Gas-phase peptide/protein cationizing agent switching via ion/ion reactions

Polypeptide ions comprising different cationizing agents show distinct fragmentation behavior in the gas phase. Thus, it is desirable to be able to form ions with different cationizing agents such as protons and metal ions. Usually, metal-cationized peptide/protein ions are introduced to the mass spectrometer by electrospraying solutions containing a mixture of the peptide/protein of interest and a metal salt. A new technique for generating metal-containing polypeptide ions that involves gas-phase ion/ion reactions is described. In this strategy, solutions of metal-containing ions and solutions of proteins are each electrosprayed into separate ion sources. The approach allows for independent maximization of ion signal and selection of ions prior to gas-phase reactions. Selected ions are stored in a quadrupole ion trap where reactions of ions of opposite polarity form metal-cationized peptides and proteins in the gas phase by cation switching. This approach affords a high degree of flexibility in forming metal-containing peptide and protein ions via the ability to mass-select reactant ions. The ability to form a variety of peptide/protein ions with various cationizing reagents in the gas phase is attractive both for the study of intrinsic interactions of metal ions with polypeptides and for maximizing the structural information available from tandem mass spectrometry of peptides and proteins.