Alzheimer's disease (AD) is a
neurodegenerative disorder of increasing concern. It belongs to diseases termed
tauopathies which are characterized by inclusions of abnormally hyperphosphorylated and truncated forms of the
protein tau. Studies of
tauopathies often focus on detection and characterization of these aberrant tau proteoforms, in particular the phosphorylation sites, which represent a significant analytical challenge for example when several phosphosites can be present on the same
peptide. Such isomers can even be difficult to fully separate chromatographically. Since recently introduced cyclic ion mobility-mass spectrometry can offer different selectivity, we have investigated the closely positioned phosphorylation sites S214, T212, and T217 of a tryptic
peptide from
proline rich region of tau-TPSLPTPPTREPK. The conformational heterogeneity of the isomeric
peptides in the gas phase hindered their separation due to their overlapping arrival time distributions. Increasing the resolution of the analysis alone is insufficient to distinguish the
peptides in a mixture typical of patient samples. We therefore developed a method based on a combination of collision-induced dissociation, isomeric product
ions (m/z 677) mobility separation and post-mobility dissociation to aid in analyzing the isomeric
phosphopeptides of tau in diseased brain extract. For all three isomers (T212, S214, and T217), the ion mobility signal of the ion at m/z 677 was still observable at the concentration of 0.1 nmol/L. This work not only offers insights into the phosphorylation of
tau protein in AD but also provides an analytical workflow for the characterization of challenging pathological
protein modifications in
neurodegenerative diseases.