The Gas-Phase Oxidation of Cationic Bioanalytes via Ion/Ion Reactions

Several solution-phase derivatizations have recently been implemented in the gas-phase through the interaction between oppositely charged ions, viz., ion/ion reactions. The work presented here primarily focuses on the oxidation of bioanalytes via ion/ion reactions with periodate and persulfate anions. Methionine and tryptophan residues in simple polypeptides are selectively oxidized upon ion/ion reactions with periodate anion. The oxidative labeling of disulfide bonds is performed via ion/ion reactions and is used to identify intermolecularly disulfide-linked peptides and further probe their primary structure. Non-modified, non-disulfide linked peptides lacking easily oxidized residues (i.e., methionine and tryptophan) can also undergo oxidation. Peptides containing neutral basic sites undergo oxidation upon ion/ion reactions with periodate anion to various forms, including the [M+H+O]+,[M-H]+, and [M-H-NH3]+ species. Furthermore, persulfate anion is a stronger oxidizing reagent than periodate and increases the amount of oxidation observed with these less-readily oxidized residues. Persulfate anion and its derivatives, sulfate radical anion and peroxymonosulfate anion are capable of generating a variety of oxidation products, including the [M+H+O]+, [M-H]+, and M+• species