Structure characterization of unexpected covalent O-sulfonation and ion-pairing on an extremely hydrophilic peptide with CE-MS and FT-ICR-MS

• Published in: Analytical and bioanalytical chemistry 2015-09, Vol.407 (22), p.6637-6655
• Main Authors: Pattky, Martin, Nicolardi, Simone, Santiago-Schübel, Beatrix, Sydes, Daniel, van der Burgt, Yuri E. M, Klein, Antonia N, Jiang, Nan, Mohrlüder, Jeannine, Hänel, Karen, Kutzsche, Janine, Funke, S. A, Willbold, D, Willbold, S, Huhn, C
• Format: Article
• Language: English
• Subjects: Alzheimer disease; Alzheimer's disease; Amino acids; Analysis; Analytical Chemistry; asparagine; bacteriophages; Binding Sites; Biochemistry; Capillarity; Capillary electrophoresis; Characterization and Evaluation of Materials; Chemical properties; Chemistry; Chemistry and Materials Science; Chromatography; coatings; Covalence; deamidation; Diagnosis; dissociation; electrolytes; electron transfer; Electrophoresis; Electrophoresis, Capillary - methods; Food Science; Fourier transforms; Fragmentation; hydrophilicity; Hydrophobic and Hydrophilic Interactions; ions; Laboratory Medicine; Liquid chromatography; mass spectrometry; Molecular weight; Monitoring/Environmental Analysis; Paper in Forefront; Peptide Mapping - methods; Peptides; Peptides - chemistry; Protein Binding; Reproducibility of Results; Residues; reversed-phase liquid chromatography; Risk factors; Sensitivity and Specificity; Spectrometry, Mass, Electrospray Ionization - methods; Spectroscopy, Fourier Transform Infrared - methods; Sulfates; Sulfates - chemistry; Sulfonic Acids - chemistry; Sulfuric acid; threonine
• ISSN: 1618-2642; 1618-2650
• DOI: 10.1007/s00216-015-8826-8

In this study, we characterized unexpected side-products in a commercially synthesized peptide with the sequence RPRTRLHTHRNR. This so-called peptide D3 was selected by mirror phage display against low molecular weight amyloid-β-peptide (Aβ) associated with Alzheimer’s disease. Capillary electrophoresis (CE) was the method of choice for structure analysis because the extreme hydrophilicity of the peptide did not allow reversed-phase liquid chromatography (RPLC) and hydrophilic interaction stationary phases (HILIC). CE-MS analysis, applying a strongly acidic background electrolyte and different statically adsorbed capillary coatings, provided fast and efficient analysis and revealed that D3 unexpectedly showed strong ion-pairing with sulfuric acid. Moreover, covalent O-sulfonation at one or two threonine residues was identified as a result of a side reaction during peptide synthesis, and deamidation was found at either the asparagine residue or at the C-terminus. In total, more than 10 different species with different m/z values were observed. Tandem-MS analysis with collision induced dissociation (CID) using a CE-quadrupole-time-of-flight (QTOF) setup predominantly resulted in sulfate losses and did not yield any further characteristic fragment ions at high collision energies. Therefore, direct infusion Fourier transform ion cyclotron resonance (FT-ICR) MS was employed to identify the covalent modification and discriminate O-sulfonation from possible O-phosphorylation by using an accurate mass analysis. Electron transfer dissociation (ETD) was used for the identification of the threonine O-sulfation sites. In this work, it is shown that the combination of CE-MS and FT-ICR-MS with ETD fragmentation was essential for the full characterization of this extremely basic peptide with labile modifications.