Generation of di-lithiated peptide ions from multiply protonated peptides via ion/ion reactions

Gas phase conversion of multiply protonated peptides to alkali di-metalated peptide ions via ion/ion reactions is demonstrated as an alternative strategy to forming alkali di-metalated ions from solution. Previously, alkali di-metalated peptide ions have been generated by direct electrospray (ESI) of solutions that contain both peptides and a salt of the metal of interest. Here, di-lithiated peptides are generated in the gas-phase via ion/ion reactions of multiply charged protonated peptides with metal containing anions (Metal2L3−) where lithium is the metal, and L is a singly charged anionic ligand. Fragmentation spectra of a series of di-lithiated model peptide ions [Pep-H + 2Metal]+ formed via ion/ion reactions are compared with fragmentation of ions of the same molecular formula formed directly from nano-electrospray ionization (nano-ESI) of solutions that contain metal salts and peptides. Di-lithiated ions formed in the gas phase fragment in a similar manner to those formed directly from nano-ESI of a solution containing metal salt and peptide. Low energy beam type collisionally induced dissociation (CID) is also demonstrated as a tool to form the ion of interest from an intermediate complex that is formed during an ion/ion reaction.