Protein interactions with silver nanoparticles: Green synthesis, and biophysical approach

Silver nanoparticles (AgNPs) with an average particle size of 20nm were synthesized by using aromatic amino acid fluorescence active, tryptophan as a reducing agent. This study aims to investigate the interaction between Bovine Serum Albumin (BSA) and AgNPs as a function of particle size and shape. UV-vis analysis implies the formation of the ground state complex between BSA and AgNPs through electrostatic interactions. The fluorescence spectra indicated that the AgNPs have a potent ability to quench the intrinsic fluorescence of BSA by static quenching mechanisms. The different parameters (the apparent association constant (K_app=2.6×10⁴mol^-1dm³₎, Stern-Volmer quenching constant (K_SV=3.5×10⁴mol^-1dm³), number of binding sites (n=1.3) and bimolecular rate constant of the quenching reaction (k_q=6.1×10¹²mol^-1dm³s^-1)) were calculated by using the UV-vis and fluorescence spectra and discussed. The indole moieties of tryptophan residues of BSA were responsible to the complex formation with AgNPs in ground and excited states via electrostatic, van der Waals, hydrogen bonding, hydrophobic and hydrophilic interactions. Adsorption of AgNPs into the core of BSA changes the tryptophan environment from hydrophobic to hydrophilic (from folding to partially folded and/or unfolded). Circular dichroism results suggested that the helicity of BSA decreased from 67.68% to 60.25% and 67.68% to 45.42% with [AgNPs] and temperature, respectively.