Gas-phase anionic complexes of alkali metal ions and peptides: Structure and collision activated decompositions.

Alkali metal ions and anionic peptides can be desorbed into the gas phase to give metal-bound peptides and bis(peptide) complexes bearing a - 1 charge. Although amide nitrogens of peptide bonds are deprotonated in the gas phase by alkali metal ions, this reacion does not occur in solution. Metal-bound dipeptide anions exist as a single structure, whereas those of tripeptide complexes have three structures as revealed by tandem mass spectrometric studies. Ions of bis(peptide) complexes of alkali metals decompose upon collisional activation principally to form deprotonated peptides, in contrast to bis(peptide) complexes of alkaline earth metal ions, which undergo elimination of a neutral peptide.