Peptide and protein ion/ion reactions in electrodynamic ion traps: tools and methods

Gas-phase reactions of oppositely charged ions can play important roles in the analysis of peptides and proteins. Electrospray ionization (ESI) can yield multiply charged versions of gaseous peptide and protein ions that can react with oppositely charged ions via several distinct mechanisms. Reagent ions that react via proton transfer can be used to facilitate protein mixture analysis and the mass assignment of product ions in a tandem mass spectrometry experiment. Proton transfer reactions can also be used to concentrate protein ion signals into one or two charge states and can be used to charge state purify a precursor ion population for subsequent dissociation. Electron transfer reactions have been shown to lead to fragmentation of gaseous protonated peptides and proteins. The extent of sequence information available from an electron transfer reaction is often greater than that obtained via conventional collision-induced dissociation. Electron transfer dissociation is particularly useful in probing the structures of polypeptides with labile posttranslational modifications. We summarize here the tools and general methods for conducting ion/ion reaction studies with emphasis on electrodynamic ion traps as the reaction vessels.