Background: In a biological system, an engineered nanomaterial (ENM) surface is altered by adsorbed proteins that modify ENM fate and toxicity. Thus far, protein corona characterizations have focused on protein adsorption, interaction strength, and downstream impacts on cell interactions. Given previous reports of Ag ENM disruption of Cu trafficking, this study focuses on Ag ENM interactions with a model Cu metalloprotein, Cu(II) azurin. The study provides evidence of otherwise overlooked ENM-protein chemical reactivity within the corona: redox activity.
Results: Citrate-coated Ag ENMs of various sizes (10-40 nm) reacted with Cu(II) azurin resulted in an order of magnitude more dissolved ionic silver (Ag(I)(aq)) than samples of Ag ENMs only, ENMs mixed Cu(II) ions, or control proteins such as cytochrome c and horse radish peroxidase. This dramatic increase in ENM oxidative dissolution was observed even when Cu(II) azurin was combined with a diverse mixture of Escherchia coli proteins to mimic the complexity of the cellular conona. SDS PAGE results confirm that the multiprotein ENM corona includes azurin. A Cu(I)(aq) colorimetric indicator confirms Cu(II) azurin reduction upon interaction with Ag ENMs, but not with the addition of ionic silver, Ag(I)(aq).
Conclusions: Cu(II) azurin and 10-40 nm Ag ENMs react to catalyze Ag ENM oxidative dissolution and reduction of the model Cu metalloprotein. Results push the current evaluation of protein-ENM characterization beyond passive binding interactions and enable the proposal of a mechanism for reactivity between a model Cu metalloprotein and Ag ENMs.