Gas-Phase Oxidation of Neutral Basic Residues in Polypeptide Cations by Periodate

Field induced displacement reactions with proton bound dimers of organophosphorus compounds in a tandem differential mobility spectrometer

Endothermic displacement reactions between proton bound dimers of organophosphorus compounds (OPCs) and isopropanol (IPA) were enabled in air at ambient pressure with tandem differential mobility spectrometry (DMS). Proton bound dimers (M2H+) were mobility isolated in purified air with a first DMS stage, mixed with IPA at ≥100 ppm in a middle reactive stage at 106 to 160 Td from a symmetrical 4 MHz waveform, and mobility analyzed in a second DMS stage. Although the enthalpy for displacement of M by IPA in M2H+ is unfavorable by +44 to 50 kJ mol-1, formation of the heterogenous proton bound dimer, MH+(IPA) arises from field induced dissociation of M2H+ to MH+ and addition of IPA. While peak dispersion for M2H+ of OPCs is limited to -2.25 to -0.5 V compensation voltage, peaks for MH+(IPA) were located at -10.5 to -8.25 V through a combination of ion transformation and mobility-based vapor modification. This inaugural use of ion reactions in air at ambient pressure demonstrates that multi-stage sequential processing of ions can improve significantly the analytical performance in a mobility spectrometer.

Gas-phase rearrangement reaction of Schiff-base-modified peptide ions

RATIONALE

Schiff base modification of peptides has been shown to facilitate their primary structural characterization via tandem mass spectrometry. However, we have discovered a novel rearrangement reaction via ion trap collisional activation involving the imine of the Schiff base and one of several functional groups, particularly the side chains of the basic residues lysine, arginine, and histidine, in the peptide.

METHODS

Gas-phase ion/ion reactions involving an aldehyde-containing reagent were used to generate Schiff-base-modified model peptides in a hybrid triple quadrupole/linear ion trap tandem mass spectrometer. Subsequent ion trap collisional activation was used to study the rearrangement reaction.

RESULTS

Schiff-base-modified peptide ions were found to undergo a rearrangement reaction that was observed to be either a major or minor contributor to the product ion spectrum, depending upon a variety of factors that include, for example, ion polarity, identity of the nucleophile in the peptide (e.g., side chains of lysine, histidine, and arginine), and the position of the nucleophile relative to the imine.

CONCLUSIONS

Relatively low-energy rearrangement reactions can occur in Schiff-base-modified peptide ions that involve the imine of the Schiff base and a nucleophile present in the polypeptide. While this rearrangement process does not appear to compromise the structural information that can be generated via collisional activation of Schiff-base-modified peptide ions, it can siphon away signal from the structurally diagnostic processes in some instances.

Gas-Phase Oxidation via Ion/Ion Reactions: Pathways and Applications

Here, we provide an overview of pathways available upon the gas-phase oxidation of peptides and DNA via ion/ion reactions and explore potential applications of these chemistries. The oxidation of thioethers (i.e., methionine residues and S-alkyl cysteine residues), disulfide bonds, S-nitrosylated cysteine residues, and DNA to the [M+H+O]+ derivative via ion/ion reactions with periodate and peroxymono-sulfate anions is demonstrated. The oxidation of neutral basic sites to various oxidized structures, including the [M+H+O]+, [M-H]+, and [M-H-NH3]+ species, via ion/ion reactions is illustrated and the oxidation characteristics of two different oxidizing reagents, periodate and persulfate anions, are compared. Lastly, the highly efficient generation of molecular radical cations via ion/ion reactions with sulfate radical anion is summarized. Activation of the newly generated molecular radical peptide cations results in losses of various neutral side chains, several of which generate dehydroalanine residues that can be used to localize the amino acid from which the dehydroalanine was generated. The chemistries presented herein result in a diverse range of structures that can be used for a variety of applications, including the identification and localization of S-alkyl cysteine residues, the oxidative cleavage of disulfide bonds, and the generation of molecular radical cations from even-electron doubly protonated peptides. Graphical Abstract ᅟ.