Strong Interactions of Anionic Peptides and Alkaline Earth Metal Ions: Metal-Ion-Bound Peptides in the Gas Phase

Tripeptides comprising amino acids with neutral side chains interact with alkaline earth metal ions to form gas-phase anionic complexes of the composition [tripept + Met²⁺ − 3H⁺]⁻ under fast atom bombardment. The metal ion binds to the deprotonated C-terminal carboxylate group and to the two amide nitrogens. Because the C-terminal and the central amino acid are tightly bound by the metal ion, they are not vulnerable to collisionally activated decompositions in a tandem mass spectrometer. Instead, the significant fragmentations occur at the N-terminal amino acid site, which is the least tightly bonded. Ions are formed by elimination of an imine and the imine plus CO from the N-terminus (product ions are assigned as x₂ + H and y₂, respectively). Other major fragmentations of this complex include dehydrogenation and loss of an ammonia molecule. Peptides with functionalized side chains, such as those of serine, threonine, and phenylalanine, lose the side chains readily when they are bound to metal ions and submitted to collisional activation. Other fragmentation channels are largely suppressed, indicating direct metal ion‒side chain interaction. Fragmentation mechanisms are proposed on the basis of results with isotopically labeled peptides and from MS/MS/MS experiments.