Characterization of pH-Dependent Reversible Self-Assembly of Amyloid Beta 1-40-Coated Gold Colloids

The characterization of Aβ1-40 coated over the nano-gold colloidal particle surfaces was conducted using surface plasmon resonance (SPR) spectroscopy and transmission electron microscopy (TEM). The observed pH-dependent shift of the SPR band of Aβ1-40-coated 20 nm gold particles was correlated with alternation of aggregation and disaggregation observed in the TEM images. A pH of ~4 induced an unfolded conformation, and a pH of ~10 induced a folded conformation of Aβ1-40 on the gold surface. This reversible aggregation process was observed by Raman imaging as the pH was gradually changed from pH 4 to pH 10. We observed a pH-dependent morphology change in Aβ1-40 on the gold surface, where clear aggregates were observed at pH 4. However, we observed very subtle differences in the surface-enhanced Raman spectroscopy (SERS) spectrum between pH 4 and pH 10 conditions, with the most striking difference being spectral density in the region of 250 cm^-1 and 1750 cm^-1. Specifically, the mode analysis of the reversible aggregation indicated that the aggregates were formed by the unfolded conformation of Aβ1-40 which involved the benzene ring section of Tyrosine and Phenylalanine. Conversely, the disassembly of the aggregates was associated with conformational changes in protein folding of Aβ1-40 involving Histidine, Glutamine, Methionine, and Aspartic acid.