchange studies on AuNPs were conducted using these HSA species and an IgG antibody which exhibits 10 free sulfhydryls. Antibody-AuNP conjugates were synthesized, purified, and dispersed in solutions containing each of the HSA species. No protein exchange was detected with the HSA or modified HSA containing 5 thiols; however, 85% of the antibody was displaced on the AuNP surface by the extensively thiolated HSA presenting 20 free thiols. Furthermore, the impact of protein adsorption sequence was probed in which each of the HSA species were pre-adsorbed onto the AuNP and dispersed in a solution of antibody. The antibody fully displaced the HSA with a single thiol from the AuNP within 3 h, requires 24 h to completely displace the modified HSA containing 5 thiols, and was unable to displace the modified HSA containing 20 thiols. These results indicate that the number of Au-S interactions governs the binding interaction between the protein and AuNP. This work provides further insight into the protein-AuNP binding mechanism and identifies important design principles for engineered proteins to optimize bioconjugates