On the Nucleation and Growth of Amyloid β -Protein Fibrils: Detection of Nuclei and Quantitation of Rate Constants

We have studied the fibrillogenesis of synthetic amyloid β -protein-(1-40) fragment (Aβ ) in 0.1 M HCl. At low pH, Aβ formed fibrils at a rate amenable to detailed monitoring by quasi-elastic light-scattering spectroscopy. Examination of the fibrils with circular dichroism spectroscopy and electron microscopy showed them to be highly similar to those found in amyloid plaques. We determined the hydrodynamic radii of Aβ aggregates during the entire process of fibril nucleation and growth. Above an Aβ concentration of ≈ 0.1 mM, the initial rate of elongation and the final size of fibrils were independent of Aβ concentration. Below an Aβ concentration of 0.1 mM, the initial elongation rate was proportional to the peptide concentration, and the resulting fibrils were significantly longer than those formed at higher concentration. We also found that the surfactant n-dodecylhexaoxyethylene glycol monoether (C₁₂E₆) slowed nucleation and elongation of fibrils in a concentration-dependent manner. Our observations are consistent with a model of Aβ fibrillogenesis that includes the following key steps: (i) peptide micelles form above a certain critical Aβ concentration, (ii) fibrils nucleate within these micelles or on heterogeneous nuclei (seeds), and (iii) fibrils grow by irreversible binding of monomers to fibril ends. Interpretation of our data enabled us to determine the sizes of fibril nuclei and Aβ micelles and the rates of fibril nucleation (from micelles) and fibril elongation. Our approach provides a powerful means for the quantitative assay of Aβ fibrillogenesis.