Probing the Secondary Structure of Individual Aβ₄₀ Amorphous Aggregates and Fibrils by AFM-IR Spectroscopy

Area of Science:

• Biochemistry
• Biophysics
• Neuroscience

Background:

• Alzheimer's disease (AD) pathogenesis involves amyloid-beta (Aβ) protein aggregation.
• Understanding the structural dynamics of Aβ aggregates and fibrils is crucial for elucidating AD mechanisms at a molecular level.

Purpose of the Study:

• To characterize the structural evolution of Aβ aggregates and fibrils over time using high-resolution spectroscopy.
• To investigate the influence of copper ions (Cu2+) on Aβ aggregation pathways.

Main Methods:

• Atomic Force Microscopy-Infrared (AFM-IR) spectroscopy was employed to analyze the secondary structure of individual Aβ aggregates and fibrils.
• Aβ aggregation was monitored at different time points (15 minutes, 2 hours, 1 week).
• The effect of Cu2+ on Aβ aggregation was examined.

Main Results:

• At 15 minutes, Aβ primarily existed as disordered structures.
• At 2 hours, large amorphous aggregates showed structural diversity, including co-existing parallel and antiparallel β-sheets, with fibril-like species present.
• By 1 week, Aβ fibrils displayed the characteristic signature of parallel β-sheets.
• Cu2+ induced rapid formation of amorphous Aβ aggregates with varied secondary structures and inhibited subsequent fibril growth.

Conclusions:

• Fibril-like structures are present within large amorphous Aβ aggregates, suggesting nucleation may occur within these species.
• This finding offers insights analogous to protein crystallization theories where nucleation occurs in large clusters.
• Copper ions significantly alter Aβ aggregation dynamics, promoting amorphous structures over fibrils.