Concanavalin A adsorption on gold nanoparticles: Influence of citrate ions on nanoparticle-protein interactions

We report in this article a dynamic overcoating of a protein on gold nanoparticles (AuNP). By exploring in depth the experimental and theoretical aspects of the interactions between AuNPs, stabilized by a layer of sodium citrate (AuNP–cit), and a lectin, Concanavalin A (ConA), we highlighted a dynamic interaction between these two entities. Experiments (physicochemical characterization, fluorescence quenching of tryptophan residues of ConA, and atomic force microscopy in force mode) were run in parallel with a numerical approach using molecular dynamics (MD) to explain in detail the different aspects of the [AuNP–cit ConA] interactions. Our results were compared to the ones obtained using human serum albumin (HSA) and bare AuNP (without the sodium citrate coating) obtained via MD. The complex [AuNP–cit ConA] was of low energy non-covalent binding via electrostatic interactions (with ionic binding) and low affinity between the two partners. Additionally, ConA was not able to displace the sodium citrate corona from AuNP–cit, leading to an important electrostatic competition between the different species in the solvent. This phenomenon suggested that a dynamic overcoating of the sodium citrate corona of the AuNPs took place rather than a ligand exchange, in which sodium citrate is displaced from the surface of AuNPs, which is more classically described in the case of NP – protein interactions.