Discrimination of Aspartic and Isoaspartic Acid Residues in Peptides by Tandem Mass Spectrometry with Hydrogen Attachment Dissociation

Long-lived proteins undergo chemical modifications that can cause age-related diseases. Among these chemical modifications, isomerization is the most difficult to identify. Isomerization often occurs at the aspartic acid (Asp) residues. In this study, we used tandem mass spectrometry equipped with a newly developed ion activation method, hydrogen attachment dissociation (HAD), to analyze peptides containing Asp isomers. Although HAD preferentially produces [cn + 2H]+ and [zm + 2H]+ via N-Cα bond cleavage, [cn + 58 + 2H]+ and [zm - 58 + 2H]+ originate from the fragmentation of the isoAsp residue. Notably, [cn + 58 + 2H]+ and [zm - 58 + 2H]+ could be used as diagnostic fragment ions for the isoAsp residue because these fragment ions did not originate from the Asp residue. The detailed fragmentation mechanism was investigated by computational analysis using density functional theory. According to the results, hydrogen attachment to the carbonyl oxygen in the isoAsp residue results in the Cα-Cβ bond cleavage. The experimental and theoretical joint study indicates that the present method allows us to discriminate Asp and isoAsp residues, including site identification of the isoAsp residue. Moreover, we demonstrated that the molar ratio of peptide isomers in the mixture could be estimated from their fragment ion abundance. Therefore, tandem mass spectrometry with HAD is a useful method for the rapid discrimination and semiquantitative analysis of peptides containing isoAsp residues.

Ion-molecule reactions of mass-selected ions

Gas-phase reactions of mass-selected ions with neutrals covers a very broad area of fundamental and applied mass spectrometry (MS). Oftentimes, ion-molecule reactions (IMR) can serve as a viable alternative to collision-induced dissociation and other ion dissociation techniques when using tandem MS. This review focuses on the literature pertaining applications of IMR since 2013. During the past decade considerable efforts have been made in analytical applications of IMR, including advances in one of the major techniques for characterization of unsaturated fatty acids and lipids, ozone-induced dissociation, and the development of a new technique for sequencing of large ions, hydrogen atom attachment/abstraction dissociation. Many advances have also been made in identifying gas-phase chemistry specific to a functional group in organic and biological compounds, which are useful in structure elucidation of analytes and differentiation of isomers/isobars. With "soft" ionization techniques like electrospray ionization having become mainstream for quite some time now, the efforts in the area of metal ion catalysis have firmly moved into exploring chemistry of ligated metal complexes in their "natural" oxidation states allowing to model individual steps of mechanisms in homogeneous catalysis, especially in combination with high-level DFT calculations. Finally, IMR continue to contribute to the body of knowledge in the area of chemistry of interstellar processes.

Mass Spectrometric Behavior and Molecular Mechanisms of Fermented Deoxyanthocyanidins to Alleviate Ulcerative Colitis Based on Network Pharmacology

Aims. Ulcerative colitis (UC) is a type of chronic idiopathic inflammatory bowel disease with a multifactorial pathogenesis and limited treatment options. The aim of the present study is to investigate the hydrogen deuterium exchange mass spectrometry (HDX-MS) behaviors of fermented deoxyanthocyanidins and their molecular mechanisms to alleviate UC by using quantum chemistry and network pharmacology. Methods. Tandem MS indicated at least two fragmentation pathways through which deuterated vinylphenol-deoxyanthocyanidins could generate different product ions. Quantum calculations were conducted to determine the transition states of the relevant molecules and analyze their optimized configuration, vibrational characteristics, intrinsic reaction coordinates, and corresponding energies. The potential targets of deoxyanthocyanidins in UC were screened from a public database. The R package was used for Gene Ontology (GO) and KEGG pathway analyses, and the protein–protein interactions (PPIs) of the targets were assessed using Search Tool for the Retrieval of Interacting Genes (STRING). Finally, molecular docking was implemented to analyze the binding energies and action modes of the target compounds through the online tool CB-Dock. Results. Quantum calculations indicated two potential fragmentation pathways involving the six-membered ring and dihydrogen cooperative transfer reactions of the vinylphenol-deoxyanthocyanidins. A total of 146 and 57 intersecting targets of natural and fermented deoxyanthocyanidins were separately screened out from the UC database and significant overlaps in GO terms and KEGG pathways were noted. Three shared hub targets (i.e., PTGS2, ESR1, and EGFR) were selected from the two PPI networks by STRING. Molecular docking results showed that all deoxyanthocyanidins have a good binding potential with the hub target proteins and that fermented deoxyanthocyanidins have lower binding energies and more stable conformations compared with natural ones. Conclusions. Deoxyanthocyanidins may provide anti-inflammatory, antioxidative, and immune system regulatory effects to suppress UC progression. It is proposed for the first time that fermentation of deoxyanthocyanidins can help adjust the structure of the intestinal microbiota and increase the biological activity of the natural compounds against UC. Furthermore, HDX-MS is a helpful strategy to analyze deoxyanthocyanidin metabolites with unknown structures.

Hot Hydrogen Atom Irradiation of Protonated/Deprotonated Peptide in an Ion Trap Facilitates Fragmentation through Heated Radical Formation

Tandem mass spectrometry with fragmentation involving the reaction with hydrogen atoms is expected to be useful for the analysis of peptides and proteins. In general, hydrogen atoms preferentially react with odd-electron radicals. The attachment of hydrogen atoms to even-electron peptide ions is barely observed because of their low reaction rate. To date, only the methodology developed by our group has successfully induced the fragmentation of even-electron peptide ions by reacting with hydrogen atoms. In the present study, we focused on the temperature of the peptide ions and hydrogen atoms in an ion trap mass spectrometer to understand the mechanism of the corresponding reaction. Because the reaction between even-electron peptide ions and hydrogen atoms has a significant transition state barrier, the use of hot hydrogen atoms is required to initiate the reaction. The reaction contributes to increase the internal energy of the resultant peptide radicals because the heat of reaction and kinetic energy of the hydrogen atom are converted to the internal energy of the product. The resultant oxygen- and carbon-centered peptide radicals undergo radical-induced fragmentation with sub-picosecond and sub-millisecond time scales, respectively.

Gas-Phase Peptide Fragmentation Induced by Hydrogen Attachment, from Principle to Sequencing of Amide Nitrogen-Methylated Peptides

Tandem mass spectrometry (MS/MS) with radical-based fragmentation was developed recently, which involves the reaction of hydrogen atoms and peptides in a process called hydrogen attachment/abstraction dissociation (HAD). HAD mainly produces [c_n + 2H]⁺ and [z_m + 2H]⁺ via hydrogen attachment to the carbonyl oxygen on the peptide backbone. In addition, HAD often generates [a_n + 2H]⁺ and [x_m + 2H]⁺. To explain the formation of [a_n + 2H]⁺ and [x_m + 2H]⁺, hydrogen attachment to the carbonyl carbon atom on the peptide backbone is proposed to initiate C_α-C bond cleavage. The resultant hydrogen-abundant oxygen-centered radical intermediate undergoes radical-induced dissociation to give [a_n + H]^+• and [x_m + 2H]⁺. Subsequently, [a_n + 2H]⁺ was produced by the reaction of [a_n + H]^+• and a hydrogen atom. The fragment ions formed by the cleavage of N-C_α and C_α-C bonds are observed in the HAD-MS/MS spectra, and the mass differences of these fragment ions correspond to the mass of peptide bonds. Consequently, HAD-MS/MS allows the identification of post-translational modifications on the peptide backbone. In addition, HAD-MS/MS provides a consecutive series of [c_n + 2H]⁺ and [a_n + 2H]⁺ as the N-terminal fragments, as well as [z_m + 2H]⁺ and [x_m + 2H]⁺, which enables the sequencing of peptides with post-translational modification, including the discrimination of modifications on the side chain and backbone.

Characterization of Polyethers Using Tandem Mass Spectrometry with Hydrogen Abstraction Dissociation and Thermal Activation

The recently developed hydrogen radical-mediated fragmentation technique using an ion trap involving hydrogen attachment/abstraction dissociation-tandem mass spectrometry (HAD-MS/MS) was applied to the analysis of polyethylene glycol (PEG) and its derivatives. HAD was found to be initiated by hydrogen abstraction from carbon atoms in the polyether. Subsequently, the produced carbon-centered radical intermediates underwent radical-induced cleavage of their C-O bonds, with this process being facilitated by heating of the ion trap. The bond cleavage resulted in the formation of b fragments containing double bonds between carbon atoms. A counterpart c^• alkoxy radical was discovered to be a fragile radical species. Consequently, c^• underwent further radical-induced dissociation to produce small fragments during HAD-MS/MS with thermal activation. As a result, HAD-MS/MS with thermal activation through ion trap heating preferentially provided b fragments, facilitating identification of repeating units and individual end groups of the polyether analytes.

Hydrogen attachment dissociation of peptides containing disulfide bonds

A combination of tandem mass spectrometry (MS/MS) and hydrogen attachment dissociation (HAD) is a useful method for peptide sequence analysis. In this study, gas-phase fragmentation induced by the attachment of hydrogen to peptides containing disulfide bonds was investigated. Hydrogen attachment induced the cleavage of either the disulfide or N-C_α bond, which competitively occurred during HAD. The disulfide bond cleavage proceeded through an intermediate, which contains a thiyl radical (-S˙) and a thiol group (-SH). In contrast, N-C_α bond cleavage produced an intermediate containing an enol-imine group and α-carbon radical. The intermediate α-carbon radical then attacked the disulfide bond, resulting in a cyclic [z]⁺ fragment. The counterpart, [c + H]⁺˙ with a thiyl radical underwent further hydrogen attachment, producing [c + 2H]⁺. Because both disulfide and N-C_α bonds were cleaved by a single hydrogen attachment event, HAD-MS/MS can provide sequence information for the backbone region in the disulfide loop.