The antibacterial activity and formation mechanism of quercetin-coated silver nanoparticles and protein complex

Positively charged quercetin-coated AgNPs (Que@AgNPs) were synthesized using quercetin as reducing agent and cetyltrimethyl ammonium bromide as stabilizer. The antibacterial activity of Que@AgNPs against Escherichia coli and S. aureus is significantly stronger than that of AgNPs without quercetin coating. It is found that the antibacterial activity of Que@AgNPs-lysozyme/γ-globulin complex is further enhanced, which is related to the concentration of Que@AgNPs and the coated lysozyme/γ-globulin. While, the antibacterial activity of Que@AgNPs-fibrinogen complex is lower than that of Que@AgNPs. The formation mechanism of Que@AgNPs-protein complex is further studied. The results show that the interaction between lysozyme/γ-globulin/fibrinogen and Que@AgNPs is a static quenching process. Hydrophobic interaction is important for lysozyme/γ-globulin, whereas in the case of fibrinogen, electrostatic force is mainly present. The presence of two different types of the binding sites for lysozyme and only one type of binding site for γ-globulin and fibrinogen on the surface of Que@AgNPs. The adsorption of lysozyme/γ-globulin/fibrinogen onto Que@AgNPs follows pseudo-second-order model, and chemisorption is the rate-limiting step. The experimental data fit well with Freundlich isotherm model. Que@AgNPs result in the loosening and unfolding of lysozyme/γ-globulin/fibrinogen backbone, and the content of α-helix (lysozyme, fibrinogen) or β-sheet (γ-globulin) of the secondary structure increases with increasing of Que@AgNPs concentrations.