The interaction between Ag+ and bovine serum albumin: A spectroscopic investigation

By using spectroscopic methods, we probed the interaction of Ag+ with bovine serum albumin (BSA) in an aqueous environment. Fluorescence of BSA quenched by Ag+ is a dynamic quenching process. Two binding modes–a strong one at low concentration of Ag+ and a weak one at high concentration were found. The association constant (KA) and the number of binding sites (n) were 4.88 × 103 M− 1 and 1.17 for strong binding, and 17.6 M− 1 and 0.547 for weak binding at 293 K. The results of thermodynamic parameters ΔHθ, ΔGθ and ΔSθ for instinct binding modes at different temperatures indicated that the hydrogen bonding and van der Waals interaction play a major role for low Ag+/BSA ratio while electrostatic association for high Ag+/BSA ratio. Data of UV–Vis and Circular dichroism (CD) suggested that with the increasing amount of Ag+, the secondary structure undergoes a decrease in α-helix and an increase in β content and the backbone of BSA experiences a micro-environmental alteration. Furthermore, the distance r between donor (Trp-212) and acceptor (Ag+) was evaluated to be 10 nm according to nonradiative energy transfer theory.

Research Highlights
► A spectroscopy-based assessment model was performed to estimate the binding of Ag+ to bovine serum albumin (BSA).
► Ag+ was found not only to change the microenvironment of fluorescent amino acid residues of BSA but also to cause alterations in the secondary and tertiary structural content of protein.
► Two binding modes occur according to Ag+ concentration, which is different other metal bindings.