Conformational-Sensitive Fast Photochemical Oxidation of Proteins and Mass Spectrometry Characterize Amyloid Beta 1-42 Aggregation

Preventing and treating Alzheimer's disease require understanding the aggregation of amyloid beta 1-42 (Aβ_1-42) to give oligomers, protofibrils, and fibrils. Here we describe footprinting of Aβ_1-42 by hydroxyl radical-based fast photochemical oxidation of proteins (FPOP) and mass spectrometry (MS) to monitor the time-course of Aβ_1-42 aggregation. We resolved five distinct stages characterized by two sigmoidal behaviors, showing the time-dependent transitions of monomers-paranuclei-protofibrils-fibrillar aggregates. Kinetic modeling allows deciphering the amounts and interconversion of the dominant Aβ_1-42 species. Moreover, the irreversible footprinting probe provides insights into the kinetics of oligomerization and subsequent fibrillar growth by allowing the conformational changes of Aβ_1-42 at subregional and even amino-acid-residue levels to be revealed. The middle domain of Aβ_1-42 plays a major role in aggregation, whereas the N-terminus retains most of its solvent-accessibility during aggregation, and the hydrophobic C-terminus is involved to an intermediate extent. This approach affords an in situ, real-time monitoring of the solvent accessibility of Aβ_1-42 at various stages of oligomerization, and provides new insights on site-specific aggregation of Aβ_1-42 for a sample state beyond the capabilities of most other biophysical methods.