Ion mobility separation coupled with MS detects two structural states of Alzheimer's disease Aβ1-40 peptide oligomers

Mounting evidence points to the soluble oligomers of amyloid β (Aβ) peptide as important neurotoxic species in Alzheimer's disease, causing synaptic dysfunction and neuronal injury, and finally leading to neuronal death. The mechanism of the Aβ peptide self-assembly is still under debate. Here, Aβ1-40 peptide oligomers were studied using mass spectrometry combined with ion mobility spectrometry, which allowed separation of the signals of numerous oligomers and measurement of their collisional cross-section values (Ω). For several oligomers, at least two different species of different Ω values were detected, indicating the presence of at least two families of conformers: compact and extended. The obtained results are rationalized by a set of molecular models of Aβ1-40 oligomer structure that provided a very good correlation between the experimental and theoretical Ω values, both for the compact and the extended forms. Our results indicate that mass spectrometry detects oligomeric species that are on-pathway in the process of fibril formation or decay, but also alternative structures which may represent off-pathway evolution of oligomers.