Generation and manipulation of sodium cationized peptides in the gas phase

Sodiated peptides are often generated by electrospray ionization (ESI) of solutions containing peptides and a sodium salt. Fragmentation of singly sodiated, singly charged peptide ions commonly provides specific sequence information. However, these ions may be difficult to form by directly electrospraying a mixture. In the application of a recently described technique for forming metal containing peptide ions in the gas phase, singly sodiated, singly charged ions are formed via cation-switching ion/ion reactions of multiply protonated peptides. Proton transfer ion/ion reactions can also be used to form [M + Na]+ through the reduction of charge states of multiply charged, singly sodiated ions. The specificity and flexibility of the techniques employed provide a highly controlled means of generating sodiated peptide and protein ions. Thus, the methodologies presented here have potential for forming ions not readily observed via ESI or MALDI. Furthermore, the use of ion/ion reactions to form sodiated peptides facilitates direct comparisons of the fragmentation behavior of [M + Na]+ peptides formed in the absence of solvent with that of [M + Na]+ peptides generated by directly electrospraying a sodium salt/peptide mixture. Thus, in addition to descriptions of the formation of [M + Na]+ peptides in the gas phase using ion/ion reactions, results from CID of reaction products are presented herein.