Protein Corona Formation and Aggregation of Amyloid β 1–40-Coated Gold Nanocolloids

Amyloid fibrillogenesis is a pathogenic protein aggregation process that occurs through a highly ordered process of protein–protein interactions. To better understand the protein–protein interactions involved in amyloid fibril formation, we formed nanogold colloid aggregates by stepwise additions of ∼2 nmol of amyloid β 1–40 peptide (Aβ_1–40) at pH ∼3.7 and ∼25 °C. The processes of protein corona formation and building of gold colloid [diameters (d) of 20 and 80 nm] aggregates were confirmed by a red-shift of the surface plasmon resonance (SPR) band, λ_peak, as the number of Aβ_1–40 peptides [N(Aβ_1–40)] increased. The normalized red-shift of λ_peak, Δλ, was correlated with the degree of protein aggregation, and this process was approximated as the adsorption isotherm explained by the Langmuir–Freundlich model. As the coverage fraction (θ) was analyzed as a function of ϕ, which is the N(Aβ_1–40) per total surface area of nanogold colloids available for adsorption, the parameters for explaining the Langmuir–Freundlich model were in good agreement for both 20 and 80 nm gold, indicating that ϕ could define the stage of the aggregation process. Surface-enhanced Raman scattering (SERS) imaging was conducted at designated values of ϕ and suggested that a protein–gold surface interaction during the initial adsorption stage may be dependent on the nanosize. The 20 nm gold case seems to prefer a relatively smaller contacting section, such as a −C–N or CC bond, but a plane of the benzene ring may play a significant role for 80 nm gold. Regardless of the size of the particles, the β-sheet and random coil conformations were considered to be used to form gold colloid aggregates. The methodology developed in this study allows for new insights into protein–protein interactions at distinct stages of aggregation.