Silver nanoparticles as inhibitors of insulin amyloid formation: A fluorescence study

Silver nanoparticles (SNPs) synthesized extracellularly using extracts of the different genus Pleurotus fungi samples, were tested for their potential to serve as the therapeutic agents targeting insulin amyloid formation at physiological conditions in vitro. The relatively monodisperse crystallites of average size ∼ 6–20 nm, and ∼ 14–38 nm hydrodynamic diameters, stabilized by a fungal protein cap and possessing zeta potential values ∼ – (39–55) mV, were obtained, as revealed by dynamic light scattering and X-ray diffraction analysis. Absorption maxima of the SNPs were within the range of 406–413 nm, corresponding to the surface plasmon resonance, while weak fluorescence bands centered at 600–610 nm were observed both in buffer and in the presence of the insulin amyloid fibrils. The Thioflavin T (ThT) fluorescence intensity at 480 nm in the protein samples incubated with the SNPs, was reduced to ∼ 29–42 % of its value in the control, revealing lower amount of the formed fibrillar species. The insulin fibrillar structures of ∼ 17 ± 5 nm in thickness and ∼ 1.4 ± 0.2 µm in length were grown in the control, while the protein spherical assemblies of ∼ 30–100 nm in diameter, as well as amorphous aggregates, both containing the SNPs in their structure, were observed in the presence of the SNPs, as revealed by transmission electron microscopy. Of these, the nanoparticles, referred to here as QD1, QD4, synthesized at NaNO3: biological medium molar ratio ∼ 1 : 1, and possessing the highest agglomeration stability, showed the pronounced anti-amyloid effect at low ratios of the SNPs to insulin ∼ 0.01 and 0.08, respectively. Based on the above results, van der Waals and hydrophobic interactions between the insulin prefibrillar assemblies and the SNPs were suggested to induce partial distortion of their structure followed by inhibition of the amyloid elongation phase and formation of the amorphous precipitates, respectively. Overall, the obtained results may represent a starting point for the application of the most effective SNPs as anti-amyloid agents.