Structure and Dynamics of Small Soluble Abeta(1-40) Oligomers Studied by Top-Down Hydrogen Exchange Mass Spectrometry.

Biochemistry. 2012 May 1;51(17):3694-703. Epub 2012 Apr 16
Pan J, Han J, Borchers CH, Konermann L.

Abeta peptides can assemble into amyloid fibrils, which represent one of the hallmarks of Alzheimer's disease. Recent studies, however, have focused on the behavior of small soluble Abeta oligomers which appear to possess a much greater neurotoxicity than mature fibrils. The structural characterization of these oligomers remains difficult due to their highly dynamic and polymorphic nature. This work explores the behavior of Abeta(1-40) in slightly basic solution (pH 9.3) at low salt concentration (10 mM ammonium acetate). These conditions lead to the formation of small oligomers, without any signs of fibrillation for several hours. The structure and dynamics of these oligomers were characterized by circular dichroism spectroscopy, size exclusion chromatography, as well as millisecond time-resolved hydrogen exchange mass spectrometry (MS). Our results reveal rapid interconversion between Abeta(1-40) oligomers and monomers. The mole fraction of monomeric molecules is on the order of 40%. Oligomers consist of ~4 Abeta(1-40) molecules on average, and the resulting assemblies have a predominantly beta-sheet secondary structure. Hydrogen exchange proceeds in the EX1 regime. This feature allows the application of conformer-specific top-down MS. Electron capture dissociation is used for interrogating the deuteration behavior of the Abeta(1-40) oligomers. This approach provides a spatial resolution of ~ 2 residues. The backbone amide deuteration pattern uncovered in this way is consistent with a beta-turn-beta motif for L17 to M35. The N-terminus is involved in hydrogen bonding as well, whereas protection gradually tapers off for the C-terminal residues 35-40. Our data are consistent with earlier proposals, according to which Abeta(1-40) oligomers adopt a beta-barrel structure. In general terms, this study demonstrates how top-down MS with precursor ion selection can be employed for structural studies on specific protein conformers within a heterogeneous mix.