Identification of Aspartic and Isoaspartic Acid Residues in Amyloid β Peptides, Including Aβ1−42, Using Electron−Ion Reactions

Abstract

Amyloid β peptides are the major components of the vascular and plaque amyloid filaments in individuals with Alzheimer’s disease (AD). Although it is still unclear what initiates the disease, isomerization of aspartic acid residues in Aβ peptides is directly related to the pathology of AD. The detection of isomerization products is analytically challenging, due to their similar chemical properties and identical molecular mass. Different methods have been applied to differentiate and quantify the isomers, including immunology, chromatography, and mass spectrometry. Typically, those methods require comparative analysis with the standard peptides and involve many sample preparation steps. To understand the role of Aβ isomerization in AD progression, a fast, simple, accurate, and reproducible method is necessary. In this work, electron capture dissociation (ECD) Fourier-transform ion cyclotron resonance mass spectrometry (FTICR MS) was applied to detect isomerization in Aβ peptides. ECD generated diagnostic fragment ions for the two isomers of Aβ17−28, [M + 2H − 60]^+• and z₆^• − 44 when aspartic acid was present and z₆^• − 57 when isoaspartic acid was present. Additionally, the z_n − 57 diagnostic ion was also observed in the electron ionization dissociation (EID) spectra of the modified Aβ17−28 fragment. ECD was further applied toward Aβ1−40 and Aβ1−42. The diagnostic ion c₆ + 57 was observed in the ECD spectra of the Aβ1−42 peptide, demonstrating isomerization at residue 7. In conclusion, both ECD and EID can clearly determine the presence and the position of isoaspartic acid residues in amyloid β peptides. The next step, therefore, is to apply this method to analyze samples of Alzheimer’s patients and healthy individuals in order to generate a better understanding of the disease.