Signatures of specificity of interactions of binary protein mixtures with citrate-stabilized gold nanoparticles

In this article we report on the observation of specificity of interactions of binary mixtures of proteins with citrate-stabilized gold nanoparticles (Cit-Au NPs), by following the changes in the optical properties of the NPs. The protein mixtures consisted of alpha-amylase and bovine serum albumin (BSA) or alpha-amylase and amyloglucosidase (AMG) or glucose oxidase (GOD) and peroxidase (POD). The results observed herein indicated that interaction between a binary protein mixture and Cit-Au NPs depended on the nature and concentration of the component proteins. For example, addition of increasing concentrations of proteins containing alpha-amylase and BSA consistently broadened the extinction spectrum of Cit-Au NPs. The area under the curves when plotted against the concentration of either of the proteins increased linearly. FTIR, fluorescence, starch agar plate assay and gel electrophoresis results indicated that both alpha-amylase and BSA were present in the agglomerated structures of proteins and NPs, indicating that both of the proteins played a role in the association of NPs. On the other hand, when the mixture contained increasing concentration either of alpha-amylase or AMG, the broadening as well as the change in the area under the curve varied randomly rather than following any linearity. Interestingly, when the mixtures of GOD and POD were used-although broadening was observed the change in the area was linear only for low concentrations of GOD in the medium and was very sensitive to its concentration. Transmission electron microscopy (TEM) results indicated agglomeration of the NPs in the presence of the protein mixtures as the primary reason behind the optical property change. Our observations indicated that the preferential attachment of one protein to Cit-Au NPs-in presence of the other-primarily depended on the overall charge of the protein in the medium.