Influence of the Global Charge of the Protein on the Stability of Lysozyme–AuNP Bioconjugates

Abstract

The structural organization of the proteins that interact with a metallic nanoparticle and form the protein corona is the relevant biological information to take into account in the design of bionanoconjugates. We have studied the interaction of the protein Lysozyme (LYZ) with gold nanoparticles (AuNPs) protected by citrate, 6-mercaptopurine and -mercaptoundecanoic acid monolayers in aqueous solution in a wide pH range. The bioconjugates are stables at pH higher than the isoelectric point (pI) of LYZ. At lower pH, flocculation occurs possibly by interactions between the exposed positive charges of the protein. The modification of the Lys residues of the protein by either succinylation or phosphopyridoxylation brings about important changes in the flocculation behavior of the bioconjugates. By taking into account the location of the modified residues in the protein three-dimensional structure, a different orientation of the protein upon binding in comparison to the native protein is proposed. Finally, the high fluorescence quenching observed in the titration experiments of the bioconjugates are discussed in terms of an amplification of the quenching efficiency by energy transfer on the self-assembled protein film and a severe effect of fluorofore interactions of the proteins attached to the nanoparticle surface.